diff options
Diffstat (limited to 'src/3rdparty/libjpeg/libjpeg.doc')
| -rw-r--r-- | src/3rdparty/libjpeg/libjpeg.doc | 3006 |
1 files changed, 3006 insertions, 0 deletions
diff --git a/src/3rdparty/libjpeg/libjpeg.doc b/src/3rdparty/libjpeg/libjpeg.doc new file mode 100644 index 0000000..689b206 --- /dev/null +++ b/src/3rdparty/libjpeg/libjpeg.doc @@ -0,0 +1,3006 @@ +USING THE IJG JPEG LIBRARY + +Copyright (C) 1994-1998, Thomas G. Lane. +This file is part of the Independent JPEG Group's software. +For conditions of distribution and use, see the accompanying README file. + + +This file describes how to use the IJG JPEG library within an application +program. Read it if you want to write a program that uses the library. + +The file example.c provides heavily commented skeleton code for calling the +JPEG library. Also see jpeglib.h (the include file to be used by application +programs) for full details about data structures and function parameter lists. +The library source code, of course, is the ultimate reference. + +Note that there have been *major* changes from the application interface +presented by IJG version 4 and earlier versions. The old design had several +inherent limitations, and it had accumulated a lot of cruft as we added +features while trying to minimize application-interface changes. We have +sacrificed backward compatibility in the version 5 rewrite, but we think the +improvements justify this. + + +TABLE OF CONTENTS +----------------- + +Overview: + Functions provided by the library + Outline of typical usage +Basic library usage: + Data formats + Compression details + Decompression details + Mechanics of usage: include files, linking, etc +Advanced features: + Compression parameter selection + Decompression parameter selection + Special color spaces + Error handling + Compressed data handling (source and destination managers) + I/O suspension + Progressive JPEG support + Buffered-image mode + Abbreviated datastreams and multiple images + Special markers + Raw (downsampled) image data + Really raw data: DCT coefficients + Progress monitoring + Memory management + Memory usage + Library compile-time options + Portability considerations + Notes for MS-DOS implementors + +You should read at least the overview and basic usage sections before trying +to program with the library. The sections on advanced features can be read +if and when you need them. + + +OVERVIEW +======== + +Functions provided by the library +--------------------------------- + +The IJG JPEG library provides C code to read and write JPEG-compressed image +files. The surrounding application program receives or supplies image data a +scanline at a time, using a straightforward uncompressed image format. All +details of color conversion and other preprocessing/postprocessing can be +handled by the library. + +The library includes a substantial amount of code that is not covered by the +JPEG standard but is necessary for typical applications of JPEG. These +functions preprocess the image before JPEG compression or postprocess it after +decompression. They include colorspace conversion, downsampling/upsampling, +and color quantization. The application indirectly selects use of this code +by specifying the format in which it wishes to supply or receive image data. +For example, if colormapped output is requested, then the decompression +library automatically invokes color quantization. + +A wide range of quality vs. speed tradeoffs are possible in JPEG processing, +and even more so in decompression postprocessing. The decompression library +provides multiple implementations that cover most of the useful tradeoffs, +ranging from very-high-quality down to fast-preview operation. On the +compression side we have generally not provided low-quality choices, since +compression is normally less time-critical. It should be understood that the +low-quality modes may not meet the JPEG standard's accuracy requirements; +nonetheless, they are useful for viewers. + +A word about functions *not* provided by the library. We handle a subset of +the ISO JPEG standard; most baseline, extended-sequential, and progressive +JPEG processes are supported. (Our subset includes all features now in common +use.) Unsupported ISO options include: + * Hierarchical storage + * Lossless JPEG + * Arithmetic entropy coding (unsupported for legal reasons) + * DNL marker + * Nonintegral subsampling ratios +We support both 8- and 12-bit data precision, but this is a compile-time +choice rather than a run-time choice; hence it is difficult to use both +precisions in a single application. + +By itself, the library handles only interchange JPEG datastreams --- in +particular the widely used JFIF file format. The library can be used by +surrounding code to process interchange or abbreviated JPEG datastreams that +are embedded in more complex file formats. (For example, this library is +used by the free LIBTIFF library to support JPEG compression in TIFF.) + + +Outline of typical usage +------------------------ + +The rough outline of a JPEG compression operation is: + + Allocate and initialize a JPEG compression object + Specify the destination for the compressed data (eg, a file) + Set parameters for compression, including image size & colorspace + jpeg_start_compress(...); + while (scan lines remain to be written) + jpeg_write_scanlines(...); + jpeg_finish_compress(...); + Release the JPEG compression object + +A JPEG compression object holds parameters and working state for the JPEG +library. We make creation/destruction of the object separate from starting +or finishing compression of an image; the same object can be re-used for a +series of image compression operations. This makes it easy to re-use the +same parameter settings for a sequence of images. Re-use of a JPEG object +also has important implications for processing abbreviated JPEG datastreams, +as discussed later. + +The image data to be compressed is supplied to jpeg_write_scanlines() from +in-memory buffers. If the application is doing file-to-file compression, +reading image data from the source file is the application's responsibility. +The library emits compressed data by calling a "data destination manager", +which typically will write the data into a file; but the application can +provide its own destination manager to do something else. + +Similarly, the rough outline of a JPEG decompression operation is: + + Allocate and initialize a JPEG decompression object + Specify the source of the compressed data (eg, a file) + Call jpeg_read_header() to obtain image info + Set parameters for decompression + jpeg_start_decompress(...); + while (scan lines remain to be read) + jpeg_read_scanlines(...); + jpeg_finish_decompress(...); + Release the JPEG decompression object + +This is comparable to the compression outline except that reading the +datastream header is a separate step. This is helpful because information +about the image's size, colorspace, etc is available when the application +selects decompression parameters. For example, the application can choose an +output scaling ratio that will fit the image into the available screen size. + +The decompression library obtains compressed data by calling a data source +manager, which typically will read the data from a file; but other behaviors +can be obtained with a custom source manager. Decompressed data is delivered +into in-memory buffers passed to jpeg_read_scanlines(). + +It is possible to abort an incomplete compression or decompression operation +by calling jpeg_abort(); or, if you do not need to retain the JPEG object, +simply release it by calling jpeg_destroy(). + +JPEG compression and decompression objects are two separate struct types. +However, they share some common fields, and certain routines such as +jpeg_destroy() can work on either type of object. + +The JPEG library has no static variables: all state is in the compression +or decompression object. Therefore it is possible to process multiple +compression and decompression operations concurrently, using multiple JPEG +objects. + +Both compression and decompression can be done in an incremental memory-to- +memory fashion, if suitable source/destination managers are used. See the +section on "I/O suspension" for more details. + + +BASIC LIBRARY USAGE +=================== + +Data formats +------------ + +Before diving into procedural details, it is helpful to understand the +image data format that the JPEG library expects or returns. + +The standard input image format is a rectangular array of pixels, with each +pixel having the same number of "component" or "sample" values (color +channels). You must specify how many components there are and the colorspace +interpretation of the components. Most applications will use RGB data +(three components per pixel) or grayscale data (one component per pixel). +PLEASE NOTE THAT RGB DATA IS THREE SAMPLES PER PIXEL, GRAYSCALE ONLY ONE. +A remarkable number of people manage to miss this, only to find that their +programs don't work with grayscale JPEG files. + +There is no provision for colormapped input. JPEG files are always full-color +or full grayscale (or sometimes another colorspace such as CMYK). You can +feed in a colormapped image by expanding it to full-color format. However +JPEG often doesn't work very well with source data that has been colormapped, +because of dithering noise. This is discussed in more detail in the JPEG FAQ +and the other references mentioned in the README file. + +Pixels are stored by scanlines, with each scanline running from left to +right. The component values for each pixel are adjacent in the row; for +example, R,G,B,R,G,B,R,G,B,... for 24-bit RGB color. Each scanline is an +array of data type JSAMPLE --- which is typically "unsigned char", unless +you've changed jmorecfg.h. (You can also change the RGB pixel layout, say +to B,G,R order, by modifying jmorecfg.h. But see the restrictions listed in +that file before doing so.) + +A 2-D array of pixels is formed by making a list of pointers to the starts of +scanlines; so the scanlines need not be physically adjacent in memory. Even +if you process just one scanline at a time, you must make a one-element +pointer array to conform to this structure. Pointers to JSAMPLE rows are of +type JSAMPROW, and the pointer to the pointer array is of type JSAMPARRAY. + +The library accepts or supplies one or more complete scanlines per call. +It is not possible to process part of a row at a time. Scanlines are always +processed top-to-bottom. You can process an entire image in one call if you +have it all in memory, but usually it's simplest to process one scanline at +a time. + +For best results, source data values should have the precision specified by +BITS_IN_JSAMPLE (normally 8 bits). For instance, if you choose to compress +data that's only 6 bits/channel, you should left-justify each value in a +byte before passing it to the compressor. If you need to compress data +that has more than 8 bits/channel, compile with BITS_IN_JSAMPLE = 12. +(See "Library compile-time options", later.) + + +The data format returned by the decompressor is the same in all details, +except that colormapped output is supported. (Again, a JPEG file is never +colormapped. But you can ask the decompressor to perform on-the-fly color +quantization to deliver colormapped output.) If you request colormapped +output then the returned data array contains a single JSAMPLE per pixel; +its value is an index into a color map. The color map is represented as +a 2-D JSAMPARRAY in which each row holds the values of one color component, +that is, colormap[i][j] is the value of the i'th color component for pixel +value (map index) j. Note that since the colormap indexes are stored in +JSAMPLEs, the maximum number of colors is limited by the size of JSAMPLE +(ie, at most 256 colors for an 8-bit JPEG library). + + +Compression details +------------------- + +Here we revisit the JPEG compression outline given in the overview. + +1. Allocate and initialize a JPEG compression object. + +A JPEG compression object is a "struct jpeg_compress_struct". (It also has +a bunch of subsidiary structures which are allocated via malloc(), but the +application doesn't control those directly.) This struct can be just a local +variable in the calling routine, if a single routine is going to execute the +whole JPEG compression sequence. Otherwise it can be static or allocated +from malloc(). + +You will also need a structure representing a JPEG error handler. The part +of this that the library cares about is a "struct jpeg_error_mgr". If you +are providing your own error handler, you'll typically want to embed the +jpeg_error_mgr struct in a larger structure; this is discussed later under +"Error handling". For now we'll assume you are just using the default error +handler. The default error handler will print JPEG error/warning messages +on stderr, and it will call exit() if a fatal error occurs. + +You must initialize the error handler structure, store a pointer to it into +the JPEG object's "err" field, and then call jpeg_create_compress() to +initialize the rest of the JPEG object. + +Typical code for this step, if you are using the default error handler, is + + struct jpeg_compress_struct cinfo; + struct jpeg_error_mgr jerr; + ... + cinfo.err = jpeg_std_error(&jerr); + jpeg_create_compress(&cinfo); + +jpeg_create_compress allocates a small amount of memory, so it could fail +if you are out of memory. In that case it will exit via the error handler; +that's why the error handler must be initialized first. + + +2. Specify the destination for the compressed data (eg, a file). + +As previously mentioned, the JPEG library delivers compressed data to a +"data destination" module. The library includes one data destination +module which knows how to write to a stdio stream. You can use your own +destination module if you want to do something else, as discussed later. + +If you use the standard destination module, you must open the target stdio +stream beforehand. Typical code for this step looks like: + + FILE * outfile; + ... + if ((outfile = fopen(filename, "wb")) == NULL) { + fprintf(stderr, "can't open %s\n", filename); + exit(1); + } + jpeg_stdio_dest(&cinfo, outfile); + +where the last line invokes the standard destination module. + +WARNING: it is critical that the binary compressed data be delivered to the +output file unchanged. On non-Unix systems the stdio library may perform +newline translation or otherwise corrupt binary data. To suppress this +behavior, you may need to use a "b" option to fopen (as shown above), or use +setmode() or another routine to put the stdio stream in binary mode. See +cjpeg.c and djpeg.c for code that has been found to work on many systems. + +You can select the data destination after setting other parameters (step 3), +if that's more convenient. You may not change the destination between +calling jpeg_start_compress() and jpeg_finish_compress(). + + +3. Set parameters for compression, including image size & colorspace. + +You must supply information about the source image by setting the following +fields in the JPEG object (cinfo structure): + + image_width Width of image, in pixels + image_height Height of image, in pixels + input_components Number of color channels (samples per pixel) + in_color_space Color space of source image + +The image dimensions are, hopefully, obvious. JPEG supports image dimensions +of 1 to 64K pixels in either direction. The input color space is typically +RGB or grayscale, and input_components is 3 or 1 accordingly. (See "Special +color spaces", later, for more info.) The in_color_space field must be +assigned one of the J_COLOR_SPACE enum constants, typically JCS_RGB or +JCS_GRAYSCALE. + +JPEG has a large number of compression parameters that determine how the +image is encoded. Most applications don't need or want to know about all +these parameters. You can set all the parameters to reasonable defaults by +calling jpeg_set_defaults(); then, if there are particular values you want +to change, you can do so after that. The "Compression parameter selection" +section tells about all the parameters. + +You must set in_color_space correctly before calling jpeg_set_defaults(), +because the defaults depend on the source image colorspace. However the +other three source image parameters need not be valid until you call +jpeg_start_compress(). There's no harm in calling jpeg_set_defaults() more +than once, if that happens to be convenient. + +Typical code for a 24-bit RGB source image is + + cinfo.image_width = Width; /* image width and height, in pixels */ + cinfo.image_height = Height; + cinfo.input_components = 3; /* # of color components per pixel */ + cinfo.in_color_space = JCS_RGB; /* colorspace of input image */ + + jpeg_set_defaults(&cinfo); + /* Make optional parameter settings here */ + + +4. jpeg_start_compress(...); + +After you have established the data destination and set all the necessary +source image info and other parameters, call jpeg_start_compress() to begin +a compression cycle. This will initialize internal state, allocate working +storage, and emit the first few bytes of the JPEG datastream header. + +Typical code: + + jpeg_start_compress(&cinfo, TRUE); + +The "TRUE" parameter ensures that a complete JPEG interchange datastream +will be written. This is appropriate in most cases. If you think you might +want to use an abbreviated datastream, read the section on abbreviated +datastreams, below. + +Once you have called jpeg_start_compress(), you may not alter any JPEG +parameters or other fields of the JPEG object until you have completed +the compression cycle. + + +5. while (scan lines remain to be written) + jpeg_write_scanlines(...); + +Now write all the required image data by calling jpeg_write_scanlines() +one or more times. You can pass one or more scanlines in each call, up +to the total image height. In most applications it is convenient to pass +just one or a few scanlines at a time. The expected format for the passed +data is discussed under "Data formats", above. + +Image data should be written in top-to-bottom scanline order. The JPEG spec +contains some weasel wording about how top and bottom are application-defined +terms (a curious interpretation of the English language...) but if you want +your files to be compatible with everyone else's, you WILL use top-to-bottom +order. If the source data must be read in bottom-to-top order, you can use +the JPEG library's virtual array mechanism to invert the data efficiently. +Examples of this can be found in the sample application cjpeg. + +The library maintains a count of the number of scanlines written so far +in the next_scanline field of the JPEG object. Usually you can just use +this variable as the loop counter, so that the loop test looks like +"while (cinfo.next_scanline < cinfo.image_height)". + +Code for this step depends heavily on the way that you store the source data. +example.c shows the following code for the case of a full-size 2-D source +array containing 3-byte RGB pixels: + + JSAMPROW row_pointer[1]; /* pointer to a single row */ + int row_stride; /* physical row width in buffer */ + + row_stride = image_width * 3; /* JSAMPLEs per row in image_buffer */ + + while (cinfo.next_scanline < cinfo.image_height) { + row_pointer[0] = & image_buffer[cinfo.next_scanline * row_stride]; + jpeg_write_scanlines(&cinfo, row_pointer, 1); + } + +jpeg_write_scanlines() returns the number of scanlines actually written. +This will normally be equal to the number passed in, so you can usually +ignore the return value. It is different in just two cases: + * If you try to write more scanlines than the declared image height, + the additional scanlines are ignored. + * If you use a suspending data destination manager, output buffer overrun + will cause the compressor to return before accepting all the passed lines. + This feature is discussed under "I/O suspension", below. The normal + stdio destination manager will NOT cause this to happen. +In any case, the return value is the same as the change in the value of +next_scanline. + + +6. jpeg_finish_compress(...); + +After all the image data has been written, call jpeg_finish_compress() to +complete the compression cycle. This step is ESSENTIAL to ensure that the +last bufferload of data is written to the data destination. +jpeg_finish_compress() also releases working memory associated with the JPEG +object. + +Typical code: + + jpeg_finish_compress(&cinfo); + +If using the stdio destination manager, don't forget to close the output +stdio stream (if necessary) afterwards. + +If you have requested a multi-pass operating mode, such as Huffman code +optimization, jpeg_finish_compress() will perform the additional passes using +data buffered by the first pass. In this case jpeg_finish_compress() may take +quite a while to complete. With the default compression parameters, this will +not happen. + +It is an error to call jpeg_finish_compress() before writing the necessary +total number of scanlines. If you wish to abort compression, call +jpeg_abort() as discussed below. + +After completing a compression cycle, you may dispose of the JPEG object +as discussed next, or you may use it to compress another image. In that case +return to step 2, 3, or 4 as appropriate. If you do not change the +destination manager, the new datastream will be written to the same target. +If you do not change any JPEG parameters, the new datastream will be written +with the same parameters as before. Note that you can change the input image +dimensions freely between cycles, but if you change the input colorspace, you +should call jpeg_set_defaults() to adjust for the new colorspace; and then +you'll need to repeat all of step 3. + + +7. Release the JPEG compression object. + +When you are done with a JPEG compression object, destroy it by calling +jpeg_destroy_compress(). This will free all subsidiary memory (regardless of +the previous state of the object). Or you can call jpeg_destroy(), which +works for either compression or decompression objects --- this may be more +convenient if you are sharing code between compression and decompression +cases. (Actually, these routines are equivalent except for the declared type +of the passed pointer. To avoid gripes from ANSI C compilers, jpeg_destroy() +should be passed a j_common_ptr.) + +If you allocated the jpeg_compress_struct structure from malloc(), freeing +it is your responsibility --- jpeg_destroy() won't. Ditto for the error +handler structure. + +Typical code: + + jpeg_destroy_compress(&cinfo); + + +8. Aborting. + +If you decide to abort a compression cycle before finishing, you can clean up +in either of two ways: + +* If you don't need the JPEG object any more, just call + jpeg_destroy_compress() or jpeg_destroy() to release memory. This is + legitimate at any point after calling jpeg_create_compress() --- in fact, + it's safe even if jpeg_create_compress() fails. + +* If you want to re-use the JPEG object, call jpeg_abort_compress(), or call + jpeg_abort() which works on both compression and decompression objects. + This will return the object to an idle state, releasing any working memory. + jpeg_abort() is allowed at any time after successful object creation. + +Note that cleaning up the data destination, if required, is your +responsibility; neither of these routines will call term_destination(). +(See "Compressed data handling", below, for more about that.) + +jpeg_destroy() and jpeg_abort() are the only safe calls to make on a JPEG +object that has reported an error by calling error_exit (see "Error handling" +for more info). The internal state of such an object is likely to be out of +whack. Either of these two routines will return the object to a known state. + + +Decompression details +--------------------- + +Here we revisit the JPEG decompression outline given in the overview. + +1. Allocate and initialize a JPEG decompression object. + +This is just like initialization for compression, as discussed above, +except that the object is a "struct jpeg_decompress_struct" and you +call jpeg_create_decompress(). Error handling is exactly the same. + +Typical code: + + struct jpeg_decompress_struct cinfo; + struct jpeg_error_mgr jerr; + ... + cinfo.err = jpeg_std_error(&jerr); + jpeg_create_decompress(&cinfo); + +(Both here and in the IJG code, we usually use variable name "cinfo" for +both compression and decompression objects.) + + +2. Specify the source of the compressed data (eg, a file). + +As previously mentioned, the JPEG library reads compressed data from a "data +source" module. The library includes one data source module which knows how +to read from a stdio stream. You can use your own source module if you want +to do something else, as discussed later. + +If you use the standard source module, you must open the source stdio stream +beforehand. Typical code for this step looks like: + + FILE * infile; + ... + if ((infile = fopen(filename, "rb")) == NULL) { + fprintf(stderr, "can't open %s\n", filename); + exit(1); + } + jpeg_stdio_src(&cinfo, infile); + +where the last line invokes the standard source module. + +WARNING: it is critical that the binary compressed data be read unchanged. +On non-Unix systems the stdio library may perform newline translation or +otherwise corrupt binary data. To suppress this behavior, you may need to use +a "b" option to fopen (as shown above), or use setmode() or another routine to +put the stdio stream in binary mode. See cjpeg.c and djpeg.c for code that +has been found to work on many systems. + +You may not change the data source between calling jpeg_read_header() and +jpeg_finish_decompress(). If you wish to read a series of JPEG images from +a single source file, you should repeat the jpeg_read_header() to +jpeg_finish_decompress() sequence without reinitializing either the JPEG +object or the data source module; this prevents buffered input data from +being discarded. + + +3. Call jpeg_read_header() to obtain image info. + +Typical code for this step is just + + jpeg_read_header(&cinfo, TRUE); + +This will read the source datastream header markers, up to the beginning +of the compressed data proper. On return, the image dimensions and other +info have been stored in the JPEG object. The application may wish to +consult this information before selecting decompression parameters. + +More complex code is necessary if + * A suspending data source is used --- in that case jpeg_read_header() + may return before it has read all the header data. See "I/O suspension", + below. The normal stdio source manager will NOT cause this to happen. + * Abbreviated JPEG files are to be processed --- see the section on + abbreviated datastreams. Standard applications that deal only in + interchange JPEG files need not be concerned with this case either. + +It is permissible to stop at this point if you just wanted to find out the +image dimensions and other header info for a JPEG file. In that case, +call jpeg_destroy() when you are done with the JPEG object, or call +jpeg_abort() to return it to an idle state before selecting a new data +source and reading another header. + + +4. Set parameters for decompression. + +jpeg_read_header() sets appropriate default decompression parameters based on +the properties of the image (in particular, its colorspace). However, you +may well want to alter these defaults before beginning the decompression. +For example, the default is to produce full color output from a color file. +If you want colormapped output you must ask for it. Other options allow the +returned image to be scaled and allow various speed/quality tradeoffs to be +selected. "Decompression parameter selection", below, gives details. + +If the defaults are appropriate, nothing need be done at this step. + +Note that all default values are set by each call to jpeg_read_header(). +If you reuse a decompression object, you cannot expect your parameter +settings to be preserved across cycles, as you can for compression. +You must set desired parameter values each time. + + +5. jpeg_start_decompress(...); + +Once the parameter values are satisfactory, call jpeg_start_decompress() to +begin decompression. This will initialize internal state, allocate working +memory, and prepare for returning data. + +Typical code is just + + jpeg_start_decompress(&cinfo); + +If you have requested a multi-pass operating mode, such as 2-pass color +quantization, jpeg_start_decompress() will do everything needed before data +output can begin. In this case jpeg_start_decompress() may take quite a while +to complete. With a single-scan (non progressive) JPEG file and default +decompression parameters, this will not happen; jpeg_start_decompress() will +return quickly. + +After this call, the final output image dimensions, including any requested +scaling, are available in the JPEG object; so is the selected colormap, if +colormapped output has been requested. Useful fields include + + output_width image width and height, as scaled + output_height + out_color_components # of color components in out_color_space + output_components # of color components returned per pixel + colormap the selected colormap, if any + actual_number_of_colors number of entries in colormap + +output_components is 1 (a colormap index) when quantizing colors; otherwise it +equals out_color_components. It is the number of JSAMPLE values that will be +emitted per pixel in the output arrays. + +Typically you will need to allocate data buffers to hold the incoming image. +You will need output_width * output_components JSAMPLEs per scanline in your +output buffer, and a total of output_height scanlines will be returned. + +Note: if you are using the JPEG library's internal memory manager to allocate +data buffers (as djpeg does), then the manager's protocol requires that you +request large buffers *before* calling jpeg_start_decompress(). This is a +little tricky since the output_XXX fields are not normally valid then. You +can make them valid by calling jpeg_calc_output_dimensions() after setting the +relevant parameters (scaling, output color space, and quantization flag). + + +6. while (scan lines remain to be read) + jpeg_read_scanlines(...); + +Now you can read the decompressed image data by calling jpeg_read_scanlines() +one or more times. At each call, you pass in the maximum number of scanlines +to be read (ie, the height of your working buffer); jpeg_read_scanlines() +will return up to that many lines. The return value is the number of lines +actually read. The format of the returned data is discussed under "Data +formats", above. Don't forget that grayscale and color JPEGs will return +different data formats! + +Image data is returned in top-to-bottom scanline order. If you must write +out the image in bottom-to-top order, you can use the JPEG library's virtual +array mechanism to invert the data efficiently. Examples of this can be +found in the sample application djpeg. + +The library maintains a count of the number of scanlines returned so far +in the output_scanline field of the JPEG object. Usually you can just use +this variable as the loop counter, so that the loop test looks like +"while (cinfo.output_scanline < cinfo.output_height)". (Note that the test +should NOT be against image_height, unless you never use scaling. The +image_height field is the height of the original unscaled image.) +The return value always equals the change in the value of output_scanline. + +If you don't use a suspending data source, it is safe to assume that +jpeg_read_scanlines() reads at least one scanline per call, until the +bottom of the image has been reached. + +If you use a buffer larger than one scanline, it is NOT safe to assume that +jpeg_read_scanlines() fills it. (The current implementation returns only a +few scanlines per call, no matter how large a buffer you pass.) So you must +always provide a loop that calls jpeg_read_scanlines() repeatedly until the +whole image has been read. + + +7. jpeg_finish_decompress(...); + +After all the image data has been read, call jpeg_finish_decompress() to +complete the decompression cycle. This causes working memory associated +with the JPEG object to be released. + +Typical code: + + jpeg_finish_decompress(&cinfo); + +If using the stdio source manager, don't forget to close the source stdio +stream if necessary. + +It is an error to call jpeg_finish_decompress() before reading the correct +total number of scanlines. If you wish to abort decompression, call +jpeg_abort() as discussed below. + +After completing a decompression cycle, you may dispose of the JPEG object as +discussed next, or you may use it to decompress another image. In that case +return to step 2 or 3 as appropriate. If you do not change the source +manager, the next image will be read from the same source. + + +8. Release the JPEG decompression object. + +When you are done with a JPEG decompression object, destroy it by calling +jpeg_destroy_decompress() or jpeg_destroy(). The previous discussion of +destroying compression objects applies here too. + +Typical code: + + jpeg_destroy_decompress(&cinfo); + + +9. Aborting. + +You can abort a decompression cycle by calling jpeg_destroy_decompress() or +jpeg_destroy() if you don't need the JPEG object any more, or +jpeg_abort_decompress() or jpeg_abort() if you want to reuse the object. +The previous discussion of aborting compression cycles applies here too. + + +Mechanics of usage: include files, linking, etc +----------------------------------------------- + +Applications using the JPEG library should include the header file jpeglib.h +to obtain declarations of data types and routines. Before including +jpeglib.h, include system headers that define at least the typedefs FILE and +size_t. On ANSI-conforming systems, including <stdio.h> is sufficient; on +older Unix systems, you may need <sys/types.h> to define size_t. + +If the application needs to refer to individual JPEG library error codes, also +include jerror.h to define those symbols. + +jpeglib.h indirectly includes the files jconfig.h and jmorecfg.h. If you are +installing the JPEG header files in a system directory, you will want to +install all four files: jpeglib.h, jerror.h, jconfig.h, jmorecfg.h. + +The most convenient way to include the JPEG code into your executable program +is to prepare a library file ("libjpeg.a", or a corresponding name on non-Unix +machines) and reference it at your link step. If you use only half of the +library (only compression or only decompression), only that much code will be +included from the library, unless your linker is hopelessly brain-damaged. +The supplied makefiles build libjpeg.a automatically (see install.doc). + +While you can build the JPEG library as a shared library if the whim strikes +you, we don't really recommend it. The trouble with shared libraries is that +at some point you'll probably try to substitute a new version of the library +without recompiling the calling applications. That generally doesn't work +because the parameter struct declarations usually change with each new +version. In other words, the library's API is *not* guaranteed binary +compatible across versions; we only try to ensure source-code compatibility. +(In hindsight, it might have been smarter to hide the parameter structs from +applications and introduce a ton of access functions instead. Too late now, +however.) + +On some systems your application may need to set up a signal handler to ensure +that temporary files are deleted if the program is interrupted. This is most +critical if you are on MS-DOS and use the jmemdos.c memory manager back end; +it will try to grab extended memory for temp files, and that space will NOT be +freed automatically. See cjpeg.c or djpeg.c for an example signal handler. + +It may be worth pointing out that the core JPEG library does not actually +require the stdio library: only the default source/destination managers and +error handler need it. You can use the library in a stdio-less environment +if you replace those modules and use jmemnobs.c (or another memory manager of +your own devising). More info about the minimum system library requirements +may be found in jinclude.h. + + +ADVANCED FEATURES +================= + +Compression parameter selection +------------------------------- + +This section describes all the optional parameters you can set for JPEG +compression, as well as the "helper" routines provided to assist in this +task. Proper setting of some parameters requires detailed understanding +of the JPEG standard; if you don't know what a parameter is for, it's best +not to mess with it! See REFERENCES in the README file for pointers to +more info about JPEG. + +It's a good idea to call jpeg_set_defaults() first, even if you plan to set +all the parameters; that way your code is more likely to work with future JPEG +libraries that have additional parameters. For the same reason, we recommend +you use a helper routine where one is provided, in preference to twiddling +cinfo fields directly. + +The helper routines are: + +jpeg_set_defaults (j_compress_ptr cinfo) + This routine sets all JPEG parameters to reasonable defaults, using + only the input image's color space (field in_color_space, which must + already be set in cinfo). Many applications will only need to use + this routine and perhaps jpeg_set_quality(). + +jpeg_set_colorspace (j_compress_ptr cinfo, J_COLOR_SPACE colorspace) + Sets the JPEG file's colorspace (field jpeg_color_space) as specified, + and sets other color-space-dependent parameters appropriately. See + "Special color spaces", below, before using this. A large number of + parameters, including all per-component parameters, are set by this + routine; if you want to twiddle individual parameters you should call + jpeg_set_colorspace() before rather than after. + +jpeg_default_colorspace (j_compress_ptr cinfo) + Selects an appropriate JPEG colorspace based on cinfo->in_color_space, + and calls jpeg_set_colorspace(). This is actually a subroutine of + jpeg_set_defaults(). It's broken out in case you want to change + just the colorspace-dependent JPEG parameters. + +jpeg_set_quality (j_compress_ptr cinfo, int quality, boolean force_baseline) + Constructs JPEG quantization tables appropriate for the indicated + quality setting. The quality value is expressed on the 0..100 scale + recommended by IJG (cjpeg's "-quality" switch uses this routine). + Note that the exact mapping from quality values to tables may change + in future IJG releases as more is learned about DCT quantization. + If the force_baseline parameter is TRUE, then the quantization table + entries are constrained to the range 1..255 for full JPEG baseline + compatibility. In the current implementation, this only makes a + difference for quality settings below 25, and it effectively prevents + very small/low quality files from being generated. The IJG decoder + is capable of reading the non-baseline files generated at low quality + settings when force_baseline is FALSE, but other decoders may not be. + +jpeg_set_linear_quality (j_compress_ptr cinfo, int scale_factor, + boolean force_baseline) + Same as jpeg_set_quality() except that the generated tables are the + sample tables given in the JPEC spec section K.1, multiplied by the + specified scale factor (which is expressed as a percentage; thus + scale_factor = 100 reproduces the spec's tables). Note that larger + scale factors give lower quality. This entry point is useful for + conforming to the Adobe PostScript DCT conventions, but we do not + recommend linear scaling as a user-visible quality scale otherwise. + force_baseline again constrains the computed table entries to 1..255. + +int jpeg_quality_scaling (int quality) + Converts a value on the IJG-recommended quality scale to a linear + scaling percentage. Note that this routine may change or go away + in future releases --- IJG may choose to adopt a scaling method that + can't be expressed as a simple scalar multiplier, in which case the + premise of this routine collapses. Caveat user. + +jpeg_add_quant_table (j_compress_ptr cinfo, int which_tbl, + const unsigned int *basic_table, + int scale_factor, boolean force_baseline) + Allows an arbitrary quantization table to be created. which_tbl + indicates which table slot to fill. basic_table points to an array + of 64 unsigned ints given in normal array order. These values are + multiplied by scale_factor/100 and then clamped to the range 1..65535 + (or to 1..255 if force_baseline is TRUE). + CAUTION: prior to library version 6a, jpeg_add_quant_table expected + the basic table to be given in JPEG zigzag order. If you need to + write code that works with either older or newer versions of this + routine, you must check the library version number. Something like + "#if JPEG_LIB_VERSION >= 61" is the right test. + +jpeg_simple_progression (j_compress_ptr cinfo) + Generates a default scan script for writing a progressive-JPEG file. + This is the recommended method of creating a progressive file, + unless you want to make a custom scan sequence. You must ensure that + the JPEG color space is set correctly before calling this routine. + + +Compression parameters (cinfo fields) include: + +J_DCT_METHOD dct_method + Selects the algorithm used for the DCT step. Choices are: + JDCT_ISLOW: slow but accurate integer algorithm + JDCT_IFAST: faster, less accurate integer method + JDCT_FLOAT: floating-point method + JDCT_DEFAULT: default method (normally JDCT_ISLOW) + JDCT_FASTEST: fastest method (normally JDCT_IFAST) + The FLOAT method is very slightly more accurate than the ISLOW method, + but may give different results on different machines due to varying + roundoff behavior. The integer methods should give the same results + on all machines. On machines with sufficiently fast FP hardware, the + floating-point method may also be the fastest. The IFAST method is + considerably less accurate than the other two; its use is not + recommended if high quality is a concern. JDCT_DEFAULT and + JDCT_FASTEST are macros configurable by each installation. + +J_COLOR_SPACE jpeg_color_space +int num_components + The JPEG color space and corresponding number of components; see + "Special color spaces", below, for more info. We recommend using + jpeg_set_color_space() if you want to change these. + +boolean optimize_coding + TRUE causes the compressor to compute optimal Huffman coding tables + for the image. This requires an extra pass over the data and + therefore costs a good deal of space and time. The default is + FALSE, which tells the compressor to use the supplied or default + Huffman tables. In most cases optimal tables save only a few percent + of file size compared to the default tables. Note that when this is + TRUE, you need not supply Huffman tables at all, and any you do + supply will be overwritten. + +unsigned int restart_interval +int restart_in_rows + To emit restart markers in the JPEG file, set one of these nonzero. + Set restart_interval to specify the exact interval in MCU blocks. + Set restart_in_rows to specify the interval in MCU rows. (If + restart_in_rows is not 0, then restart_interval is set after the + image width in MCUs is computed.) Defaults are zero (no restarts). + One restart marker per MCU row is often a good choice. + NOTE: the overhead of restart markers is higher in grayscale JPEG + files than in color files, and MUCH higher in progressive JPEGs. + If you use restarts, you may want to use larger intervals in those + cases. + +const jpeg_scan_info * scan_info +int num_scans + By default, scan_info is NULL; this causes the compressor to write a + single-scan sequential JPEG file. If not NULL, scan_info points to + an array of scan definition records of length num_scans. The + compressor will then write a JPEG file having one scan for each scan + definition record. This is used to generate noninterleaved or + progressive JPEG files. The library checks that the scan array + defines a valid JPEG scan sequence. (jpeg_simple_progression creates + a suitable scan definition array for progressive JPEG.) This is + discussed further under "Progressive JPEG support". + +int smoothing_factor + If non-zero, the input image is smoothed; the value should be 1 for + minimal smoothing to 100 for maximum smoothing. Consult jcsample.c + for details of the smoothing algorithm. The default is zero. + +boolean write_JFIF_header + If TRUE, a JFIF APP0 marker is emitted. jpeg_set_defaults() and + jpeg_set_colorspace() set this TRUE if a JFIF-legal JPEG color space + (ie, YCbCr or grayscale) is selected, otherwise FALSE. + +UINT8 JFIF_major_version +UINT8 JFIF_minor_version + The version number to be written into the JFIF marker. + jpeg_set_defaults() initializes the version to 1.01 (major=minor=1). + You should set it to 1.02 (major=1, minor=2) if you plan to write + any JFIF 1.02 extension markers. + +UINT8 density_unit +UINT16 X_density +UINT16 Y_density + The resolution information to be written into the JFIF marker; + not used otherwise. density_unit may be 0 for unknown, + 1 for dots/inch, or 2 for dots/cm. The default values are 0,1,1 + indicating square pixels of unknown size. + +boolean write_Adobe_marker + If TRUE, an Adobe APP14 marker is emitted. jpeg_set_defaults() and + jpeg_set_colorspace() set this TRUE if JPEG color space RGB, CMYK, + or YCCK is selected, otherwise FALSE. It is generally a bad idea + to set both write_JFIF_header and write_Adobe_marker. In fact, + you probably shouldn't change the default settings at all --- the + default behavior ensures that the JPEG file's color space can be + recognized by the decoder. + +JQUANT_TBL * quant_tbl_ptrs[NUM_QUANT_TBLS] + Pointers to coefficient quantization tables, one per table slot, + or NULL if no table is defined for a slot. Usually these should + be set via one of the above helper routines; jpeg_add_quant_table() + is general enough to define any quantization table. The other + routines will set up table slot 0 for luminance quality and table + slot 1 for chrominance. + +JHUFF_TBL * dc_huff_tbl_ptrs[NUM_HUFF_TBLS] +JHUFF_TBL * ac_huff_tbl_ptrs[NUM_HUFF_TBLS] + Pointers to Huffman coding tables, one per table slot, or NULL if + no table is defined for a slot. Slots 0 and 1 are filled with the + JPEG sample tables by jpeg_set_defaults(). If you need to allocate + more table structures, jpeg_alloc_huff_table() may be used. + Note that optimal Huffman tables can be computed for an image + by setting optimize_coding, as discussed above; there's seldom + any need to mess with providing your own Huffman tables. + +There are some additional cinfo fields which are not documented here +because you currently can't change them; for example, you can't set +arith_code TRUE because arithmetic coding is unsupported. + + +Per-component parameters are stored in the struct cinfo.comp_info[i] for +component number i. Note that components here refer to components of the +JPEG color space, *not* the source image color space. A suitably large +comp_info[] array is allocated by jpeg_set_defaults(); if you choose not +to use that routine, it's up to you to allocate the array. + +int component_id + The one-byte identifier code to be recorded in the JPEG file for + this component. For the standard color spaces, we recommend you + leave the default values alone. + +int h_samp_factor +int v_samp_factor + Horizontal and vertical sampling factors for the component; must + be 1..4 according to the JPEG standard. Note that larger sampling + factors indicate a higher-resolution component; many people find + this behavior quite unintuitive. The default values are 2,2 for + luminance components and 1,1 for chrominance components, except + for grayscale where 1,1 is used. + +int quant_tbl_no + Quantization table number for component. The default value is + 0 for luminance components and 1 for chrominance components. + +int dc_tbl_no +int ac_tbl_no + DC and AC entropy coding table numbers. The default values are + 0 for luminance components and 1 for chrominance components. + +int component_index + Must equal the component's index in comp_info[]. (Beginning in + release v6, the compressor library will fill this in automatically; + you don't have to.) + + +Decompression parameter selection +--------------------------------- + +Decompression parameter selection is somewhat simpler than compression +parameter selection, since all of the JPEG internal parameters are +recorded in the source file and need not be supplied by the application. +(Unless you are working with abbreviated files, in which case see +"Abbreviated datastreams", below.) Decompression parameters control +the postprocessing done on the image to deliver it in a format suitable +for the application's use. Many of the parameters control speed/quality +tradeoffs, in which faster decompression may be obtained at the price of +a poorer-quality image. The defaults select the highest quality (slowest) +processing. + +The following fields in the JPEG object are set by jpeg_read_header() and +may be useful to the application in choosing decompression parameters: + +JDIMENSION image_width Width and height of image +JDIMENSION image_height +int num_components Number of color components +J_COLOR_SPACE jpeg_color_space Colorspace of image +boolean saw_JFIF_marker TRUE if a JFIF APP0 marker was seen + UINT8 JFIF_major_version Version information from JFIF marker + UINT8 JFIF_minor_version + UINT8 density_unit Resolution data from JFIF marker + UINT16 X_density + UINT16 Y_density +boolean saw_Adobe_marker TRUE if an Adobe APP14 marker was seen + UINT8 Adobe_transform Color transform code from Adobe marker + +The JPEG color space, unfortunately, is something of a guess since the JPEG +standard proper does not provide a way to record it. In practice most files +adhere to the JFIF or Adobe conventions, and the decoder will recognize these +correctly. See "Special color spaces", below, for more info. + + +The decompression parameters that determine the basic properties of the +returned image are: + +J_COLOR_SPACE out_color_space + Output color space. jpeg_read_header() sets an appropriate default + based on jpeg_color_space; typically it will be RGB or grayscale. + The application can change this field to request output in a different + colorspace. For example, set it to JCS_GRAYSCALE to get grayscale + output from a color file. (This is useful for previewing: grayscale + output is faster than full color since the color components need not + be processed.) Note that not all possible color space transforms are + currently implemented; you may need to extend jdcolor.c if you want an + unusual conversion. + +unsigned int scale_num, scale_denom + Scale the image by the fraction scale_num/scale_denom. Default is + 1/1, or no scaling. Currently, the only supported scaling ratios + are 1/1, 1/2, 1/4, and 1/8. (The library design allows for arbitrary + scaling ratios but this is not likely to be implemented any time soon.) + Smaller scaling ratios permit significantly faster decoding since + fewer pixels need be processed and a simpler IDCT method can be used. + +boolean quantize_colors + If set TRUE, colormapped output will be delivered. Default is FALSE, + meaning that full-color output will be delivered. + +The next three parameters are relevant only if quantize_colors is TRUE. + +int desired_number_of_colors + Maximum number of colors to use in generating a library-supplied color + map (the actual number of colors is returned in a different field). + Default 256. Ignored when the application supplies its own color map. + +boolean two_pass_quantize + If TRUE, an extra pass over the image is made to select a custom color + map for the image. This usually looks a lot better than the one-size- + fits-all colormap that is used otherwise. Default is TRUE. Ignored + when the application supplies its own color map. + +J_DITHER_MODE dither_mode + Selects color dithering method. Supported values are: + JDITHER_NONE no dithering: fast, very low quality + JDITHER_ORDERED ordered dither: moderate speed and quality + JDITHER_FS Floyd-Steinberg dither: slow, high quality + Default is JDITHER_FS. (At present, ordered dither is implemented + only in the single-pass, standard-colormap case. If you ask for + ordered dither when two_pass_quantize is TRUE or when you supply + an external color map, you'll get F-S dithering.) + +When quantize_colors is TRUE, the target color map is described by the next +two fields. colormap is set to NULL by jpeg_read_header(). The application +can supply a color map by setting colormap non-NULL and setting +actual_number_of_colors to the map size. Otherwise, jpeg_start_decompress() +selects a suitable color map and sets these two fields itself. +[Implementation restriction: at present, an externally supplied colormap is +only accepted for 3-component output color spaces.] + +JSAMPARRAY colormap + The color map, represented as a 2-D pixel array of out_color_components + rows and actual_number_of_colors columns. Ignored if not quantizing. + CAUTION: if the JPEG library creates its own colormap, the storage + pointed to by this field is released by jpeg_finish_decompress(). + Copy the colormap somewhere else first, if you want to save it. + +int actual_number_of_colors + The number of colors in the color map. + +Additional decompression parameters that the application may set include: + +J_DCT_METHOD dct_method + Selects the algorithm used for the DCT step. Choices are the same + as described above for compression. + +boolean do_fancy_upsampling + If TRUE, do careful upsampling of chroma components. If FALSE, + a faster but sloppier method is used. Default is TRUE. The visual + impact of the sloppier method is often very small. + +boolean do_block_smoothing + If TRUE, interblock smoothing is applied in early stages of decoding + progressive JPEG files; if FALSE, not. Default is TRUE. Early + progression stages look "fuzzy" with smoothing, "blocky" without. + In any case, block smoothing ceases to be applied after the first few + AC coefficients are known to full accuracy, so it is relevant only + when using buffered-image mode for progressive images. + +boolean enable_1pass_quant +boolean enable_external_quant +boolean enable_2pass_quant + These are significant only in buffered-image mode, which is + described in its own section below. + + +The output image dimensions are given by the following fields. These are +computed from the source image dimensions and the decompression parameters +by jpeg_start_decompress(). You can also call jpeg_calc_output_dimensions() +to obtain the values that will result from the current parameter settings. +This can be useful if you are trying to pick a scaling ratio that will get +close to a desired target size. It's also important if you are using the +JPEG library's memory manager to allocate output buffer space, because you +are supposed to request such buffers *before* jpeg_start_decompress(). + +JDIMENSION output_width Actual dimensions of output image. +JDIMENSION output_height +int out_color_components Number of color components in out_color_space. +int output_components Number of color components returned. +int rec_outbuf_height Recommended height of scanline buffer. + +When quantizing colors, output_components is 1, indicating a single color map +index per pixel. Otherwise it equals out_color_components. The output arrays +are required to be output_width * output_components JSAMPLEs wide. + +rec_outbuf_height is the recommended minimum height (in scanlines) of the +buffer passed to jpeg_read_scanlines(). If the buffer is smaller, the +library will still work, but time will be wasted due to unnecessary data +copying. In high-quality modes, rec_outbuf_height is always 1, but some +faster, lower-quality modes set it to larger values (typically 2 to 4). +If you are going to ask for a high-speed processing mode, you may as well +go to the trouble of honoring rec_outbuf_height so as to avoid data copying. +(An output buffer larger than rec_outbuf_height lines is OK, but won't +provide any material speed improvement over that height.) + + +Special color spaces +-------------------- + +The JPEG standard itself is "color blind" and doesn't specify any particular +color space. It is customary to convert color data to a luminance/chrominance +color space before compressing, since this permits greater compression. The +existing de-facto JPEG file format standards specify YCbCr or grayscale data +(JFIF), or grayscale, RGB, YCbCr, CMYK, or YCCK (Adobe). For special +applications such as multispectral images, other color spaces can be used, +but it must be understood that such files will be unportable. + +The JPEG library can handle the most common colorspace conversions (namely +RGB <=> YCbCr and CMYK <=> YCCK). It can also deal with data of an unknown +color space, passing it through without conversion. If you deal extensively +with an unusual color space, you can easily extend the library to understand +additional color spaces and perform appropriate conversions. + +For compression, the source data's color space is specified by field +in_color_space. This is transformed to the JPEG file's color space given +by jpeg_color_space. jpeg_set_defaults() chooses a reasonable JPEG color +space depending on in_color_space, but you can override this by calling +jpeg_set_colorspace(). Of course you must select a supported transformation. +jccolor.c currently supports the following transformations: + RGB => YCbCr + RGB => GRAYSCALE + YCbCr => GRAYSCALE + CMYK => YCCK +plus the null transforms: GRAYSCALE => GRAYSCALE, RGB => RGB, +YCbCr => YCbCr, CMYK => CMYK, YCCK => YCCK, and UNKNOWN => UNKNOWN. + +The de-facto file format standards (JFIF and Adobe) specify APPn markers that +indicate the color space of the JPEG file. It is important to ensure that +these are written correctly, or omitted if the JPEG file's color space is not +one of the ones supported by the de-facto standards. jpeg_set_colorspace() +will set the compression parameters to include or omit the APPn markers +properly, so long as it is told the truth about the JPEG color space. +For example, if you are writing some random 3-component color space without +conversion, don't try to fake out the library by setting in_color_space and +jpeg_color_space to JCS_YCbCr; use JCS_UNKNOWN. You may want to write an +APPn marker of your own devising to identify the colorspace --- see "Special +markers", below. + +When told that the color space is UNKNOWN, the library will default to using +luminance-quality compression parameters for all color components. You may +well want to change these parameters. See the source code for +jpeg_set_colorspace(), in jcparam.c, for details. + +For decompression, the JPEG file's color space is given in jpeg_color_space, +and this is transformed to the output color space out_color_space. +jpeg_read_header's setting of jpeg_color_space can be relied on if the file +conforms to JFIF or Adobe conventions, but otherwise it is no better than a +guess. If you know the JPEG file's color space for certain, you can override +jpeg_read_header's guess by setting jpeg_color_space. jpeg_read_header also +selects a default output color space based on (its guess of) jpeg_color_space; +set out_color_space to override this. Again, you must select a supported +transformation. jdcolor.c currently supports + YCbCr => GRAYSCALE + YCbCr => RGB + GRAYSCALE => RGB + YCCK => CMYK +as well as the null transforms. (Since GRAYSCALE=>RGB is provided, an +application can force grayscale JPEGs to look like color JPEGs if it only +wants to handle one case.) + +The two-pass color quantizer, jquant2.c, is specialized to handle RGB data +(it weights distances appropriately for RGB colors). You'll need to modify +the code if you want to use it for non-RGB output color spaces. Note that +jquant2.c is used to map to an application-supplied colormap as well as for +the normal two-pass colormap selection process. + +CAUTION: it appears that Adobe Photoshop writes inverted data in CMYK JPEG +files: 0 represents 100% ink coverage, rather than 0% ink as you'd expect. +This is arguably a bug in Photoshop, but if you need to work with Photoshop +CMYK files, you will have to deal with it in your application. We cannot +"fix" this in the library by inverting the data during the CMYK<=>YCCK +transform, because that would break other applications, notably Ghostscript. +Photoshop versions prior to 3.0 write EPS files containing JPEG-encoded CMYK +data in the same inverted-YCCK representation used in bare JPEG files, but +the surrounding PostScript code performs an inversion using the PS image +operator. I am told that Photoshop 3.0 will write uninverted YCCK in +EPS/JPEG files, and will omit the PS-level inversion. (But the data +polarity used in bare JPEG files will not change in 3.0.) In either case, +the JPEG library must not invert the data itself, or else Ghostscript would +read these EPS files incorrectly. + + +Error handling +-------------- + +When the default error handler is used, any error detected inside the JPEG +routines will cause a message to be printed on stderr, followed by exit(). +You can supply your own error handling routines to override this behavior +and to control the treatment of nonfatal warnings and trace/debug messages. +The file example.c illustrates the most common case, which is to have the +application regain control after an error rather than exiting. + +The JPEG library never writes any message directly; it always goes through +the error handling routines. Three classes of messages are recognized: + * Fatal errors: the library cannot continue. + * Warnings: the library can continue, but the data is corrupt, and a + damaged output image is likely to result. + * Trace/informational messages. These come with a trace level indicating + the importance of the message; you can control the verbosity of the + program by adjusting the maximum trace level that will be displayed. + +You may, if you wish, simply replace the entire JPEG error handling module +(jerror.c) with your own code. However, you can avoid code duplication by +only replacing some of the routines depending on the behavior you need. +This is accomplished by calling jpeg_std_error() as usual, but then overriding +some of the method pointers in the jpeg_error_mgr struct, as illustrated by +example.c. + +All of the error handling routines will receive a pointer to the JPEG object +(a j_common_ptr which points to either a jpeg_compress_struct or a +jpeg_decompress_struct; if you need to tell which, test the is_decompressor +field). This struct includes a pointer to the error manager struct in its +"err" field. Frequently, custom error handler routines will need to access +additional data which is not known to the JPEG library or the standard error +handler. The most convenient way to do this is to embed either the JPEG +object or the jpeg_error_mgr struct in a larger structure that contains +additional fields; then casting the passed pointer provides access to the +additional fields. Again, see example.c for one way to do it. (Beginning +with IJG version 6b, there is also a void pointer "client_data" in each +JPEG object, which the application can also use to find related data. +The library does not touch client_data at all.) + +The individual methods that you might wish to override are: + +error_exit (j_common_ptr cinfo) + Receives control for a fatal error. Information sufficient to + generate the error message has been stored in cinfo->err; call + output_message to display it. Control must NOT return to the caller; + generally this routine will exit() or longjmp() somewhere. + Typically you would override this routine to get rid of the exit() + default behavior. Note that if you continue processing, you should + clean up the JPEG object with jpeg_abort() or jpeg_destroy(). + +output_message (j_common_ptr cinfo) + Actual output of any JPEG message. Override this to send messages + somewhere other than stderr. Note that this method does not know + how to generate a message, only where to send it. + +format_message (j_common_ptr cinfo, char * buffer) + Constructs a readable error message string based on the error info + stored in cinfo->err. This method is called by output_message. Few + applications should need to override this method. One possible + reason for doing so is to implement dynamic switching of error message + language. + +emit_message (j_common_ptr cinfo, int msg_level) + Decide whether or not to emit a warning or trace message; if so, + calls output_message. The main reason for overriding this method + would be to abort on warnings. msg_level is -1 for warnings, + 0 and up for trace messages. + +Only error_exit() and emit_message() are called from the rest of the JPEG +library; the other two are internal to the error handler. + +The actual message texts are stored in an array of strings which is pointed to +by the field err->jpeg_message_table. The messages are numbered from 0 to +err->last_jpeg_message, and it is these code numbers that are used in the +JPEG library code. You could replace the message texts (for instance, with +messages in French or German) by changing the message table pointer. See +jerror.h for the default texts. CAUTION: this table will almost certainly +change or grow from one library version to the next. + +It may be useful for an application to add its own message texts that are +handled by the same mechanism. The error handler supports a second "add-on" +message table for this purpose. To define an addon table, set the pointer +err->addon_message_table and the message numbers err->first_addon_message and +err->last_addon_message. If you number the addon messages beginning at 1000 +or so, you won't have to worry about conflicts with the library's built-in +messages. See the sample applications cjpeg/djpeg for an example of using +addon messages (the addon messages are defined in cderror.h). + +Actual invocation of the error handler is done via macros defined in jerror.h: + ERREXITn(...) for fatal errors + WARNMSn(...) for corrupt-data warnings + TRACEMSn(...) for trace and informational messages. +These macros store the message code and any additional parameters into the +error handler struct, then invoke the error_exit() or emit_message() method. +The variants of each macro are for varying numbers of additional parameters. +The additional parameters are inserted into the generated message using +standard printf() format codes. + +See jerror.h and jerror.c for further details. + + +Compressed data handling (source and destination managers) +---------------------------------------------------------- + +The JPEG compression library sends its compressed data to a "destination +manager" module. The default destination manager just writes the data to a +stdio stream, but you can provide your own manager to do something else. +Similarly, the decompression library calls a "source manager" to obtain the +compressed data; you can provide your own source manager if you want the data +to come from somewhere other than a stdio stream. + +In both cases, compressed data is processed a bufferload at a time: the +destination or source manager provides a work buffer, and the library invokes +the manager only when the buffer is filled or emptied. (You could define a +one-character buffer to force the manager to be invoked for each byte, but +that would be rather inefficient.) The buffer's size and location are +controlled by the manager, not by the library. For example, if you desired to +decompress a JPEG datastream that was all in memory, you could just make the +buffer pointer and length point to the original data in memory. Then the +buffer-reload procedure would be invoked only if the decompressor ran off the +end of the datastream, which would indicate an erroneous datastream. + +The work buffer is defined as an array of datatype JOCTET, which is generally +"char" or "unsigned char". On a machine where char is not exactly 8 bits +wide, you must define JOCTET as a wider data type and then modify the data +source and destination modules to transcribe the work arrays into 8-bit units +on external storage. + +A data destination manager struct contains a pointer and count defining the +next byte to write in the work buffer and the remaining free space: + + JOCTET * next_output_byte; /* => next byte to write in buffer */ + size_t free_in_buffer; /* # of byte spaces remaining in buffer */ + +The library increments the pointer and decrements the count until the buffer +is filled. The manager's empty_output_buffer method must reset the pointer +and count. The manager is expected to remember the buffer's starting address +and total size in private fields not visible to the library. + +A data destination manager provides three methods: + +init_destination (j_compress_ptr cinfo) + Initialize destination. This is called by jpeg_start_compress() + before any data is actually written. It must initialize + next_output_byte and free_in_buffer. free_in_buffer must be + initialized to a positive value. + +empty_output_buffer (j_compress_ptr cinfo) + This is called whenever the buffer has filled (free_in_buffer + reaches zero). In typical applications, it should write out the + *entire* buffer (use the saved start address and buffer length; + ignore the current state of next_output_byte and free_in_buffer). + Then reset the pointer & count to the start of the buffer, and + return TRUE indicating that the buffer has been dumped. + free_in_buffer must be set to a positive value when TRUE is + returned. A FALSE return should only be used when I/O suspension is + desired (this operating mode is discussed in the next section). + +term_destination (j_compress_ptr cinfo) + Terminate destination --- called by jpeg_finish_compress() after all + data has been written. In most applications, this must flush any + data remaining in the buffer. Use either next_output_byte or + free_in_buffer to determine how much data is in the buffer. + +term_destination() is NOT called by jpeg_abort() or jpeg_destroy(). If you +want the destination manager to be cleaned up during an abort, you must do it +yourself. + +You will also need code to create a jpeg_destination_mgr struct, fill in its +method pointers, and insert a pointer to the struct into the "dest" field of +the JPEG compression object. This can be done in-line in your setup code if +you like, but it's probably cleaner to provide a separate routine similar to +the jpeg_stdio_dest() routine of the supplied destination manager. + +Decompression source managers follow a parallel design, but with some +additional frammishes. The source manager struct contains a pointer and count +defining the next byte to read from the work buffer and the number of bytes +remaining: + + const JOCTET * next_input_byte; /* => next byte to read from buffer */ + size_t bytes_in_buffer; /* # of bytes remaining in buffer */ + +The library increments the pointer and decrements the count until the buffer +is emptied. The manager's fill_input_buffer method must reset the pointer and +count. In most applications, the manager must remember the buffer's starting +address and total size in private fields not visible to the library. + +A data source manager provides five methods: + +init_source (j_decompress_ptr cinfo) + Initialize source. This is called by jpeg_read_header() before any + data is actually read. Unlike init_destination(), it may leave + bytes_in_buffer set to 0 (in which case a fill_input_buffer() call + will occur immediately). + +fill_input_buffer (j_decompress_ptr cinfo) + This is called whenever bytes_in_buffer has reached zero and more + data is wanted. In typical applications, it should read fresh data + into the buffer (ignoring the current state of next_input_byte and + bytes_in_buffer), reset the pointer & count to the start of the + buffer, and return TRUE indicating that the buffer has been reloaded. + It is not necessary to fill the buffer entirely, only to obtain at + least one more byte. bytes_in_buffer MUST be set to a positive value + if TRUE is returned. A FALSE return should only be used when I/O + suspension is desired (this mode is discussed in the next section). + +skip_input_data (j_decompress_ptr cinfo, long num_bytes) + Skip num_bytes worth of data. The buffer pointer and count should + be advanced over num_bytes input bytes, refilling the buffer as + needed. This is used to skip over a potentially large amount of + uninteresting data (such as an APPn marker). In some applications + it may be possible to optimize away the reading of the skipped data, + but it's not clear that being smart is worth much trouble; large + skips are uncommon. bytes_in_buffer may be zero on return. + A zero or negative skip count should be treated as a no-op. + +resync_to_restart (j_decompress_ptr cinfo, int desired) + This routine is called only when the decompressor has failed to find + a restart (RSTn) marker where one is expected. Its mission is to + find a suitable point for resuming decompression. For most + applications, we recommend that you just use the default resync + procedure, jpeg_resync_to_restart(). However, if you are able to back + up in the input data stream, or if you have a-priori knowledge about + the likely location of restart markers, you may be able to do better. + Read the read_restart_marker() and jpeg_resync_to_restart() routines + in jdmarker.c if you think you'd like to implement your own resync + procedure. + +term_source (j_decompress_ptr cinfo) + Terminate source --- called by jpeg_finish_decompress() after all + data has been read. Often a no-op. + +For both fill_input_buffer() and skip_input_data(), there is no such thing +as an EOF return. If the end of the file has been reached, the routine has +a choice of exiting via ERREXIT() or inserting fake data into the buffer. +In most cases, generating a warning message and inserting a fake EOI marker +is the best course of action --- this will allow the decompressor to output +however much of the image is there. In pathological cases, the decompressor +may swallow the EOI and again demand data ... just keep feeding it fake EOIs. +jdatasrc.c illustrates the recommended error recovery behavior. + +term_source() is NOT called by jpeg_abort() or jpeg_destroy(). If you want +the source manager to be cleaned up during an abort, you must do it yourself. + +You will also need code to create a jpeg_source_mgr struct, fill in its method +pointers, and insert a pointer to the struct into the "src" field of the JPEG +decompression object. This can be done in-line in your setup code if you +like, but it's probably cleaner to provide a separate routine similar to the +jpeg_stdio_src() routine of the supplied source manager. + +For more information, consult the stdio source and destination managers +in jdatasrc.c and jdatadst.c. + + +I/O suspension +-------------- + +Some applications need to use the JPEG library as an incremental memory-to- +memory filter: when the compressed data buffer is filled or emptied, they want +control to return to the outer loop, rather than expecting that the buffer can +be emptied or reloaded within the data source/destination manager subroutine. +The library supports this need by providing an "I/O suspension" mode, which we +describe in this section. + +The I/O suspension mode is not a panacea: nothing is guaranteed about the +maximum amount of time spent in any one call to the library, so it will not +eliminate response-time problems in single-threaded applications. If you +need guaranteed response time, we suggest you "bite the bullet" and implement +a real multi-tasking capability. + +To use I/O suspension, cooperation is needed between the calling application +and the data source or destination manager; you will always need a custom +source/destination manager. (Please read the previous section if you haven't +already.) The basic idea is that the empty_output_buffer() or +fill_input_buffer() routine is a no-op, merely returning FALSE to indicate +that it has done nothing. Upon seeing this, the JPEG library suspends +operation and returns to its caller. The surrounding application is +responsible for emptying or refilling the work buffer before calling the +JPEG library again. + +Compression suspension: + +For compression suspension, use an empty_output_buffer() routine that returns +FALSE; typically it will not do anything else. This will cause the +compressor to return to the caller of jpeg_write_scanlines(), with the return +value indicating that not all the supplied scanlines have been accepted. +The application must make more room in the output buffer, adjust the output +buffer pointer/count appropriately, and then call jpeg_write_scanlines() +again, pointing to the first unconsumed scanline. + +When forced to suspend, the compressor will backtrack to a convenient stopping +point (usually the start of the current MCU); it will regenerate some output +data when restarted. Therefore, although empty_output_buffer() is only +called when the buffer is filled, you should NOT write out the entire buffer +after a suspension. Write only the data up to the current position of +next_output_byte/free_in_buffer. The data beyond that point will be +regenerated after resumption. + +Because of the backtracking behavior, a good-size output buffer is essential +for efficiency; you don't want the compressor to suspend often. (In fact, an +overly small buffer could lead to infinite looping, if a single MCU required +more data than would fit in the buffer.) We recommend a buffer of at least +several Kbytes. You may want to insert explicit code to ensure that you don't +call jpeg_write_scanlines() unless there is a reasonable amount of space in +the output buffer; in other words, flush the buffer before trying to compress +more data. + +The compressor does not allow suspension while it is trying to write JPEG +markers at the beginning and end of the file. This means that: + * At the beginning of a compression operation, there must be enough free + space in the output buffer to hold the header markers (typically 600 or + so bytes). The recommended buffer size is bigger than this anyway, so + this is not a problem as long as you start with an empty buffer. However, + this restriction might catch you if you insert large special markers, such + as a JFIF thumbnail image, without flushing the buffer afterwards. + * When you call jpeg_finish_compress(), there must be enough space in the + output buffer to emit any buffered data and the final EOI marker. In the + current implementation, half a dozen bytes should suffice for this, but + for safety's sake we recommend ensuring that at least 100 bytes are free + before calling jpeg_finish_compress(). + +A more significant restriction is that jpeg_finish_compress() cannot suspend. +This means you cannot use suspension with multi-pass operating modes, namely +Huffman code optimization and multiple-scan output. Those modes write the +whole file during jpeg_finish_compress(), which will certainly result in +buffer overrun. (Note that this restriction applies only to compression, +not decompression. The decompressor supports input suspension in all of its +operating modes.) + +Decompression suspension: + +For decompression suspension, use a fill_input_buffer() routine that simply +returns FALSE (except perhaps during error recovery, as discussed below). +This will cause the decompressor to return to its caller with an indication +that suspension has occurred. This can happen at four places: + * jpeg_read_header(): will return JPEG_SUSPENDED. + * jpeg_start_decompress(): will return FALSE, rather than its usual TRUE. + * jpeg_read_scanlines(): will return the number of scanlines already + completed (possibly 0). + * jpeg_finish_decompress(): will return FALSE, rather than its usual TRUE. +The surrounding application must recognize these cases, load more data into +the input buffer, and repeat the call. In the case of jpeg_read_scanlines(), +increment the passed pointers past any scanlines successfully read. + +Just as with compression, the decompressor will typically backtrack to a +convenient restart point before suspending. When fill_input_buffer() is +called, next_input_byte/bytes_in_buffer point to the current restart point, +which is where the decompressor will backtrack to if FALSE is returned. +The data beyond that position must NOT be discarded if you suspend; it needs +to be re-read upon resumption. In most implementations, you'll need to shift +this data down to the start of your work buffer and then load more data after +it. Again, this behavior means that a several-Kbyte work buffer is essential +for decent performance; furthermore, you should load a reasonable amount of +new data before resuming decompression. (If you loaded, say, only one new +byte each time around, you could waste a LOT of cycles.) + +The skip_input_data() source manager routine requires special care in a +suspension scenario. This routine is NOT granted the ability to suspend the +decompressor; it can decrement bytes_in_buffer to zero, but no more. If the +requested skip distance exceeds the amount of data currently in the input +buffer, then skip_input_data() must set bytes_in_buffer to zero and record the +additional skip distance somewhere else. The decompressor will immediately +call fill_input_buffer(), which should return FALSE, which will cause a +suspension return. The surrounding application must then arrange to discard +the recorded number of bytes before it resumes loading the input buffer. +(Yes, this design is rather baroque, but it avoids complexity in the far more +common case where a non-suspending source manager is used.) + +If the input data has been exhausted, we recommend that you emit a warning +and insert dummy EOI markers just as a non-suspending data source manager +would do. This can be handled either in the surrounding application logic or +within fill_input_buffer(); the latter is probably more efficient. If +fill_input_buffer() knows that no more data is available, it can set the +pointer/count to point to a dummy EOI marker and then return TRUE just as +though it had read more data in a non-suspending situation. + +The decompressor does not attempt to suspend within standard JPEG markers; +instead it will backtrack to the start of the marker and reprocess the whole +marker next time. Hence the input buffer must be large enough to hold the +longest standard marker in the file. Standard JPEG markers should normally +not exceed a few hundred bytes each (DHT tables are typically the longest). +We recommend at least a 2K buffer for performance reasons, which is much +larger than any correct marker is likely to be. For robustness against +damaged marker length counts, you may wish to insert a test in your +application for the case that the input buffer is completely full and yet +the decoder has suspended without consuming any data --- otherwise, if this +situation did occur, it would lead to an endless loop. (The library can't +provide this test since it has no idea whether "the buffer is full", or +even whether there is a fixed-size input buffer.) + +The input buffer would need to be 64K to allow for arbitrary COM or APPn +markers, but these are handled specially: they are either saved into allocated +memory, or skipped over by calling skip_input_data(). In the former case, +suspension is handled correctly, and in the latter case, the problem of +buffer overrun is placed on skip_input_data's shoulders, as explained above. +Note that if you provide your own marker handling routine for large markers, +you should consider how to deal with buffer overflow. + +Multiple-buffer management: + +In some applications it is desirable to store the compressed data in a linked +list of buffer areas, so as to avoid data copying. This can be handled by +having empty_output_buffer() or fill_input_buffer() set the pointer and count +to reference the next available buffer; FALSE is returned only if no more +buffers are available. Although seemingly straightforward, there is a +pitfall in this approach: the backtrack that occurs when FALSE is returned +could back up into an earlier buffer. For example, when fill_input_buffer() +is called, the current pointer & count indicate the backtrack restart point. +Since fill_input_buffer() will set the pointer and count to refer to a new +buffer, the restart position must be saved somewhere else. Suppose a second +call to fill_input_buffer() occurs in the same library call, and no +additional input data is available, so fill_input_buffer must return FALSE. +If the JPEG library has not moved the pointer/count forward in the current +buffer, then *the correct restart point is the saved position in the prior +buffer*. Prior buffers may be discarded only after the library establishes +a restart point within a later buffer. Similar remarks apply for output into +a chain of buffers. + +The library will never attempt to backtrack over a skip_input_data() call, +so any skipped data can be permanently discarded. You still have to deal +with the case of skipping not-yet-received data, however. + +It's much simpler to use only a single buffer; when fill_input_buffer() is +called, move any unconsumed data (beyond the current pointer/count) down to +the beginning of this buffer and then load new data into the remaining buffer +space. This approach requires a little more data copying but is far easier +to get right. + + +Progressive JPEG support +------------------------ + +Progressive JPEG rearranges the stored data into a series of scans of +increasing quality. In situations where a JPEG file is transmitted across a +slow communications link, a decoder can generate a low-quality image very +quickly from the first scan, then gradually improve the displayed quality as +more scans are received. The final image after all scans are complete is +identical to that of a regular (sequential) JPEG file of the same quality +setting. Progressive JPEG files are often slightly smaller than equivalent +sequential JPEG files, but the possibility of incremental display is the main +reason for using progressive JPEG. + +The IJG encoder library generates progressive JPEG files when given a +suitable "scan script" defining how to divide the data into scans. +Creation of progressive JPEG files is otherwise transparent to the encoder. +Progressive JPEG files can also be read transparently by the decoder library. +If the decoding application simply uses the library as defined above, it +will receive a final decoded image without any indication that the file was +progressive. Of course, this approach does not allow incremental display. +To perform incremental display, an application needs to use the decoder +library's "buffered-image" mode, in which it receives a decoded image +multiple times. + +Each displayed scan requires about as much work to decode as a full JPEG +image of the same size, so the decoder must be fairly fast in relation to the +data transmission rate in order to make incremental display useful. However, +it is possible to skip displaying the image and simply add the incoming bits +to the decoder's coefficient buffer. This is fast because only Huffman +decoding need be done, not IDCT, upsampling, colorspace conversion, etc. +The IJG decoder library allows the application to switch dynamically between +displaying the image and simply absorbing the incoming bits. A properly +coded application can automatically adapt the number of display passes to +suit the time available as the image is received. Also, a final +higher-quality display cycle can be performed from the buffered data after +the end of the file is reached. + +Progressive compression: + +To create a progressive JPEG file (or a multiple-scan sequential JPEG file), +set the scan_info cinfo field to point to an array of scan descriptors, and +perform compression as usual. Instead of constructing your own scan list, +you can call the jpeg_simple_progression() helper routine to create a +recommended progression sequence; this method should be used by all +applications that don't want to get involved in the nitty-gritty of +progressive scan sequence design. (If you want to provide user control of +scan sequences, you may wish to borrow the scan script reading code found +in rdswitch.c, so that you can read scan script files just like cjpeg's.) +When scan_info is not NULL, the compression library will store DCT'd data +into a buffer array as jpeg_write_scanlines() is called, and will emit all +the requested scans during jpeg_finish_compress(). This implies that +multiple-scan output cannot be created with a suspending data destination +manager, since jpeg_finish_compress() does not support suspension. We +should also note that the compressor currently forces Huffman optimization +mode when creating a progressive JPEG file, because the default Huffman +tables are unsuitable for progressive files. + +Progressive decompression: + +When buffered-image mode is not used, the decoder library will read all of +a multi-scan file during jpeg_start_decompress(), so that it can provide a +final decoded image. (Here "multi-scan" means either progressive or +multi-scan sequential.) This makes multi-scan files transparent to the +decoding application. However, existing applications that used suspending +input with version 5 of the IJG library will need to be modified to check +for a suspension return from jpeg_start_decompress(). + +To perform incremental display, an application must use the library's +buffered-image mode. This is described in the next section. + + +Buffered-image mode +------------------- + +In buffered-image mode, the library stores the partially decoded image in a +coefficient buffer, from which it can be read out as many times as desired. +This mode is typically used for incremental display of progressive JPEG files, +but it can be used with any JPEG file. Each scan of a progressive JPEG file +adds more data (more detail) to the buffered image. The application can +display in lockstep with the source file (one display pass per input scan), +or it can allow input processing to outrun display processing. By making +input and display processing run independently, it is possible for the +application to adapt progressive display to a wide range of data transmission +rates. + +The basic control flow for buffered-image decoding is + + jpeg_create_decompress() + set data source + jpeg_read_header() + set overall decompression parameters + cinfo.buffered_image = TRUE; /* select buffered-image mode */ + jpeg_start_decompress() + for (each output pass) { + adjust output decompression parameters if required + jpeg_start_output() /* start a new output pass */ + for (all scanlines in image) { + jpeg_read_scanlines() + display scanlines + } + jpeg_finish_output() /* terminate output pass */ + } + jpeg_finish_decompress() + jpeg_destroy_decompress() + +This differs from ordinary unbuffered decoding in that there is an additional +level of looping. The application can choose how many output passes to make +and how to display each pass. + +The simplest approach to displaying progressive images is to do one display +pass for each scan appearing in the input file. In this case the outer loop +condition is typically + while (! jpeg_input_complete(&cinfo)) +and the start-output call should read + jpeg_start_output(&cinfo, cinfo.input_scan_number); +The second parameter to jpeg_start_output() indicates which scan of the input +file is to be displayed; the scans are numbered starting at 1 for this +purpose. (You can use a loop counter starting at 1 if you like, but using +the library's input scan counter is easier.) The library automatically reads +data as necessary to complete each requested scan, and jpeg_finish_output() +advances to the next scan or end-of-image marker (hence input_scan_number +will be incremented by the time control arrives back at jpeg_start_output()). +With this technique, data is read from the input file only as needed, and +input and output processing run in lockstep. + +After reading the final scan and reaching the end of the input file, the +buffered image remains available; it can be read additional times by +repeating the jpeg_start_output()/jpeg_read_scanlines()/jpeg_finish_output() +sequence. For example, a useful technique is to use fast one-pass color +quantization for display passes made while the image is arriving, followed by +a final display pass using two-pass quantization for highest quality. This +is done by changing the library parameters before the final output pass. +Changing parameters between passes is discussed in detail below. + +In general the last scan of a progressive file cannot be recognized as such +until after it is read, so a post-input display pass is the best approach if +you want special processing in the final pass. + +When done with the image, be sure to call jpeg_finish_decompress() to release +the buffered image (or just use jpeg_destroy_decompress()). + +If input data arrives faster than it can be displayed, the application can +cause the library to decode input data in advance of what's needed to produce +output. This is done by calling the routine jpeg_consume_input(). +The return value is one of the following: + JPEG_REACHED_SOS: reached an SOS marker (the start of a new scan) + JPEG_REACHED_EOI: reached the EOI marker (end of image) + JPEG_ROW_COMPLETED: completed reading one MCU row of compressed data + JPEG_SCAN_COMPLETED: completed reading last MCU row of current scan + JPEG_SUSPENDED: suspended before completing any of the above +(JPEG_SUSPENDED can occur only if a suspending data source is used.) This +routine can be called at any time after initializing the JPEG object. It +reads some additional data and returns when one of the indicated significant +events occurs. (If called after the EOI marker is reached, it will +immediately return JPEG_REACHED_EOI without attempting to read more data.) + +The library's output processing will automatically call jpeg_consume_input() +whenever the output processing overtakes the input; thus, simple lockstep +display requires no direct calls to jpeg_consume_input(). But by adding +calls to jpeg_consume_input(), you can absorb data in advance of what is +being displayed. This has two benefits: + * You can limit buildup of unprocessed data in your input buffer. + * You can eliminate extra display passes by paying attention to the + state of the library's input processing. + +The first of these benefits only requires interspersing calls to +jpeg_consume_input() with your display operations and any other processing +you may be doing. To avoid wasting cycles due to backtracking, it's best to +call jpeg_consume_input() only after a hundred or so new bytes have arrived. +This is discussed further under "I/O suspension", above. (Note: the JPEG +library currently is not thread-safe. You must not call jpeg_consume_input() +from one thread of control if a different library routine is working on the +same JPEG object in another thread.) + +When input arrives fast enough that more than one new scan is available +before you start a new output pass, you may as well skip the output pass +corresponding to the completed scan. This occurs for free if you pass +cinfo.input_scan_number as the target scan number to jpeg_start_output(). +The input_scan_number field is simply the index of the scan currently being +consumed by the input processor. You can ensure that this is up-to-date by +emptying the input buffer just before calling jpeg_start_output(): call +jpeg_consume_input() repeatedly until it returns JPEG_SUSPENDED or +JPEG_REACHED_EOI. + +The target scan number passed to jpeg_start_output() is saved in the +cinfo.output_scan_number field. The library's output processing calls +jpeg_consume_input() whenever the current input scan number and row within +that scan is less than or equal to the current output scan number and row. +Thus, input processing can "get ahead" of the output processing but is not +allowed to "fall behind". You can achieve several different effects by +manipulating this interlock rule. For example, if you pass a target scan +number greater than the current input scan number, the output processor will +wait until that scan starts to arrive before producing any output. (To avoid +an infinite loop, the target scan number is automatically reset to the last +scan number when the end of image is reached. Thus, if you specify a large +target scan number, the library will just absorb the entire input file and +then perform an output pass. This is effectively the same as what +jpeg_start_decompress() does when you don't select buffered-image mode.) +When you pass a target scan number equal to the current input scan number, +the image is displayed no faster than the current input scan arrives. The +final possibility is to pass a target scan number less than the current input +scan number; this disables the input/output interlock and causes the output +processor to simply display whatever it finds in the image buffer, without +waiting for input. (However, the library will not accept a target scan +number less than one, so you can't avoid waiting for the first scan.) + +When data is arriving faster than the output display processing can advance +through the image, jpeg_consume_input() will store data into the buffered +image beyond the point at which the output processing is reading data out +again. If the input arrives fast enough, it may "wrap around" the buffer to +the point where the input is more than one whole scan ahead of the output. +If the output processing simply proceeds through its display pass without +paying attention to the input, the effect seen on-screen is that the lower +part of the image is one or more scans better in quality than the upper part. +Then, when the next output scan is started, you have a choice of what target +scan number to use. The recommended choice is to use the current input scan +number at that time, which implies that you've skipped the output scans +corresponding to the input scans that were completed while you processed the +previous output scan. In this way, the decoder automatically adapts its +speed to the arriving data, by skipping output scans as necessary to keep up +with the arriving data. + +When using this strategy, you'll want to be sure that you perform a final +output pass after receiving all the data; otherwise your last display may not +be full quality across the whole screen. So the right outer loop logic is +something like this: + do { + absorb any waiting input by calling jpeg_consume_input() + final_pass = jpeg_input_complete(&cinfo); + adjust output decompression parameters if required + jpeg_start_output(&cinfo, cinfo.input_scan_number); + ... + jpeg_finish_output() + } while (! final_pass); +rather than quitting as soon as jpeg_input_complete() returns TRUE. This +arrangement makes it simple to use higher-quality decoding parameters +for the final pass. But if you don't want to use special parameters for +the final pass, the right loop logic is like this: + for (;;) { + absorb any waiting input by calling jpeg_consume_input() + jpeg_start_output(&cinfo, cinfo.input_scan_number); + ... + jpeg_finish_output() + if (jpeg_input_complete(&cinfo) && + cinfo.input_scan_number == cinfo.output_scan_number) + break; + } +In this case you don't need to know in advance whether an output pass is to +be the last one, so it's not necessary to have reached EOF before starting +the final output pass; rather, what you want to test is whether the output +pass was performed in sync with the final input scan. This form of the loop +will avoid an extra output pass whenever the decoder is able (or nearly able) +to keep up with the incoming data. + +When the data transmission speed is high, you might begin a display pass, +then find that much or all of the file has arrived before you can complete +the pass. (You can detect this by noting the JPEG_REACHED_EOI return code +from jpeg_consume_input(), or equivalently by testing jpeg_input_complete().) +In this situation you may wish to abort the current display pass and start a +new one using the newly arrived information. To do so, just call +jpeg_finish_output() and then start a new pass with jpeg_start_output(). + +A variant strategy is to abort and restart display if more than one complete +scan arrives during an output pass; this can be detected by noting +JPEG_REACHED_SOS returns and/or examining cinfo.input_scan_number. This +idea should be employed with caution, however, since the display process +might never get to the bottom of the image before being aborted, resulting +in the lower part of the screen being several passes worse than the upper. +In most cases it's probably best to abort an output pass only if the whole +file has arrived and you want to begin the final output pass immediately. + +When receiving data across a communication link, we recommend always using +the current input scan number for the output target scan number; if a +higher-quality final pass is to be done, it should be started (aborting any +incomplete output pass) as soon as the end of file is received. However, +many other strategies are possible. For example, the application can examine +the parameters of the current input scan and decide whether to display it or +not. If the scan contains only chroma data, one might choose not to use it +as the target scan, expecting that the scan will be small and will arrive +quickly. To skip to the next scan, call jpeg_consume_input() until it +returns JPEG_REACHED_SOS or JPEG_REACHED_EOI. Or just use the next higher +number as the target scan for jpeg_start_output(); but that method doesn't +let you inspect the next scan's parameters before deciding to display it. + + +In buffered-image mode, jpeg_start_decompress() never performs input and +thus never suspends. An application that uses input suspension with +buffered-image mode must be prepared for suspension returns from these +routines: +* jpeg_start_output() performs input only if you request 2-pass quantization + and the target scan isn't fully read yet. (This is discussed below.) +* jpeg_read_scanlines(), as always, returns the number of scanlines that it + was able to produce before suspending. +* jpeg_finish_output() will read any markers following the target scan, + up to the end of the file or the SOS marker that begins another scan. + (But it reads no input if jpeg_consume_input() has already reached the + end of the file or a SOS marker beyond the target output scan.) +* jpeg_finish_decompress() will read until the end of file, and thus can + suspend if the end hasn't already been reached (as can be tested by + calling jpeg_input_complete()). +jpeg_start_output(), jpeg_finish_output(), and jpeg_finish_decompress() +all return TRUE if they completed their tasks, FALSE if they had to suspend. +In the event of a FALSE return, the application must load more input data +and repeat the call. Applications that use non-suspending data sources need +not check the return values of these three routines. + + +It is possible to change decoding parameters between output passes in the +buffered-image mode. The decoder library currently supports only very +limited changes of parameters. ONLY THE FOLLOWING parameter changes are +allowed after jpeg_start_decompress() is called: +* dct_method can be changed before each call to jpeg_start_output(). + For example, one could use a fast DCT method for early scans, changing + to a higher quality method for the final scan. +* dither_mode can be changed before each call to jpeg_start_output(); + of course this has no impact if not using color quantization. Typically + one would use ordered dither for initial passes, then switch to + Floyd-Steinberg dither for the final pass. Caution: changing dither mode + can cause more memory to be allocated by the library. Although the amount + of memory involved is not large (a scanline or so), it may cause the + initial max_memory_to_use specification to be exceeded, which in the worst + case would result in an out-of-memory failure. +* do_block_smoothing can be changed before each call to jpeg_start_output(). + This setting is relevant only when decoding a progressive JPEG image. + During the first DC-only scan, block smoothing provides a very "fuzzy" look + instead of the very "blocky" look seen without it; which is better seems a + matter of personal taste. But block smoothing is nearly always a win + during later stages, especially when decoding a successive-approximation + image: smoothing helps to hide the slight blockiness that otherwise shows + up on smooth gradients until the lowest coefficient bits are sent. +* Color quantization mode can be changed under the rules described below. + You *cannot* change between full-color and quantized output (because that + would alter the required I/O buffer sizes), but you can change which + quantization method is used. + +When generating color-quantized output, changing quantization method is a +very useful way of switching between high-speed and high-quality display. +The library allows you to change among its three quantization methods: +1. Single-pass quantization to a fixed color cube. + Selected by cinfo.two_pass_quantize = FALSE and cinfo.colormap = NULL. +2. Single-pass quantization to an application-supplied colormap. + Selected by setting cinfo.colormap to point to the colormap (the value of + two_pass_quantize is ignored); also set cinfo.actual_number_of_colors. +3. Two-pass quantization to a colormap chosen specifically for the image. + Selected by cinfo.two_pass_quantize = TRUE and cinfo.colormap = NULL. + (This is the default setting selected by jpeg_read_header, but it is + probably NOT what you want for the first pass of progressive display!) +These methods offer successively better quality and lesser speed. However, +only the first method is available for quantizing in non-RGB color spaces. + +IMPORTANT: because the different quantizer methods have very different +working-storage requirements, the library requires you to indicate which +one(s) you intend to use before you call jpeg_start_decompress(). (If we did +not require this, the max_memory_to_use setting would be a complete fiction.) +You do this by setting one or more of these three cinfo fields to TRUE: + enable_1pass_quant Fixed color cube colormap + enable_external_quant Externally-supplied colormap + enable_2pass_quant Two-pass custom colormap +All three are initialized FALSE by jpeg_read_header(). But +jpeg_start_decompress() automatically sets TRUE the one selected by the +current two_pass_quantize and colormap settings, so you only need to set the +enable flags for any other quantization methods you plan to change to later. + +After setting the enable flags correctly at jpeg_start_decompress() time, you +can change to any enabled quantization method by setting two_pass_quantize +and colormap properly just before calling jpeg_start_output(). The following +special rules apply: +1. You must explicitly set cinfo.colormap to NULL when switching to 1-pass + or 2-pass mode from a different mode, or when you want the 2-pass + quantizer to be re-run to generate a new colormap. +2. To switch to an external colormap, or to change to a different external + colormap than was used on the prior pass, you must call + jpeg_new_colormap() after setting cinfo.colormap. +NOTE: if you want to use the same colormap as was used in the prior pass, +you should not do either of these things. This will save some nontrivial +switchover costs. +(These requirements exist because cinfo.colormap will always be non-NULL +after completing a prior output pass, since both the 1-pass and 2-pass +quantizers set it to point to their output colormaps. Thus you have to +do one of these two things to notify the library that something has changed. +Yup, it's a bit klugy, but it's necessary to do it this way for backwards +compatibility.) + +Note that in buffered-image mode, the library generates any requested colormap +during jpeg_start_output(), not during jpeg_start_decompress(). + +When using two-pass quantization, jpeg_start_output() makes a pass over the +buffered image to determine the optimum color map; it therefore may take a +significant amount of time, whereas ordinarily it does little work. The +progress monitor hook is called during this pass, if defined. It is also +important to realize that if the specified target scan number is greater than +or equal to the current input scan number, jpeg_start_output() will attempt +to consume input as it makes this pass. If you use a suspending data source, +you need to check for a FALSE return from jpeg_start_output() under these +conditions. The combination of 2-pass quantization and a not-yet-fully-read +target scan is the only case in which jpeg_start_output() will consume input. + + +Application authors who support buffered-image mode may be tempted to use it +for all JPEG images, even single-scan ones. This will work, but it is +inefficient: there is no need to create an image-sized coefficient buffer for +single-scan images. Requesting buffered-image mode for such an image wastes +memory. Worse, it can cost time on large images, since the buffered data has +to be swapped out or written to a temporary file. If you are concerned about +maximum performance on baseline JPEG files, you should use buffered-image +mode only when the incoming file actually has multiple scans. This can be +tested by calling jpeg_has_multiple_scans(), which will return a correct +result at any time after jpeg_read_header() completes. + +It is also worth noting that when you use jpeg_consume_input() to let input +processing get ahead of output processing, the resulting pattern of access to +the coefficient buffer is quite nonsequential. It's best to use the memory +manager jmemnobs.c if you can (ie, if you have enough real or virtual main +memory). If not, at least make sure that max_memory_to_use is set as high as +possible. If the JPEG memory manager has to use a temporary file, you will +probably see a lot of disk traffic and poor performance. (This could be +improved with additional work on the memory manager, but we haven't gotten +around to it yet.) + +In some applications it may be convenient to use jpeg_consume_input() for all +input processing, including reading the initial markers; that is, you may +wish to call jpeg_consume_input() instead of jpeg_read_header() during +startup. This works, but note that you must check for JPEG_REACHED_SOS and +JPEG_REACHED_EOI return codes as the equivalent of jpeg_read_header's codes. +Once the first SOS marker has been reached, you must call +jpeg_start_decompress() before jpeg_consume_input() will consume more input; +it'll just keep returning JPEG_REACHED_SOS until you do. If you read a +tables-only file this way, jpeg_consume_input() will return JPEG_REACHED_EOI +without ever returning JPEG_REACHED_SOS; be sure to check for this case. +If this happens, the decompressor will not read any more input until you call +jpeg_abort() to reset it. It is OK to call jpeg_consume_input() even when not +using buffered-image mode, but in that case it's basically a no-op after the +initial markers have been read: it will just return JPEG_SUSPENDED. + + +Abbreviated datastreams and multiple images +------------------------------------------- + +A JPEG compression or decompression object can be reused to process multiple +images. This saves a small amount of time per image by eliminating the +"create" and "destroy" operations, but that isn't the real purpose of the +feature. Rather, reuse of an object provides support for abbreviated JPEG +datastreams. Object reuse can also simplify processing a series of images in +a single input or output file. This section explains these features. + +A JPEG file normally contains several hundred bytes worth of quantization +and Huffman tables. In a situation where many images will be stored or +transmitted with identical tables, this may represent an annoying overhead. +The JPEG standard therefore permits tables to be omitted. The standard +defines three classes of JPEG datastreams: + * "Interchange" datastreams contain an image and all tables needed to decode + the image. These are the usual kind of JPEG file. + * "Abbreviated image" datastreams contain an image, but are missing some or + all of the tables needed to decode that image. + * "Abbreviated table specification" (henceforth "tables-only") datastreams + contain only table specifications. +To decode an abbreviated image, it is necessary to load the missing table(s) +into the decoder beforehand. This can be accomplished by reading a separate +tables-only file. A variant scheme uses a series of images in which the first +image is an interchange (complete) datastream, while subsequent ones are +abbreviated and rely on the tables loaded by the first image. It is assumed +that once the decoder has read a table, it will remember that table until a +new definition for the same table number is encountered. + +It is the application designer's responsibility to figure out how to associate +the correct tables with an abbreviated image. While abbreviated datastreams +can be useful in a closed environment, their use is strongly discouraged in +any situation where data exchange with other applications might be needed. +Caveat designer. + +The JPEG library provides support for reading and writing any combination of +tables-only datastreams and abbreviated images. In both compression and +decompression objects, a quantization or Huffman table will be retained for +the lifetime of the object, unless it is overwritten by a new table definition. + + +To create abbreviated image datastreams, it is only necessary to tell the +compressor not to emit some or all of the tables it is using. Each +quantization and Huffman table struct contains a boolean field "sent_table", +which normally is initialized to FALSE. For each table used by the image, the +header-writing process emits the table and sets sent_table = TRUE unless it is +already TRUE. (In normal usage, this prevents outputting the same table +definition multiple times, as would otherwise occur because the chroma +components typically share tables.) Thus, setting this field to TRUE before +calling jpeg_start_compress() will prevent the table from being written at +all. + +If you want to create a "pure" abbreviated image file containing no tables, +just call "jpeg_suppress_tables(&cinfo, TRUE)" after constructing all the +tables. If you want to emit some but not all tables, you'll need to set the +individual sent_table fields directly. + +To create an abbreviated image, you must also call jpeg_start_compress() +with a second parameter of FALSE, not TRUE. Otherwise jpeg_start_compress() +will force all the sent_table fields to FALSE. (This is a safety feature to +prevent abbreviated images from being created accidentally.) + +To create a tables-only file, perform the same parameter setup that you +normally would, but instead of calling jpeg_start_compress() and so on, call +jpeg_write_tables(&cinfo). This will write an abbreviated datastream +containing only SOI, DQT and/or DHT markers, and EOI. All the quantization +and Huffman tables that are currently defined in the compression object will +be emitted unless their sent_tables flag is already TRUE, and then all the +sent_tables flags will be set TRUE. + +A sure-fire way to create matching tables-only and abbreviated image files +is to proceed as follows: + + create JPEG compression object + set JPEG parameters + set destination to tables-only file + jpeg_write_tables(&cinfo); + set destination to image file + jpeg_start_compress(&cinfo, FALSE); + write data... + jpeg_finish_compress(&cinfo); + +Since the JPEG parameters are not altered between writing the table file and +the abbreviated image file, the same tables are sure to be used. Of course, +you can repeat the jpeg_start_compress() ... jpeg_finish_compress() sequence +many times to produce many abbreviated image files matching the table file. + +You cannot suppress output of the computed Huffman tables when Huffman +optimization is selected. (If you could, there'd be no way to decode the +image...) Generally, you don't want to set optimize_coding = TRUE when +you are trying to produce abbreviated files. + +In some cases you might want to compress an image using tables which are +not stored in the application, but are defined in an interchange or +tables-only file readable by the application. This can be done by setting up +a JPEG decompression object to read the specification file, then copying the +tables into your compression object. See jpeg_copy_critical_parameters() +for an example of copying quantization tables. + + +To read abbreviated image files, you simply need to load the proper tables +into the decompression object before trying to read the abbreviated image. +If the proper tables are stored in the application program, you can just +allocate the table structs and fill in their contents directly. For example, +to load a fixed quantization table into table slot "n": + + if (cinfo.quant_tbl_ptrs[n] == NULL) + cinfo.quant_tbl_ptrs[n] = jpeg_alloc_quant_table((j_common_ptr) &cinfo); + quant_ptr = cinfo.quant_tbl_ptrs[n]; /* quant_ptr is JQUANT_TBL* */ + for (i = 0; i < 64; i++) { + /* Qtable[] is desired quantization table, in natural array order */ + quant_ptr->quantval[i] = Qtable[i]; + } + +Code to load a fixed Huffman table is typically (for AC table "n"): + + if (cinfo.ac_huff_tbl_ptrs[n] == NULL) + cinfo.ac_huff_tbl_ptrs[n] = jpeg_alloc_huff_table((j_common_ptr) &cinfo); + huff_ptr = cinfo.ac_huff_tbl_ptrs[n]; /* huff_ptr is JHUFF_TBL* */ + for (i = 1; i <= 16; i++) { + /* counts[i] is number of Huffman codes of length i bits, i=1..16 */ + huff_ptr->bits[i] = counts[i]; + } + for (i = 0; i < 256; i++) { + /* symbols[] is the list of Huffman symbols, in code-length order */ + huff_ptr->huffval[i] = symbols[i]; + } + +(Note that trying to set cinfo.quant_tbl_ptrs[n] to point directly at a +constant JQUANT_TBL object is not safe. If the incoming file happened to +contain a quantization table definition, your master table would get +overwritten! Instead allocate a working table copy and copy the master table +into it, as illustrated above. Ditto for Huffman tables, of course.) + +You might want to read the tables from a tables-only file, rather than +hard-wiring them into your application. The jpeg_read_header() call is +sufficient to read a tables-only file. You must pass a second parameter of +FALSE to indicate that you do not require an image to be present. Thus, the +typical scenario is + + create JPEG decompression object + set source to tables-only file + jpeg_read_header(&cinfo, FALSE); + set source to abbreviated image file + jpeg_read_header(&cinfo, TRUE); + set decompression parameters + jpeg_start_decompress(&cinfo); + read data... + jpeg_finish_decompress(&cinfo); + +In some cases, you may want to read a file without knowing whether it contains +an image or just tables. In that case, pass FALSE and check the return value +from jpeg_read_header(): it will be JPEG_HEADER_OK if an image was found, +JPEG_HEADER_TABLES_ONLY if only tables were found. (A third return value, +JPEG_SUSPENDED, is possible when using a suspending data source manager.) +Note that jpeg_read_header() will not complain if you read an abbreviated +image for which you haven't loaded the missing tables; the missing-table check +occurs later, in jpeg_start_decompress(). + + +It is possible to read a series of images from a single source file by +repeating the jpeg_read_header() ... jpeg_finish_decompress() sequence, +without releasing/recreating the JPEG object or the data source module. +(If you did reinitialize, any partial bufferload left in the data source +buffer at the end of one image would be discarded, causing you to lose the +start of the next image.) When you use this method, stored tables are +automatically carried forward, so some of the images can be abbreviated images +that depend on tables from earlier images. + +If you intend to write a series of images into a single destination file, +you might want to make a specialized data destination module that doesn't +flush the output buffer at term_destination() time. This would speed things +up by some trifling amount. Of course, you'd need to remember to flush the +buffer after the last image. You can make the later images be abbreviated +ones by passing FALSE to jpeg_start_compress(). + + +Special markers +--------------- + +Some applications may need to insert or extract special data in the JPEG +datastream. The JPEG standard provides marker types "COM" (comment) and +"APP0" through "APP15" (application) to hold application-specific data. +Unfortunately, the use of these markers is not specified by the standard. +COM markers are fairly widely used to hold user-supplied text. The JFIF file +format spec uses APP0 markers with specified initial strings to hold certain +data. Adobe applications use APP14 markers beginning with the string "Adobe" +for miscellaneous data. Other APPn markers are rarely seen, but might +contain almost anything. + +If you wish to store user-supplied text, we recommend you use COM markers +and place readable 7-bit ASCII text in them. Newline conventions are not +standardized --- expect to find LF (Unix style), CR/LF (DOS style), or CR +(Mac style). A robust COM reader should be able to cope with random binary +garbage, including nulls, since some applications generate COM markers +containing non-ASCII junk. (But yours should not be one of them.) + +For program-supplied data, use an APPn marker, and be sure to begin it with an +identifying string so that you can tell whether the marker is actually yours. +It's probably best to avoid using APP0 or APP14 for any private markers. +(NOTE: the upcoming SPIFF standard will use APP8 markers; we recommend you +not use APP8 markers for any private purposes, either.) + +Keep in mind that at most 65533 bytes can be put into one marker, but you +can have as many markers as you like. + +By default, the IJG compression library will write a JFIF APP0 marker if the +selected JPEG colorspace is grayscale or YCbCr, or an Adobe APP14 marker if +the selected colorspace is RGB, CMYK, or YCCK. You can disable this, but +we don't recommend it. The decompression library will recognize JFIF and +Adobe markers and will set the JPEG colorspace properly when one is found. + + +You can write special markers immediately following the datastream header by +calling jpeg_write_marker() after jpeg_start_compress() and before the first +call to jpeg_write_scanlines(). When you do this, the markers appear after +the SOI and the JFIF APP0 and Adobe APP14 markers (if written), but before +all else. Specify the marker type parameter as "JPEG_COM" for COM or +"JPEG_APP0 + n" for APPn. (Actually, jpeg_write_marker will let you write +any marker type, but we don't recommend writing any other kinds of marker.) +For example, to write a user comment string pointed to by comment_text: + jpeg_write_marker(cinfo, JPEG_COM, comment_text, strlen(comment_text)); + +If it's not convenient to store all the marker data in memory at once, +you can instead call jpeg_write_m_header() followed by multiple calls to +jpeg_write_m_byte(). If you do it this way, it's your responsibility to +call jpeg_write_m_byte() exactly the number of times given in the length +parameter to jpeg_write_m_header(). (This method lets you empty the +output buffer partway through a marker, which might be important when +using a suspending data destination module. In any case, if you are using +a suspending destination, you should flush its buffer after inserting +any special markers. See "I/O suspension".) + +Or, if you prefer to synthesize the marker byte sequence yourself, +you can just cram it straight into the data destination module. + +If you are writing JFIF 1.02 extension markers (thumbnail images), don't +forget to set cinfo.JFIF_minor_version = 2 so that the encoder will write the +correct JFIF version number in the JFIF header marker. The library's default +is to write version 1.01, but that's wrong if you insert any 1.02 extension +markers. (We could probably get away with just defaulting to 1.02, but there +used to be broken decoders that would complain about unknown minor version +numbers. To reduce compatibility risks it's safest not to write 1.02 unless +you are actually using 1.02 extensions.) + + +When reading, two methods of handling special markers are available: +1. You can ask the library to save the contents of COM and/or APPn markers +into memory, and then examine them at your leisure afterwards. +2. You can supply your own routine to process COM and/or APPn markers +on-the-fly as they are read. +The first method is simpler to use, especially if you are using a suspending +data source; writing a marker processor that copes with input suspension is +not easy (consider what happens if the marker is longer than your available +input buffer). However, the second method conserves memory since the marker +data need not be kept around after it's been processed. + +For either method, you'd normally set up marker handling after creating a +decompression object and before calling jpeg_read_header(), because the +markers of interest will typically be near the head of the file and so will +be scanned by jpeg_read_header. Once you've established a marker handling +method, it will be used for the life of that decompression object +(potentially many datastreams), unless you change it. Marker handling is +determined separately for COM markers and for each APPn marker code. + + +To save the contents of special markers in memory, call + jpeg_save_markers(cinfo, marker_code, length_limit) +where marker_code is the marker type to save, JPEG_COM or JPEG_APP0+n. +(To arrange to save all the special marker types, you need to call this +routine 17 times, for COM and APP0-APP15.) If the incoming marker is longer +than length_limit data bytes, only length_limit bytes will be saved; this +parameter allows you to avoid chewing up memory when you only need to see the +first few bytes of a potentially large marker. If you want to save all the +data, set length_limit to 0xFFFF; that is enough since marker lengths are only +16 bits. As a special case, setting length_limit to 0 prevents that marker +type from being saved at all. (That is the default behavior, in fact.) + +After jpeg_read_header() completes, you can examine the special markers by +following the cinfo->marker_list pointer chain. All the special markers in +the file appear in this list, in order of their occurrence in the file (but +omitting any markers of types you didn't ask for). Both the original data +length and the saved data length are recorded for each list entry; the latter +will not exceed length_limit for the particular marker type. Note that these +lengths exclude the marker length word, whereas the stored representation +within the JPEG file includes it. (Hence the maximum data length is really +only 65533.) + +It is possible that additional special markers appear in the file beyond the +SOS marker at which jpeg_read_header stops; if so, the marker list will be +extended during reading of the rest of the file. This is not expected to be +common, however. If you are short on memory you may want to reset the length +limit to zero for all marker types after finishing jpeg_read_header, to +ensure that the max_memory_to_use setting cannot be exceeded due to addition +of later markers. + +The marker list remains stored until you call jpeg_finish_decompress or +jpeg_abort, at which point the memory is freed and the list is set to empty. +(jpeg_destroy also releases the storage, of course.) + +Note that the library is internally interested in APP0 and APP14 markers; +if you try to set a small nonzero length limit on these types, the library +will silently force the length up to the minimum it wants. (But you can set +a zero length limit to prevent them from being saved at all.) Also, in a +16-bit environment, the maximum length limit may be constrained to less than +65533 by malloc() limitations. It is therefore best not to assume that the +effective length limit is exactly what you set it to be. + + +If you want to supply your own marker-reading routine, you do it by calling +jpeg_set_marker_processor(). A marker processor routine must have the +signature + boolean jpeg_marker_parser_method (j_decompress_ptr cinfo) +Although the marker code is not explicitly passed, the routine can find it +in cinfo->unread_marker. At the time of call, the marker proper has been +read from the data source module. The processor routine is responsible for +reading the marker length word and the remaining parameter bytes, if any. +Return TRUE to indicate success. (FALSE should be returned only if you are +using a suspending data source and it tells you to suspend. See the standard +marker processors in jdmarker.c for appropriate coding methods if you need to +use a suspending data source.) + +If you override the default APP0 or APP14 processors, it is up to you to +recognize JFIF and Adobe markers if you want colorspace recognition to occur +properly. We recommend copying and extending the default processors if you +want to do that. (A better idea is to save these marker types for later +examination by calling jpeg_save_markers(); that method doesn't interfere +with the library's own processing of these markers.) + +jpeg_set_marker_processor() and jpeg_save_markers() are mutually exclusive +--- if you call one it overrides any previous call to the other, for the +particular marker type specified. + +A simple example of an external COM processor can be found in djpeg.c. +Also, see jpegtran.c for an example of using jpeg_save_markers. + + +Raw (downsampled) image data +---------------------------- + +Some applications need to supply already-downsampled image data to the JPEG +compressor, or to receive raw downsampled data from the decompressor. The +library supports this requirement by allowing the application to write or +read raw data, bypassing the normal preprocessing or postprocessing steps. +The interface is different from the standard one and is somewhat harder to +use. If your interest is merely in bypassing color conversion, we recommend +that you use the standard interface and simply set jpeg_color_space = +in_color_space (or jpeg_color_space = out_color_space for decompression). +The mechanism described in this section is necessary only to supply or +receive downsampled image data, in which not all components have the same +dimensions. + + +To compress raw data, you must supply the data in the colorspace to be used +in the JPEG file (please read the earlier section on Special color spaces) +and downsampled to the sampling factors specified in the JPEG parameters. +You must supply the data in the format used internally by the JPEG library, +namely a JSAMPIMAGE array. This is an array of pointers to two-dimensional +arrays, each of type JSAMPARRAY. Each 2-D array holds the values for one +color component. This structure is necessary since the components are of +different sizes. If the image dimensions are not a multiple of the MCU size, +you must also pad the data correctly (usually, this is done by replicating +the last column and/or row). The data must be padded to a multiple of a DCT +block in each component: that is, each downsampled row must contain a +multiple of 8 valid samples, and there must be a multiple of 8 sample rows +for each component. (For applications such as conversion of digital TV +images, the standard image size is usually a multiple of the DCT block size, +so that no padding need actually be done.) + +The procedure for compression of raw data is basically the same as normal +compression, except that you call jpeg_write_raw_data() in place of +jpeg_write_scanlines(). Before calling jpeg_start_compress(), you must do +the following: + * Set cinfo->raw_data_in to TRUE. (It is set FALSE by jpeg_set_defaults().) + This notifies the library that you will be supplying raw data. + * Ensure jpeg_color_space is correct --- an explicit jpeg_set_colorspace() + call is a good idea. Note that since color conversion is bypassed, + in_color_space is ignored, except that jpeg_set_defaults() uses it to + choose the default jpeg_color_space setting. + * Ensure the sampling factors, cinfo->comp_info[i].h_samp_factor and + cinfo->comp_info[i].v_samp_factor, are correct. Since these indicate the + dimensions of the data you are supplying, it's wise to set them + explicitly, rather than assuming the library's defaults are what you want. + +To pass raw data to the library, call jpeg_write_raw_data() in place of +jpeg_write_scanlines(). The two routines work similarly except that +jpeg_write_raw_data takes a JSAMPIMAGE data array rather than JSAMPARRAY. +The scanlines count passed to and returned from jpeg_write_raw_data is +measured in terms of the component with the largest v_samp_factor. + +jpeg_write_raw_data() processes one MCU row per call, which is to say +v_samp_factor*DCTSIZE sample rows of each component. The passed num_lines +value must be at least max_v_samp_factor*DCTSIZE, and the return value will +be exactly that amount (or possibly some multiple of that amount, in future +library versions). This is true even on the last call at the bottom of the +image; don't forget to pad your data as necessary. + +The required dimensions of the supplied data can be computed for each +component as + cinfo->comp_info[i].width_in_blocks*DCTSIZE samples per row + cinfo->comp_info[i].height_in_blocks*DCTSIZE rows in image +after jpeg_start_compress() has initialized those fields. If the valid data +is smaller than this, it must be padded appropriately. For some sampling +factors and image sizes, additional dummy DCT blocks are inserted to make +the image a multiple of the MCU dimensions. The library creates such dummy +blocks itself; it does not read them from your supplied data. Therefore you +need never pad by more than DCTSIZE samples. An example may help here. +Assume 2h2v downsampling of YCbCr data, that is + cinfo->comp_info[0].h_samp_factor = 2 for Y + cinfo->comp_info[0].v_samp_factor = 2 + cinfo->comp_info[1].h_samp_factor = 1 for Cb + cinfo->comp_info[1].v_samp_factor = 1 + cinfo->comp_info[2].h_samp_factor = 1 for Cr + cinfo->comp_info[2].v_samp_factor = 1 +and suppose that the nominal image dimensions (cinfo->image_width and +cinfo->image_height) are 101x101 pixels. Then jpeg_start_compress() will +compute downsampled_width = 101 and width_in_blocks = 13 for Y, +downsampled_width = 51 and width_in_blocks = 7 for Cb and Cr (and the same +for the height fields). You must pad the Y data to at least 13*8 = 104 +columns and rows, the Cb/Cr data to at least 7*8 = 56 columns and rows. The +MCU height is max_v_samp_factor = 2 DCT rows so you must pass at least 16 +scanlines on each call to jpeg_write_raw_data(), which is to say 16 actual +sample rows of Y and 8 each of Cb and Cr. A total of 7 MCU rows are needed, +so you must pass a total of 7*16 = 112 "scanlines". The last DCT block row +of Y data is dummy, so it doesn't matter what you pass for it in the data +arrays, but the scanlines count must total up to 112 so that all of the Cb +and Cr data gets passed. + +Output suspension is supported with raw-data compression: if the data +destination module suspends, jpeg_write_raw_data() will return 0. +In this case the same data rows must be passed again on the next call. + + +Decompression with raw data output implies bypassing all postprocessing: +you cannot ask for rescaling or color quantization, for instance. More +seriously, you must deal with the color space and sampling factors present in +the incoming file. If your application only handles, say, 2h1v YCbCr data, +you must check for and fail on other color spaces or other sampling factors. +The library will not convert to a different color space for you. + +To obtain raw data output, set cinfo->raw_data_out = TRUE before +jpeg_start_decompress() (it is set FALSE by jpeg_read_header()). Be sure to +verify that the color space and sampling factors are ones you can handle. +Then call jpeg_read_raw_data() in place of jpeg_read_scanlines(). The +decompression process is otherwise the same as usual. + +jpeg_read_raw_data() returns one MCU row per call, and thus you must pass a +buffer of at least max_v_samp_factor*DCTSIZE scanlines (scanline counting is +the same as for raw-data compression). The buffer you pass must be large +enough to hold the actual data plus padding to DCT-block boundaries. As with +compression, any entirely dummy DCT blocks are not processed so you need not +allocate space for them, but the total scanline count includes them. The +above example of computing buffer dimensions for raw-data compression is +equally valid for decompression. + +Input suspension is supported with raw-data decompression: if the data source +module suspends, jpeg_read_raw_data() will return 0. You can also use +buffered-image mode to read raw data in multiple passes. + + +Really raw data: DCT coefficients +--------------------------------- + +It is possible to read or write the contents of a JPEG file as raw DCT +coefficients. This facility is mainly intended for use in lossless +transcoding between different JPEG file formats. Other possible applications +include lossless cropping of a JPEG image, lossless reassembly of a +multi-strip or multi-tile TIFF/JPEG file into a single JPEG datastream, etc. + +To read the contents of a JPEG file as DCT coefficients, open the file and do +jpeg_read_header() as usual. But instead of calling jpeg_start_decompress() +and jpeg_read_scanlines(), call jpeg_read_coefficients(). This will read the +entire image into a set of virtual coefficient-block arrays, one array per +component. The return value is a pointer to an array of virtual-array +descriptors. Each virtual array can be accessed directly using the JPEG +memory manager's access_virt_barray method (see Memory management, below, +and also read structure.doc's discussion of virtual array handling). Or, +for simple transcoding to a different JPEG file format, the array list can +just be handed directly to jpeg_write_coefficients(). + +Each block in the block arrays contains quantized coefficient values in +normal array order (not JPEG zigzag order). The block arrays contain only +DCT blocks containing real data; any entirely-dummy blocks added to fill out +interleaved MCUs at the right or bottom edges of the image are discarded +during reading and are not stored in the block arrays. (The size of each +block array can be determined from the width_in_blocks and height_in_blocks +fields of the component's comp_info entry.) This is also the data format +expected by jpeg_write_coefficients(). + +When you are done using the virtual arrays, call jpeg_finish_decompress() +to release the array storage and return the decompression object to an idle +state; or just call jpeg_destroy() if you don't need to reuse the object. + +If you use a suspending data source, jpeg_read_coefficients() will return +NULL if it is forced to suspend; a non-NULL return value indicates successful +completion. You need not test for a NULL return value when using a +non-suspending data source. + +It is also possible to call jpeg_read_coefficients() to obtain access to the +decoder's coefficient arrays during a normal decode cycle in buffered-image +mode. This frammish might be useful for progressively displaying an incoming +image and then re-encoding it without loss. To do this, decode in buffered- +image mode as discussed previously, then call jpeg_read_coefficients() after +the last jpeg_finish_output() call. The arrays will be available for your use +until you call jpeg_finish_decompress(). + + +To write the contents of a JPEG file as DCT coefficients, you must provide +the DCT coefficients stored in virtual block arrays. You can either pass +block arrays read from an input JPEG file by jpeg_read_coefficients(), or +allocate virtual arrays from the JPEG compression object and fill them +yourself. In either case, jpeg_write_coefficients() is substituted for +jpeg_start_compress() and jpeg_write_scanlines(). Thus the sequence is + * Create compression object + * Set all compression parameters as necessary + * Request virtual arrays if needed + * jpeg_write_coefficients() + * jpeg_finish_compress() + * Destroy or re-use compression object +jpeg_write_coefficients() is passed a pointer to an array of virtual block +array descriptors; the number of arrays is equal to cinfo.num_components. + +The virtual arrays need only have been requested, not realized, before +jpeg_write_coefficients() is called. A side-effect of +jpeg_write_coefficients() is to realize any virtual arrays that have been +requested from the compression object's memory manager. Thus, when obtaining +the virtual arrays from the compression object, you should fill the arrays +after calling jpeg_write_coefficients(). The data is actually written out +when you call jpeg_finish_compress(); jpeg_write_coefficients() only writes +the file header. + +When writing raw DCT coefficients, it is crucial that the JPEG quantization +tables and sampling factors match the way the data was encoded, or the +resulting file will be invalid. For transcoding from an existing JPEG file, +we recommend using jpeg_copy_critical_parameters(). This routine initializes +all the compression parameters to default values (like jpeg_set_defaults()), +then copies the critical information from a source decompression object. +The decompression object should have just been used to read the entire +JPEG input file --- that is, it should be awaiting jpeg_finish_decompress(). + +jpeg_write_coefficients() marks all tables stored in the compression object +as needing to be written to the output file (thus, it acts like +jpeg_start_compress(cinfo, TRUE)). This is for safety's sake, to avoid +emitting abbreviated JPEG files by accident. If you really want to emit an +abbreviated JPEG file, call jpeg_suppress_tables(), or set the tables' +individual sent_table flags, between calling jpeg_write_coefficients() and +jpeg_finish_compress(). + + +Progress monitoring +------------------- + +Some applications may need to regain control from the JPEG library every so +often. The typical use of this feature is to produce a percent-done bar or +other progress display. (For a simple example, see cjpeg.c or djpeg.c.) +Although you do get control back frequently during the data-transferring pass +(the jpeg_read_scanlines or jpeg_write_scanlines loop), any additional passes +will occur inside jpeg_finish_compress or jpeg_start_decompress; those +routines may take a long time to execute, and you don't get control back +until they are done. + +You can define a progress-monitor routine which will be called periodically +by the library. No guarantees are made about how often this call will occur, +so we don't recommend you use it for mouse tracking or anything like that. +At present, a call will occur once per MCU row, scanline, or sample row +group, whichever unit is convenient for the current processing mode; so the +wider the image, the longer the time between calls. During the data +transferring pass, only one call occurs per call of jpeg_read_scanlines or +jpeg_write_scanlines, so don't pass a large number of scanlines at once if +you want fine resolution in the progress count. (If you really need to use +the callback mechanism for time-critical tasks like mouse tracking, you could +insert additional calls inside some of the library's inner loops.) + +To establish a progress-monitor callback, create a struct jpeg_progress_mgr, +fill in its progress_monitor field with a pointer to your callback routine, +and set cinfo->progress to point to the struct. The callback will be called +whenever cinfo->progress is non-NULL. (This pointer is set to NULL by +jpeg_create_compress or jpeg_create_decompress; the library will not change +it thereafter. So if you allocate dynamic storage for the progress struct, +make sure it will live as long as the JPEG object does. Allocating from the +JPEG memory manager with lifetime JPOOL_PERMANENT will work nicely.) You +can use the same callback routine for both compression and decompression. + +The jpeg_progress_mgr struct contains four fields which are set by the library: + long pass_counter; /* work units completed in this pass */ + long pass_limit; /* total number of work units in this pass */ + int completed_passes; /* passes completed so far */ + int total_passes; /* total number of passes expected */ +During any one pass, pass_counter increases from 0 up to (not including) +pass_limit; the step size is usually but not necessarily 1. The pass_limit +value may change from one pass to another. The expected total number of +passes is in total_passes, and the number of passes already completed is in +completed_passes. Thus the fraction of work completed may be estimated as + completed_passes + (pass_counter/pass_limit) + -------------------------------------------- + total_passes +ignoring the fact that the passes may not be equal amounts of work. + +When decompressing, pass_limit can even change within a pass, because it +depends on the number of scans in the JPEG file, which isn't always known in +advance. The computed fraction-of-work-done may jump suddenly (if the library +discovers it has overestimated the number of scans) or even decrease (in the +opposite case). It is not wise to put great faith in the work estimate. + +When using the decompressor's buffered-image mode, the progress monitor work +estimate is likely to be completely unhelpful, because the library has no way +to know how many output passes will be demanded of it. Currently, the library +sets total_passes based on the assumption that there will be one more output +pass if the input file end hasn't yet been read (jpeg_input_complete() isn't +TRUE), but no more output passes if the file end has been reached when the +output pass is started. This means that total_passes will rise as additional +output passes are requested. If you have a way of determining the input file +size, estimating progress based on the fraction of the file that's been read +will probably be more useful than using the library's value. + + +Memory management +----------------- + +This section covers some key facts about the JPEG library's built-in memory +manager. For more info, please read structure.doc's section about the memory +manager, and consult the source code if necessary. + +All memory and temporary file allocation within the library is done via the +memory manager. If necessary, you can replace the "back end" of the memory +manager to control allocation yourself (for example, if you don't want the +library to use malloc() and free() for some reason). + +Some data is allocated "permanently" and will not be freed until the JPEG +object is destroyed. Most data is allocated "per image" and is freed by +jpeg_finish_compress, jpeg_finish_decompress, or jpeg_abort. You can call the +memory manager yourself to allocate structures that will automatically be +freed at these times. Typical code for this is + ptr = (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE, size); +Use JPOOL_PERMANENT to get storage that lasts as long as the JPEG object. +Use alloc_large instead of alloc_small for anything bigger than a few Kbytes. +There are also alloc_sarray and alloc_barray routines that automatically +build 2-D sample or block arrays. + +The library's minimum space requirements to process an image depend on the +image's width, but not on its height, because the library ordinarily works +with "strip" buffers that are as wide as the image but just a few rows high. +Some operating modes (eg, two-pass color quantization) require full-image +buffers. Such buffers are treated as "virtual arrays": only the current strip +need be in memory, and the rest can be swapped out to a temporary file. + +If you use the simplest memory manager back end (jmemnobs.c), then no +temporary files are used; virtual arrays are simply malloc()'d. Images bigger +than memory can be processed only if your system supports virtual memory. +The other memory manager back ends support temporary files of various flavors +and thus work in machines without virtual memory. They may also be useful on +Unix machines if you need to process images that exceed available swap space. + +When using temporary files, the library will make the in-memory buffers for +its virtual arrays just big enough to stay within a "maximum memory" setting. +Your application can set this limit by setting cinfo->mem->max_memory_to_use +after creating the JPEG object. (Of course, there is still a minimum size for +the buffers, so the max-memory setting is effective only if it is bigger than +the minimum space needed.) If you allocate any large structures yourself, you +must allocate them before jpeg_start_compress() or jpeg_start_decompress() in +order to have them counted against the max memory limit. Also keep in mind +that space allocated with alloc_small() is ignored, on the assumption that +it's too small to be worth worrying about; so a reasonable safety margin +should be left when setting max_memory_to_use. + +If you use the jmemname.c or jmemdos.c memory manager back end, it is +important to clean up the JPEG object properly to ensure that the temporary +files get deleted. (This is especially crucial with jmemdos.c, where the +"temporary files" may be extended-memory segments; if they are not freed, +DOS will require a reboot to recover the memory.) Thus, with these memory +managers, it's a good idea to provide a signal handler that will trap any +early exit from your program. The handler should call either jpeg_abort() +or jpeg_destroy() for any active JPEG objects. A handler is not needed with +jmemnobs.c, and shouldn't be necessary with jmemansi.c or jmemmac.c either, +since the C library is supposed to take care of deleting files made with +tmpfile(). + + +Memory usage +------------ + +Working memory requirements while performing compression or decompression +depend on image dimensions, image characteristics (such as colorspace and +JPEG process), and operating mode (application-selected options). + +As of v6b, the decompressor requires: + 1. About 24K in more-or-less-fixed-size data. This varies a bit depending + on operating mode and image characteristics (particularly color vs. + grayscale), but it doesn't depend on image dimensions. + 2. Strip buffers (of size proportional to the image width) for IDCT and + upsampling results. The worst case for commonly used sampling factors + is about 34 bytes * width in pixels for a color image. A grayscale image + only needs about 8 bytes per pixel column. + 3. A full-image DCT coefficient buffer is needed to decode a multi-scan JPEG + file (including progressive JPEGs), or whenever you select buffered-image + mode. This takes 2 bytes/coefficient. At typical 2x2 sampling, that's + 3 bytes per pixel for a color image. Worst case (1x1 sampling) requires + 6 bytes/pixel. For grayscale, figure 2 bytes/pixel. + 4. To perform 2-pass color quantization, the decompressor also needs a + 128K color lookup table and a full-image pixel buffer (3 bytes/pixel). +This does not count any memory allocated by the application, such as a +buffer to hold the final output image. + +The above figures are valid for 8-bit JPEG data precision and a machine with +32-bit ints. For 12-bit JPEG data, double the size of the strip buffers and +quantization pixel buffer. The "fixed-size" data will be somewhat smaller +with 16-bit ints, larger with 64-bit ints. Also, CMYK or other unusual +color spaces will require different amounts of space. + +The full-image coefficient and pixel buffers, if needed at all, do not +have to be fully RAM resident; you can have the library use temporary +files instead when the total memory usage would exceed a limit you set. +(But if your OS supports virtual memory, it's probably better to just use +jmemnobs and let the OS do the swapping.) + +The compressor's memory requirements are similar, except that it has no need +for color quantization. Also, it needs a full-image DCT coefficient buffer +if Huffman-table optimization is asked for, even if progressive mode is not +requested. + +If you need more detailed information about memory usage in a particular +situation, you can enable the MEM_STATS code in jmemmgr.c. + + +Library compile-time options +---------------------------- + +A number of compile-time options are available by modifying jmorecfg.h. + +The JPEG standard provides for both the baseline 8-bit DCT process and +a 12-bit DCT process. The IJG code supports 12-bit lossy JPEG if you define +BITS_IN_JSAMPLE as 12 rather than 8. Note that this causes JSAMPLE to be +larger than a char, so it affects the surrounding application's image data. +The sample applications cjpeg and djpeg can support 12-bit mode only for PPM +and GIF file formats; you must disable the other file formats to compile a +12-bit cjpeg or djpeg. (install.doc has more information about that.) +At present, a 12-bit library can handle *only* 12-bit images, not both +precisions. (If you need to include both 8- and 12-bit libraries in a single +application, you could probably do it by defining NEED_SHORT_EXTERNAL_NAMES +for just one of the copies. You'd have to access the 8-bit and 12-bit copies +from separate application source files. This is untested ... if you try it, +we'd like to hear whether it works!) + +Note that a 12-bit library always compresses in Huffman optimization mode, +in order to generate valid Huffman tables. This is necessary because our +default Huffman tables only cover 8-bit data. If you need to output 12-bit +files in one pass, you'll have to supply suitable default Huffman tables. +You may also want to supply your own DCT quantization tables; the existing +quality-scaling code has been developed for 8-bit use, and probably doesn't +generate especially good tables for 12-bit. + +The maximum number of components (color channels) in the image is determined +by MAX_COMPONENTS. The JPEG standard allows up to 255 components, but we +expect that few applications will need more than four or so. + +On machines with unusual data type sizes, you may be able to improve +performance or reduce memory space by tweaking the various typedefs in +jmorecfg.h. In particular, on some RISC CPUs, access to arrays of "short"s +is quite slow; consider trading memory for speed by making JCOEF, INT16, and +UINT16 be "int" or "unsigned int". UINT8 is also a candidate to become int. +You probably don't want to make JSAMPLE be int unless you have lots of memory +to burn. + +You can reduce the size of the library by compiling out various optional +functions. To do this, undefine xxx_SUPPORTED symbols as necessary. + +You can also save a few K by not having text error messages in the library; +the standard error message table occupies about 5Kb. This is particularly +reasonable for embedded applications where there's no good way to display +a message anyway. To do this, remove the creation of the message table +(jpeg_std_message_table[]) from jerror.c, and alter format_message to do +something reasonable without it. You could output the numeric value of the +message code number, for example. If you do this, you can also save a couple +more K by modifying the TRACEMSn() macros in jerror.h to expand to nothing; +you don't need trace capability anyway, right? + + +Portability considerations +-------------------------- + +The JPEG library has been written to be extremely portable; the sample +applications cjpeg and djpeg are slightly less so. This section summarizes +the design goals in this area. (If you encounter any bugs that cause the +library to be less portable than is claimed here, we'd appreciate hearing +about them.) + +The code works fine on ANSI C, C++, and pre-ANSI C compilers, using any of +the popular system include file setups, and some not-so-popular ones too. +See install.doc for configuration procedures. + +The code is not dependent on the exact sizes of the C data types. As +distributed, we make the assumptions that + char is at least 8 bits wide + short is at least 16 bits wide + int is at least 16 bits wide + long is at least 32 bits wide +(These are the minimum requirements of the ANSI C standard.) Wider types will +work fine, although memory may be used inefficiently if char is much larger +than 8 bits or short is much bigger than 16 bits. The code should work +equally well with 16- or 32-bit ints. + +In a system where these assumptions are not met, you may be able to make the +code work by modifying the typedefs in jmorecfg.h. However, you will probably +have difficulty if int is less than 16 bits wide, since references to plain +int abound in the code. + +char can be either signed or unsigned, although the code runs faster if an +unsigned char type is available. If char is wider than 8 bits, you will need +to redefine JOCTET and/or provide custom data source/destination managers so +that JOCTET represents exactly 8 bits of data on external storage. + +The JPEG library proper does not assume ASCII representation of characters. +But some of the image file I/O modules in cjpeg/djpeg do have ASCII +dependencies in file-header manipulation; so does cjpeg's select_file_type() +routine. + +The JPEG library does not rely heavily on the C library. In particular, C +stdio is used only by the data source/destination modules and the error +handler, all of which are application-replaceable. (cjpeg/djpeg are more +heavily dependent on stdio.) malloc and free are called only from the memory +manager "back end" module, so you can use a different memory allocator by +replacing that one file. + +The code generally assumes that C names must be unique in the first 15 +characters. However, global function names can be made unique in the +first 6 characters by defining NEED_SHORT_EXTERNAL_NAMES. + +More info about porting the code may be gleaned by reading jconfig.doc, +jmorecfg.h, and jinclude.h. + + +Notes for MS-DOS implementors +----------------------------- + +The IJG code is designed to work efficiently in 80x86 "small" or "medium" +memory models (i.e., data pointers are 16 bits unless explicitly declared +"far"; code pointers can be either size). You may be able to use small +model to compile cjpeg or djpeg by itself, but you will probably have to use +medium model for any larger application. This won't make much difference in +performance. You *will* take a noticeable performance hit if you use a +large-data memory model (perhaps 10%-25%), and you should avoid "huge" model +if at all possible. + +The JPEG library typically needs 2Kb-3Kb of stack space. It will also +malloc about 20K-30K of near heap space while executing (and lots of far +heap, but that doesn't count in this calculation). This figure will vary +depending on selected operating mode, and to a lesser extent on image size. +There is also about 5Kb-6Kb of constant data which will be allocated in the +near data segment (about 4Kb of this is the error message table). +Thus you have perhaps 20K available for other modules' static data and near +heap space before you need to go to a larger memory model. The C library's +static data will account for several K of this, but that still leaves a good +deal for your needs. (If you are tight on space, you could reduce the sizes +of the I/O buffers allocated by jdatasrc.c and jdatadst.c, say from 4K to +1K. Another possibility is to move the error message table to far memory; +this should be doable with only localized hacking on jerror.c.) + +About 2K of the near heap space is "permanent" memory that will not be +released until you destroy the JPEG object. This is only an issue if you +save a JPEG object between compression or decompression operations. + +Far data space may also be a tight resource when you are dealing with large +images. The most memory-intensive case is decompression with two-pass color +quantization, or single-pass quantization to an externally supplied color +map. This requires a 128Kb color lookup table plus strip buffers amounting +to about 40 bytes per column for typical sampling ratios (eg, about 25600 +bytes for a 640-pixel-wide image). You may not be able to process wide +images if you have large data structures of your own. + +Of course, all of these concerns vanish if you use a 32-bit flat-memory-model +compiler, such as DJGPP or Watcom C. We highly recommend flat model if you +can use it; the JPEG library is significantly faster in flat model. |