path: root/tk8.6/generic/tkImgPPM.c

/*
 * tkImgPPM.c --
 *
 *	A photo image file handler for PPM (Portable PixMap) files.
 *
 * Copyright (c) 1994 The Australian National University.
 * Copyright (c) 1994-1997 Sun Microsystems, Inc.
 *
 * See the file "license.terms" for information on usage and redistribution of
 * this file, and for a DISCLAIMER OF ALL WARRANTIES.
 *
 * Author: Paul Mackerras (paulus@cs.anu.edu.au),
 *	Department of Computer Science,
 *	Australian National University.
 */

#include "tkInt.h"

/*
 * The maximum amount of memory to allocate for data read from the file. If we
 * need more than this, we do it in pieces.
 */

#define MAX_MEMORY	10000		/* don't allocate > 10KB */

/*
 * Define PGM and PPM, i.e. gray images and color images.
 */

#define PGM 1
#define PPM 2

/*
 * The format record for the PPM file format:
 */

static int		FileMatchPPM(Tcl_Channel chan, const char *fileName,
			    Tcl_Obj *format, int *widthPtr, int *heightPtr,
			    Tcl_Interp *interp);
static int		FileReadPPM(Tcl_Interp *interp, Tcl_Channel chan,
			    const char *fileName, Tcl_Obj *format,
			    Tk_PhotoHandle imageHandle, int destX, int destY,
			    int width, int height, int srcX, int srcY);
static int		FileWritePPM(Tcl_Interp *interp, const char *fileName,
			    Tcl_Obj *format, Tk_PhotoImageBlock *blockPtr);
static int		StringWritePPM(Tcl_Interp *interp, Tcl_Obj *format,
			    Tk_PhotoImageBlock *blockPtr);
static int		StringMatchPPM(Tcl_Obj *dataObj, Tcl_Obj *format,
			    int *widthPtr, int *heightPtr, Tcl_Interp *interp);
static int		StringReadPPM(Tcl_Interp *interp, Tcl_Obj *dataObj,
			    Tcl_Obj *format, Tk_PhotoHandle imageHandle,
			    int destX, int destY, int width, int height,
			    int srcX, int srcY);

Tk_PhotoImageFormat tkImgFmtPPM = {
    "ppm",			/* name */
    FileMatchPPM,		/* fileMatchProc */
    StringMatchPPM,		/* stringMatchProc */
    FileReadPPM,		/* fileReadProc */
    StringReadPPM,		/* stringReadProc */
    FileWritePPM,		/* fileWriteProc */
    StringWritePPM,		/* stringWriteProc */
    NULL
};

/*
 * Prototypes for local functions defined in this file:
 */

static int		ReadPPMFileHeader(Tcl_Channel chan, int *widthPtr,
			    int *heightPtr, int *maxIntensityPtr);
static int		ReadPPMStringHeader(Tcl_Obj *dataObj, int *widthPtr,
			    int *heightPtr, int *maxIntensityPtr,
			    unsigned char **dataBufferPtr, int *dataSizePtr);

/*
 *----------------------------------------------------------------------
 *
 * FileMatchPPM --
 *
 *	This function is invoked by the photo image type to see if a file
 *	contains image data in PPM format.
 *
 * Results:
 *	The return value is >0 if the first characters in file "f" look like
 *	PPM data, and 0 otherwise.
 *
 * Side effects:
 *	The access position in f may change.
 *
 *----------------------------------------------------------------------
 */

static int
FileMatchPPM(
    Tcl_Channel chan,		/* The image file, open for reading. */
    const char *fileName,	/* The name of the image file. */
    Tcl_Obj *format,		/* User-specified format string, or NULL. */
    int *widthPtr, int *heightPtr,
				/* The dimensions of the image are returned
				 * here if the file is a valid raw PPM
				 * file. */
    Tcl_Interp *interp)		/* unused */
{
    int dummy;

    return ReadPPMFileHeader(chan, widthPtr, heightPtr, &dummy);
}

/*
 *----------------------------------------------------------------------
 *
 * FileReadPPM --
 *
 *	This function is called by the photo image type to read PPM format
 *	data from a file and write it into a given photo image.
 *
 * Results:
 *	A standard TCL completion code. If TCL_ERROR is returned then an error
 *	message is left in the interp's result.
 *
 * Side effects:
 *	The access position in file f is changed, and new data is added to the
 *	image given by imageHandle.
 *
 *----------------------------------------------------------------------
 */

static int
FileReadPPM(
    Tcl_Interp *interp,		/* Interpreter to use for reporting errors. */
    Tcl_Channel chan,		/* The image file, open for reading. */
    const char *fileName,	/* The name of the image file. */
    Tcl_Obj *format,		/* User-specified format string, or NULL. */
    Tk_PhotoHandle imageHandle,	/* The photo image to write into. */
    int destX, int destY,	/* Coordinates of top-left pixel in photo
				 * image to be written to. */
    int width, int height,	/* Dimensions of block of photo image to be
				 * written to. */
    int srcX, int srcY)		/* Coordinates of top-left pixel to be used in
				 * image being read. */
{
    int fileWidth, fileHeight, maxIntensity;
    int nLines, nBytes, h, type, count, bytesPerChannel = 1;
    unsigned char *pixelPtr;
    Tk_PhotoImageBlock block;

    type = ReadPPMFileHeader(chan, &fileWidth, &fileHeight, &maxIntensity);
    if (type == 0) {
	Tcl_SetObjResult(interp, Tcl_ObjPrintf(
		"couldn't read raw PPM header from file \"%s\"", fileName));
	Tcl_SetErrorCode(interp, "TK", "IMAGE", "PPM", "NO_HEADER", NULL);
	return TCL_ERROR;
    }
    if ((fileWidth <= 0) || (fileHeight <= 0)) {
	Tcl_SetObjResult(interp, Tcl_ObjPrintf(
		"PPM image file \"%s\" has dimension(s) <= 0", fileName));
	Tcl_SetErrorCode(interp, "TK", "IMAGE", "PPM", "DIMENSIONS", NULL);
	return TCL_ERROR;
    }
    if ((maxIntensity <= 0) || (maxIntensity > 0xffff)) {
	Tcl_SetObjResult(interp, Tcl_ObjPrintf(
		"PPM image file \"%s\" has bad maximum intensity value %d",
		fileName, maxIntensity));
	Tcl_SetErrorCode(interp, "TK", "IMAGE", "PPM", "INTENSITY", NULL);
	return TCL_ERROR;
    } else if (maxIntensity > 0x00ff) {
	bytesPerChannel = 2;
    }

    if ((srcX + width) > fileWidth) {
	width = fileWidth - srcX;
    }
    if ((srcY + height) > fileHeight) {
	height = fileHeight - srcY;
    }
    if ((width <= 0) || (height <= 0)
	    || (srcX >= fileWidth) || (srcY >= fileHeight)) {
	return TCL_OK;
    }

    if (type == PGM) {
	block.pixelSize = 1 * bytesPerChannel;
	block.offset[0] = 0;
	block.offset[1] = 0;
	block.offset[2] = 0;
    } else {
	block.pixelSize = 3 * bytesPerChannel;
	block.offset[0] = 0;
	block.offset[1] = 1 * bytesPerChannel;
	block.offset[2] = 2 * bytesPerChannel;
    }
    block.offset[3] = 0;
    block.width = width;
    block.pitch = block.pixelSize * fileWidth;

    if (Tk_PhotoExpand(interp, imageHandle,
	    destX + width, destY + height) != TCL_OK) {
	return TCL_ERROR;
    }

    if (srcY > 0) {
	Tcl_Seek(chan, (Tcl_WideInt)(srcY * block.pitch), SEEK_CUR);
    }

    nLines = (MAX_MEMORY + block.pitch - 1) / block.pitch;
    if (nLines > height) {
	nLines = height;
    }
    if (nLines <= 0) {
	nLines = 1;
    }
    nBytes = nLines * block.pitch;
    pixelPtr = ckalloc(nBytes);
    block.pixelPtr = pixelPtr + srcX * block.pixelSize;

    for (h = height; h > 0; h -= nLines) {
	if (nLines > h) {
	    nLines = h;
	    nBytes = nLines * block.pitch;
	}
	count = Tcl_Read(chan, (char *) pixelPtr, nBytes);
	if (count != nBytes) {
	    Tcl_SetObjResult(interp, Tcl_ObjPrintf(
		    "error reading PPM image file \"%s\": %s", fileName,
		    Tcl_Eof(chan)?"not enough data":Tcl_PosixError(interp)));
	    if (Tcl_Eof(chan)) {
		Tcl_SetErrorCode(interp, "TK", "IMAGE", "PPM", "EOF", NULL);
	    }
	    ckfree(pixelPtr);
	    return TCL_ERROR;
	}
	if (maxIntensity < 0x00ff) {
	    unsigned char *p;

	    for (p = pixelPtr; count > 0; count--, p++) {
		*p = (((int) *p) * 255)/maxIntensity;
	    }
	} else if (maxIntensity > 0x00ff) {
	    unsigned char *p;
	    unsigned int value;

	    for (p = pixelPtr; count > 0; count--, p += 2) {
		value = ((unsigned int) p[0]) * 256 + ((unsigned int) p[1]);
		value = value * 255 / maxIntensity;
		p[0] = p[1] = (unsigned char) value;
	    }
	}
	block.height = nLines;
	if (Tk_PhotoPutBlock(interp, imageHandle, &block, destX, destY,
		width, nLines, TK_PHOTO_COMPOSITE_SET) != TCL_OK) {
	    ckfree(pixelPtr);
	    return TCL_ERROR;
	}
	destY += nLines;
    }

    ckfree(pixelPtr);
    return TCL_OK;
}

/*
 *----------------------------------------------------------------------
 *
 * FileWritePPM --
 *
 *	This function is invoked to write image data to a file in PPM format
 *	(although we can read PGM files, we never write them).
 *
 * Results:
 *	A standard TCL completion code. If TCL_ERROR is returned then an error
 *	message is left in the interp's result.
 *
 * Side effects:
 *	Data is written to the file given by "fileName".
 *
 *----------------------------------------------------------------------
 */

static int
FileWritePPM(
    Tcl_Interp *interp,
    const char *fileName,
    Tcl_Obj *format,
    Tk_PhotoImageBlock *blockPtr)
{
    Tcl_Channel chan;
    int w, h, greenOffset, blueOffset, nBytes;
    unsigned char *pixelPtr, *pixLinePtr;
    char header[16 + TCL_INTEGER_SPACE * 2];

    chan = Tcl_OpenFileChannel(interp, fileName, "w", 0666);
    if (chan == NULL) {
	return TCL_ERROR;
    }

    if (Tcl_SetChannelOption(interp, chan, "-translation", "binary")
	    != TCL_OK) {
	Tcl_Close(NULL, chan);
	return TCL_ERROR;
    }
    if (Tcl_SetChannelOption(interp, chan, "-encoding", "binary")
	    != TCL_OK) {
	Tcl_Close(NULL, chan);
	return TCL_ERROR;
    }

    sprintf(header, "P6\n%d %d\n255\n", blockPtr->width, blockPtr->height);
    Tcl_Write(chan, header, -1);

    pixLinePtr = blockPtr->pixelPtr + blockPtr->offset[0];
    greenOffset = blockPtr->offset[1] - blockPtr->offset[0];
    blueOffset = blockPtr->offset[2] - blockPtr->offset[0];

    if ((greenOffset == 1) && (blueOffset == 2) && (blockPtr->pixelSize == 3)
	    && (blockPtr->pitch == (blockPtr->width * 3))) {
	nBytes = blockPtr->height * blockPtr->pitch;
	if (Tcl_Write(chan, (char *) pixLinePtr, nBytes) != nBytes) {
	    goto writeerror;
	}
    } else {
	for (h = blockPtr->height; h > 0; h--) {
	    pixelPtr = pixLinePtr;
	    for (w = blockPtr->width; w > 0; w--) {
		if (    Tcl_Write(chan,(char *)&pixelPtr[0], 1) == -1 ||
			Tcl_Write(chan,(char *)&pixelPtr[greenOffset],1)==-1 ||
			Tcl_Write(chan,(char *)&pixelPtr[blueOffset],1) ==-1) {
		    goto writeerror;
		}
		pixelPtr += blockPtr->pixelSize;
	    }
	    pixLinePtr += blockPtr->pitch;
	}
    }

    if (Tcl_Close(NULL, chan) == 0) {
	return TCL_OK;
    }
    chan = NULL;

  writeerror:
    Tcl_SetObjResult(interp, Tcl_ObjPrintf("error writing \"%s\": %s",
	    fileName, Tcl_PosixError(interp)));
    if (chan != NULL) {
	Tcl_Close(NULL, chan);
    }
    return TCL_ERROR;
}

/*
 *----------------------------------------------------------------------
 *
 * StringWritePPM --
 *
 *	This function is invoked to write image data to a string in PPM
 *	format.
 *
 * Results:
 *	A standard TCL completion code. If TCL_ERROR is returned then an error
 *	message is left in the interp's result.
 *
 * Side effects:
 *	None.
 *
 *----------------------------------------------------------------------
 */

static int
StringWritePPM(
    Tcl_Interp *interp,
    Tcl_Obj *format,
    Tk_PhotoImageBlock *blockPtr)
{
    int w, h, size, greenOffset, blueOffset;
    unsigned char *pixLinePtr, *byteArray;
    char header[16 + TCL_INTEGER_SPACE * 2];
    Tcl_Obj *byteArrayObj;

    sprintf(header, "P6\n%d %d\n255\n", blockPtr->width, blockPtr->height);

    /*
     * Construct a byte array of the right size with the header and
     * get a pointer to the data part of it.
     */

    size = strlen(header);
    byteArrayObj = Tcl_NewByteArrayObj((unsigned char *)header, size);
    byteArray = Tcl_SetByteArrayLength(byteArrayObj,
	    size + 3*blockPtr->width*blockPtr->height);
    byteArray += size;

    pixLinePtr = blockPtr->pixelPtr + blockPtr->offset[0];
    greenOffset = blockPtr->offset[1] - blockPtr->offset[0];
    blueOffset = blockPtr->offset[2] - blockPtr->offset[0];

    /*
     * Check if we can do the data move in single action.
     */

    if ((greenOffset == 1) && (blueOffset == 2) && (blockPtr->pixelSize == 3)
	    && (blockPtr->pitch == (blockPtr->width * 3))) {
	memcpy(byteArray, pixLinePtr,
		(unsigned)blockPtr->height * blockPtr->pitch);
    } else {
	for (h = blockPtr->height; h > 0; h--) {
	    unsigned char *pixelPtr = pixLinePtr;

	    for (w = blockPtr->width; w > 0; w--) {
		*byteArray++ = pixelPtr[0];
		*byteArray++ = pixelPtr[greenOffset];
		*byteArray++ = pixelPtr[blueOffset];
		pixelPtr += blockPtr->pixelSize;
	    }
	    pixLinePtr += blockPtr->pitch;
	}
    }

    /*
     * Return the object in the interpreter result.
     */

    Tcl_SetObjResult(interp, byteArrayObj);
    return TCL_OK;
}

/*
 *----------------------------------------------------------------------
 *
 * StringMatchPPM --
 *
 *	This function is invoked by the photo image type to see if a string
 *	contains image data in PPM format.
 *
 * Results:
 *	The return value is >0 if the first characters in file "f" look like
 *	PPM data, and 0 otherwise.
 *
 * Side effects:
 *	The access position in f may change.
 *
 *----------------------------------------------------------------------
 */

static int
StringMatchPPM(
    Tcl_Obj *dataObj,		/* The image data. */
    Tcl_Obj *format,		/* User-specified format string, or NULL. */
    int *widthPtr, int *heightPtr,
				/* The dimensions of the image are returned
				 * here if the file is a valid raw PPM
				 * file. */
    Tcl_Interp *interp)		/* unused */
{
    int dummy;

    return ReadPPMStringHeader(dataObj, widthPtr, heightPtr,
	    &dummy, NULL, NULL);
}

/*
 *----------------------------------------------------------------------
 *
 * StringReadPPM --
 *
 *	This function is called by the photo image type to read PPM format
 *	data from a string and write it into a given photo image.
 *
 * Results:
 *	A standard TCL completion code. If TCL_ERROR is returned then an error
 *	message is left in the interp's result.
 *
 * Side effects:
 *	New data is added to the image given by imageHandle.
 *
 *----------------------------------------------------------------------
 */

static int
StringReadPPM(
    Tcl_Interp *interp,		/* Interpreter to use for reporting errors. */
    Tcl_Obj *dataObj,		/* The image data. */
    Tcl_Obj *format,		/* User-specified format string, or NULL. */
    Tk_PhotoHandle imageHandle,	/* The photo image to write into. */
    int destX, int destY,	/* Coordinates of top-left pixel in photo
				 * image to be written to. */
    int width, int height,	/* Dimensions of block of photo image to be
				 * written to. */
    int srcX, int srcY)		/* Coordinates of top-left pixel to be used in
				 * image being read. */
{
    int fileWidth, fileHeight, maxIntensity;
    int nLines, nBytes, h, type, count, dataSize, bytesPerChannel = 1;
    unsigned char *pixelPtr, *dataBuffer;
    Tk_PhotoImageBlock block;

    type = ReadPPMStringHeader(dataObj, &fileWidth, &fileHeight,
	    &maxIntensity, &dataBuffer, &dataSize);
    if (type == 0) {
	Tcl_SetObjResult(interp, Tcl_NewStringObj(
		"couldn't read raw PPM header from string", -1));
	Tcl_SetErrorCode(interp, "TK", "IMAGE", "PPM", "NO_HEADER", NULL);
	return TCL_ERROR;
    }
    if ((fileWidth <= 0) || (fileHeight <= 0)) {
	Tcl_SetObjResult(interp, Tcl_NewStringObj(
		"PPM image data has dimension(s) <= 0", -1));
	Tcl_SetErrorCode(interp, "TK", "IMAGE", "PPM", "DIMENSIONS", NULL);
	return TCL_ERROR;
    }
    if ((maxIntensity <= 0) || (maxIntensity > 0xffff)) {
	Tcl_SetObjResult(interp, Tcl_ObjPrintf(
		"PPM image data has bad maximum intensity value %d",
		maxIntensity));
	Tcl_SetErrorCode(interp, "TK", "IMAGE", "PPM", "INTENSITY", NULL);
	return TCL_ERROR;
    } else if (maxIntensity > 0x00ff) {
	bytesPerChannel = 2;
    }

    if ((srcX + width) > fileWidth) {
	width = fileWidth - srcX;
    }
    if ((srcY + height) > fileHeight) {
	height = fileHeight - srcY;
    }
    if ((width <= 0) || (height <= 0)
	|| (srcX >= fileWidth) || (srcY >= fileHeight)) {
	return TCL_OK;
    }

    if (type == PGM) {
	block.pixelSize = 1 * bytesPerChannel;
	block.offset[0] = 0;
	block.offset[1] = 0;
	block.offset[2] = 0;
    } else {
	block.pixelSize = 3 * bytesPerChannel;
	block.offset[0] = 0;
	block.offset[1] = 1 * bytesPerChannel;
	block.offset[2] = 2 * bytesPerChannel;
    }
    block.offset[3] = 0;
    block.width = width;
    block.pitch = block.pixelSize * fileWidth;

    if (srcY > 0) {
	dataBuffer += srcY * block.pitch;
	dataSize -= srcY * block.pitch;
    }

    if (maxIntensity == 0x00ff) {
	/*
	 * We have all the data in memory, so write everything in one go.
	 */

	if (block.pitch*height > dataSize) {
	    Tcl_SetObjResult(interp, Tcl_NewStringObj(
		    "truncated PPM data", -1));
	    Tcl_SetErrorCode(interp, "TK", "IMAGE", "PPM", "TRUNCATED", NULL);
	    return TCL_ERROR;
	}
	block.pixelPtr = dataBuffer + srcX * block.pixelSize;
	block.height = height;
	return Tk_PhotoPutBlock(interp, imageHandle, &block, destX, destY,
		width, height, TK_PHOTO_COMPOSITE_SET);
    }

    if (Tk_PhotoExpand(interp, imageHandle,
	    destX + width, destY + height) != TCL_OK) {
	return TCL_ERROR;
    }

    nLines = (MAX_MEMORY + block.pitch - 1) / block.pitch;
    if (nLines > height) {
	nLines = height;
    }
    if (nLines <= 0) {
	nLines = 1;
    }
    nBytes = nLines * block.pitch;
    pixelPtr = ckalloc(nBytes);
    block.pixelPtr = pixelPtr + srcX * block.pixelSize;

    for (h = height; h > 0; h -= nLines) {
	unsigned char *p;

	if (nLines > h) {
	    nLines = h;
	    nBytes = nLines * block.pitch;
	}
	if (dataSize < nBytes) {
	    ckfree(pixelPtr);
	    Tcl_SetObjResult(interp, Tcl_NewStringObj(
		    "truncated PPM data", -1));
	    Tcl_SetErrorCode(interp, "TK", "IMAGE", "PPM", "TRUNCATED", NULL);
	    return TCL_ERROR;
	}
	if (maxIntensity < 0x00ff) {
	    for (p=pixelPtr,count=nBytes ; count>0 ; count--,p++,dataBuffer++) {
		*p = (((int) *dataBuffer) * 255)/maxIntensity;
	    }
	} else {
	    unsigned char *p;
	    unsigned int value;

	    for (p = pixelPtr,count=nBytes; count > 1; count-=2, p += 2) {
		value = ((unsigned int) p[0]) * 256 + ((unsigned int) p[1]);
		value = value * 255 / maxIntensity;
		p[0] = p[1] = (unsigned char) value;
	    }
	}
	dataSize -= nBytes;
	block.height = nLines;
	if (Tk_PhotoPutBlock(interp, imageHandle, &block, destX, destY,
		width, nLines, TK_PHOTO_COMPOSITE_SET) != TCL_OK) {
	    ckfree(pixelPtr);
	    return TCL_ERROR;
	}
	destY += nLines;
    }

    ckfree(pixelPtr);
    return TCL_OK;
}

/*
 *----------------------------------------------------------------------
 *
 * ReadPPMFileHeader --
 *
 *	This function reads the PPM header from the beginning of a PPM file
 *	and returns information from the header.
 *
 * Results:
 *	The return value is PGM if file "f" appears to start with a valid PGM
 *	header, PPM if "f" appears to start with a valid PPM header, and 0
 *	otherwise. If the header is valid, then *widthPtr and *heightPtr are
 *	modified to hold the dimensions of the image and *maxIntensityPtr is
 *	modified to hold the value of a "fully on" intensity value.
 *
 * Side effects:
 *	The access position in f advances.
 *
 *----------------------------------------------------------------------
 */

static int
ReadPPMFileHeader(
    Tcl_Channel chan,		/* Image file to read the header from. */
    int *widthPtr, int *heightPtr,
				/* The dimensions of the image are returned
				 * here. */
    int *maxIntensityPtr)	/* The maximum intensity value for the image
				 * is stored here. */
{
#define BUFFER_SIZE 1000
    char buffer[BUFFER_SIZE], c;
    int i, numFields, type = 0;

    /*
     * Read 4 space-separated fields from the file, ignoring comments (any
     * line that starts with "#").
     */

    if (Tcl_Read(chan, &c, 1) != 1) {
	return 0;
    }
    i = 0;
    for (numFields = 0; numFields < 4; numFields++) {
	/*
	 * Skip comments and white space.
	 */

	while (1) {
	    while (isspace(UCHAR(c))) {
		if (Tcl_Read(chan, &c, 1) != 1) {
		    return 0;
		}
	    }
	    if (c != '#') {
		break;
	    }
	    do {
		if (Tcl_Read(chan, &c, 1) != 1) {
		    return 0;
		}
	    } while (c != '\n');
	}

	/*
	 * Read a field (everything up to the next white space).
	 */

	while (!isspace(UCHAR(c))) {
	    if (i < (BUFFER_SIZE-2)) {
		buffer[i] = c;
		i++;
	    }
	    if (Tcl_Read(chan, &c, 1) != 1) {
		goto done;
	    }
	}
	if (i < (BUFFER_SIZE-1)) {
	    buffer[i] = ' ';
	    i++;
	}
    }

  done:
    buffer[i] = 0;

    /*
     * Parse the fields, which are: id, width, height, maxIntensity.
     */

    if (strncmp(buffer, "P6 ", 3) == 0) {
	type = PPM;
    } else if (strncmp(buffer, "P5 ", 3) == 0) {
	type = PGM;
    } else {
	return 0;
    }
    if (sscanf(buffer+3, "%d %d %d", widthPtr, heightPtr, maxIntensityPtr)
	    != 3) {
	return 0;
    }
    return type;
}

/*
 *----------------------------------------------------------------------
 *
 * ReadPPMStringHeader --
 *
 *	This function reads the PPM header from the beginning of a PPM-format
 *	string and returns information from the header.
 *
 * Results:
 *	The return value is PGM if the string appears to start with a valid
 *	PGM header, PPM if the string appears to start with a valid PPM
 *	header, and 0 otherwise. If the header is valid, then *widthPtr and
 *	*heightPtr are modified to hold the dimensions of the image and
 *	*maxIntensityPtr is modified to hold the value of a "fully on"
 *	intensity value.
 *
 * Side effects:
 *	None
 *
 *----------------------------------------------------------------------
 */

static int
ReadPPMStringHeader(
    Tcl_Obj *dataPtr,		/* Object to read the header from. */
    int *widthPtr, int *heightPtr,
				/* The dimensions of the image are returned
				 * here. */
    int *maxIntensityPtr,	/* The maximum intensity value for the image
				 * is stored here. */
    unsigned char **dataBufferPtr,
    int *dataSizePtr)
{
#define BUFFER_SIZE 1000
    char buffer[BUFFER_SIZE], c;
    int i, numFields, dataSize, type = 0;
    unsigned char *dataBuffer;

    dataBuffer = Tcl_GetByteArrayFromObj(dataPtr, &dataSize);

    /*
     * Read 4 space-separated fields from the string, ignoring comments (any
     * line that starts with "#").
     */

    if (dataSize-- < 1) {
	return 0;
    }
    c = (char) (*dataBuffer++);
    i = 0;
    for (numFields = 0; numFields < 4; numFields++) {
	/*
	 * Skip comments and white space.
	 */

	while (1) {
	    while (isspace(UCHAR(c))) {
		if (dataSize-- < 1) {
		    return 0;
		}
		c = (char) (*dataBuffer++);
	    }
	    if (c != '#') {
		break;
	    }
	    do {
		if (dataSize-- < 1) {
		    return 0;
		}
		c = (char) (*dataBuffer++);
	    } while (c != '\n');
	}

	/*
	 * Read a field (everything up to the next white space).
	 */

	while (!isspace(UCHAR(c))) {
	    if (i < (BUFFER_SIZE-2)) {
		buffer[i] = c;
		i++;
	    }
	    if (dataSize-- < 1) {
		goto done;
	    }
	    c = (char) (*dataBuffer++);
	}
	if (i < (BUFFER_SIZE-1)) {
	    buffer[i] = ' ';
	    i++;
	}
    }

  done:
    buffer[i] = 0;

    /*
     * Parse the fields, which are: id, width, height, maxIntensity.
     */

    if (strncmp(buffer, "P6 ", 3) == 0) {
	type = PPM;
    } else if (strncmp(buffer, "P5 ", 3) == 0) {
	type = PGM;
    } else {
	return 0;
    }
    if (sscanf(buffer+3, "%d %d %d", widthPtr, heightPtr, maxIntensityPtr)
	    != 3) {
	return 0;
    }
    if (dataBufferPtr != NULL) {
	*dataBufferPtr = dataBuffer;
	*dataSizePtr = dataSize;
    }
    return type;
}

/*
 * Local Variables:
 * mode: c
 * c-basic-offset: 4
 * fill-column: 78
 * End:
 */