path: root/doc/FindPhoto.3

'\"
'\" Copyright (c) 1994 The Australian National University
'\" Copyright (c) 1994-1996 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.
'\"
.TH Tk_FindPhoto 3 8.0 Tk "Tk Library Procedures"
.so man.macros
.BS
.SH NAME
Tk_FindPhoto, Tk_PhotoPutBlock, Tk_PhotoPutZoomedBlock, Tk_PhotoGetImage, Tk_PhotoBlank, Tk_PhotoExpand, Tk_PhotoGetSize, Tk_PhotoSetSize \- manipulate the image data stored in a photo image.
.SH SYNOPSIS
.nf
\fB#include <tk.h>\fR
.sp
Tk_PhotoHandle
\fBTk_FindPhoto\fR(\fIinterp, imageName\fR)
.sp
int
\fBTk_PhotoPutBlock\fR(\fIinterp, handle, blockPtr, x, y, width, height,\
compRule\fR)
.sp
int
\fBTk_PhotoPutZoomedBlock\fR(\fIinterp, handle, blockPtr, x, y, width, height,\
zoomX, zoomY, subsampleX, subsampleY, compRule\fR)
.sp
int
\fBTk_PhotoGetImage\fR(\fIhandle, blockPtr\fR)
.sp
void
\fBTk_PhotoBlank\fR(\fIhandle\fR)
.sp
int
\fBTk_PhotoExpand\fR(\fIinterp, handle, width, height\fR)
.sp
void
\fBTk_PhotoGetSize\fR(\fIhandle, widthPtr, heightPtr\fR)
.sp
int
\fBTk_PhotoSetSize\fR(\fIinterp. handle, width, height\fR)
.SH ARGUMENTS
.AS Tk_PhotoImageBlock window_path
.AP Tcl_Interp *interp in
Interpreter in which image was created and in which error reporting is
to be done.
.AP "const char" *imageName in
Name of the photo image.
.AP Tk_PhotoHandle handle in
Opaque handle identifying the photo image to be affected.
.AP Tk_PhotoImageBlock *blockPtr in
Specifies the address and storage layout of image data.
.AP int x in
Specifies the X coordinate where the top-left corner of the block is
to be placed within the image.
.AP int y in
Specifies the Y coordinate where the top-left corner of the block is
to be placed within the image.
.AP int width in
Specifies the width of the image area to be affected (for
\fBTk_PhotoPutBlock\fR) or the desired image width (for
\fBTk_PhotoExpand\fR and \fBTk_PhotoSetSize\fR).
.AP int compRule in
Specifies the compositing rule used when combining transparent pixels
in a block of data with a photo image.  Must be one of
\fBTK_PHOTO_COMPOSITE_OVERLAY\fR (which puts the block of data over the top
of the existing photo image, with the previous contents showing
through in the transparent bits) or \fBTK_PHOTO_COMPOSITE_SET\fR (which
discards the existing photo image contents in the rectangle covered by
the data block.)
.AP int height in
Specifies the height of the image area to be affected (for
\fBTk_PhotoPutBlock\fR) or the desired image height (for
\fBTk_PhotoExpand\fR and \fBTk_PhotoSetSize\fR).
.AP int *widthPtr out
Pointer to location in which to store the image width.
.AP int *heightPtr out
Pointer to location in which to store the image height.
.AP int subsampleX in
Specifies the subsampling factor in the X direction for input
image data.
.AP int subsampleY in
Specifies the subsampling factor in the Y direction for input
image data.
.AP int zoomX in
Specifies the zoom factor to be applied in the X direction to pixels
being written to the photo image.
.AP int zoomY in
Specifies the zoom factor to be applied in the Y direction to pixels
being written to the photo image.
.BE
.SH DESCRIPTION
.PP
\fBTk_FindPhoto\fR returns an opaque handle that is used to identify a
particular photo image to the other procedures.  The parameter is the
name of the image, that is, the name specified to the \fBimage create
photo\fR command, or assigned by that command if no name was specified.
If \fIimageName\fR does not exist or is not a photo image, 
\fBTk_FindPhoto\fR returns NULL. 
.PP
\fBTk_PhotoPutBlock\fR is used to supply blocks of image data to be
displayed.  The call affects an area of the image of size
\fIwidth\fR x \fIheight\fR pixels, with its top-left corner at
coordinates (\fIx\fR,\fIy\fR).  All of \fIwidth\fR, \fIheight\fR,
\fIx\fR, and \fIy\fR must be non-negative.
If part of this area lies outside the
current bounds of the image, the image will be expanded to include the
area, unless the user has specified an explicit image size with the
\fB\-width\fR and/or \fB\-height\fR widget configuration options
(see photo(n)); in that
case the area is silently clipped to the image boundaries.
.PP
The \fIblock\fR parameter is a pointer to a
\fBTk_PhotoImageBlock\fR structure, defined as follows:
.CS
typedef struct {
    unsigned char *\fIpixelPtr\fR;
    int \fIwidth\fR;
    int \fIheight\fR;
    int \fIpitch\fR;
    int \fIpixelSize\fR;
    int \fIoffset\fR[4];
} \fBTk_PhotoImageBlock\fR;
.CE
The \fIpixelPtr\fR field points to the first pixel, that is, the
top-left pixel in the block.
The \fIwidth\fR and \fIheight\fR fields specify the dimensions of the
block of pixels.  The \fIpixelSize\fR field specifies the address
difference between two horizontally adjacent pixels.  It should be 4 for
RGB and 2 for grayscale image data.  Other values are possible, if the
offsets in the \fIoffset\fR array are adjusted accordingly (e.g. for
red, green and blue data stored in different planes).  Using such a
layout is strongly discouraged, though. Due to a bug, it might not work
correctly if an alpha channel is provided. (see the \fBBUGS\fR section
below). The \fIpitch\fR field specifies the
address difference between two vertically adjacent pixels.  The
\fIoffset\fR array contains the offsets from the address of a pixel
to the addresses of the bytes containing the red, green, blue and alpha
(transparency) components.  If the offsets for red, green and blue are
equal, the image is interpreted as grayscale. If they differ, RGB data
is assumed. Normally the offsets will be 0, 1, 2, 3 for RGB data
and 0, 0, 0, 1 for grayscale.  It is possible to provide image data
without an alpha channel by setting the offset for alpha to a negative
value and adjusting the \fIpixelSize\fR field accordingly. This use is
discouraged, though (see the \fBBUGS\fR section below).
.PP
The \fIcompRule\fR parameter to \fBTk_PhotoPutBlock\fR specifies a
compositing rule that says what to do with transparent pixels.  The
value \fBTK_PHOTO_COMPOSITE_OVERLAY\fR says that the previous contents of
the photo image should show through, and the value
\fBTK_PHOTO_COMPOSITE_SET\fR says that the previous contents of the photo
image should be completely ignored, and the values from the block be
copied directly across.  The behavior in Tk8.3 and earlier was
equivalent to having \fBTK_PHOTO_COMPOSITE_OVERLAY\fR as a compositing rule.
.PP
The value given for the \fIwidth\fR and \fIheight\fR parameters to
\fBTk_PhotoPutBlock\fR do not have to correspond to the values specified
in \fIblock\fR.  If they are smaller, \fBTk_PhotoPutBlock\fR extracts a
sub-block from the image data supplied.  If they are larger, the data
given are replicated (in a tiled fashion) to fill the specified area.
These rules operate independently in the horizontal and vertical
directions.
.PP
\fBTk_PhotoPutBlock\fR normally returns \fBTCL_OK\fR, though if it cannot
allocate sufficient memory to hold the resulting image, \fBTCL_ERROR\fR is
returned instead and, if the \fIinterp\fR argument is non-NULL, an
error message is placed in the interpreter's result.
.PP
\fBTk_PhotoPutZoomedBlock\fR works like \fBTk_PhotoPutBlock\fR except that
the image can be reduced or enlarged for display.  The
\fIsubsampleX\fR and \fIsubsampleY\fR parameters allow the size of the
image to be reduced by subsampling.
\fBTk_PhotoPutZoomedBlock\fR will use only pixels from the input image
whose X coordinates are multiples of \fIsubsampleX\fR, and whose Y
coordinates are multiples of \fIsubsampleY\fR.  For example, an image
of 512x512 pixels can be reduced to 256x256 by setting
\fIsubsampleX\fR and \fIsubsampleY\fR to 2.
.PP
The \fIzoomX\fR and \fIzoomY\fR parameters allow the image to be
enlarged by pixel replication.  Each pixel of the (possibly subsampled)
input image will be written to a block \fIzoomX\fR pixels wide and
\fIzoomY\fR pixels high of the displayed image.  Subsampling and
zooming can be used together for special effects.
.PP
\fBTk_PhotoGetImage\fR can be used to retrieve image data from a photo
image.  \fBTk_PhotoGetImage\fR fills
in the structure pointed to by the \fIblockPtr\fR parameter with values
that describe the address and layout of the image data that the
photo image has stored internally.  The values are valid
until the image is destroyed or its size is changed.
.PP
It is possible to modify an image by writing directly to the data 
the \fIpixelPtr\fR field points to. The size of the image cannot be
changed this way, though.
Also, changes made by writing directly to \fIpixelPtr\fR will not be 
immediately visible, but only after a call to 
\fBTk_ImageChanged\fR or after an event that causes the interested 
widgets to redraw themselves.
For these reasons usually it is preferable to make changes to 
a copy of the image data and write it back with 
\fBTk_PhotoPutBlock\fR or \fBTk_PhotoPutZoomedBlock\fR. 
.PP
\fBTk_PhotoGetImage\fR returns 1 for compatibility with the
corresponding procedure in the old photo widget.
.PP
\fBTk_PhotoBlank\fR blanks the entire area of the
photo image.  Blank areas of a photo image are transparent.
.PP
\fBTk_PhotoExpand\fR requests that the widget's image be expanded to be
at least \fIwidth\fR x \fIheight\fR pixels in size.  The width and/or
height are unchanged if the user has specified an explicit image width
or height with the \fB\-width\fR and/or \fB\-height\fR configuration
options, respectively.
If the image data
are being supplied in many small blocks, it is more efficient to use
\fBTk_PhotoExpand\fR or \fBTk_PhotoSetSize\fR at the beginning rather than
allowing the image to expand in many small increments as image blocks
are supplied.
.PP
\fBTk_PhotoExpand\fR normally returns \fBTCL_OK\fR, though if it cannot
allocate sufficient memory to hold the resulting image, \fBTCL_ERROR\fR is
returned instead and, if the \fIinterp\fR argument is non-NULL, an
error message is placed in the interpreter's result.
.PP
\fBTk_PhotoSetSize\fR specifies the size of the image, as if the user
had specified the given \fIwidth\fR and \fIheight\fR values to the
\fB\-width\fR and \fB\-height\fR configuration options.  A value of
zero for \fIwidth\fR or \fIheight\fR does not change the image's width
or height, but allows the width or height to be changed by subsequent
calls to \fBTk_PhotoPutBlock\fR, \fBTk_PhotoPutZoomedBlock\fR or
\fBTk_PhotoExpand\fR.
.PP
\fBTk_PhotoSetSize\fR normally returns \fBTCL_OK\fR, though if it cannot
allocate sufficient memory to hold the resulting image, \fBTCL_ERROR\fR is
returned instead and, if the \fIinterp\fR argument is non-NULL, an
error message is placed in the interpreter's result.
.PP
\fBTk_PhotoGetSize\fR returns the dimensions of the image in
*\fIwidthPtr\fR and *\fIheightPtr\fR.
.SH PORTABILITY
.PP
In Tk 8.3 and earlier, \fBTk_PhotoPutBlock\fR and
\fBTk_PhotoPutZoomedBlock\fR had different signatures. If you want to
compile code that uses the old interface against 8.4 without updating
your code, compile it with the flag
-DUSE_COMPOSITELESS_PHOTO_PUT_BLOCK.  Code linked using Stubs against
older versions of Tk will continue to work.
.PP
In Tk 8.4, \fBTk_PhotoPutBlock\fR, \fBTk_PhotoPutZoomedBlock\fR,
\fBTk_PhotoExpand\fR and \fBTk_PhotoSetSize\fR did not take an
\fIinterp\fR argument or return any result code.  If insufficient
memory was available for an image, Tk would panic.  This behaviour is
still supported if you compile your extension with the additional flag
-DUSE_PANIC_ON_PHOTO_ALLOC_FAILURE.  Code linked using Stubs against
older versions of Tk will continue to work.
.SH BUGS
The \fBTk_PhotoImageBlock\fR structure used to provide image data to
\fBTk_PhotoPutBlock\fR promises great flexibility in the layout of the
data (e.g. separate planes for the red, green, blue and alpha
channels).  Unfortunately, the implementation fails to hold this
promise.  The problem is that the \fIpixelSize\fR field is
(incorrectly) used to determine wehter the image has an alpha channel.
Currently, if the offset for the alpha channel is greater or equal than
\fIpixelSize\fR, \fBtk_PhotoPutblock\fR assumes no alpha data is
present and makes the image fully opaque.  This means that for layouts
where the channels are separate (or any other exotic layout where
\fIpixelSize\fR has to be smaller than the alpha offset), the alpha
channel will not be read correctly.  In order to be on the safe side
if this issue will be corrected in a future release, it is strongly
recommended you always provide alpha data - even if the image has no
transparency - and only use the "standard" layout with a
\fIpixelSize\fR of 2 for grayscale and 4 for RGB data with
\fIoffset\fRs of 0, 0, 0, 1 or 0, 1, 2, 3 respectively.
.SH CREDITS
.PP
The code for the photo image type was developed by Paul Mackerras,
based on his earlier photo widget code.
.SH KEYWORDS
photo, image