Provided by: tk8.6-doc_8.6.13-2_all 
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.
SYNOPSIS
#include <tk.h>
Tk_PhotoHandle
Tk_FindPhoto(interp, imageName)
int
Tk_PhotoPutBlock(interp, handle, blockPtr, x, y, width, height,compRule)
int
Tk_PhotoPutZoomedBlock(interp, handle, blockPtr, x, y, width, height,zoomX, zoomY, subsampleX, subsampleY, compRule)
int
Tk_PhotoGetImage(handle, blockPtr)
void
Tk_PhotoBlank(handle)
int
Tk_PhotoExpand(interp, handle, width, height)
void
Tk_PhotoGetSize(handle, widthPtr, heightPtr)
int
Tk_PhotoSetSize(interp. handle, width, height)
ARGUMENTS
Tcl_Interp *interp (in) Interpreter in which image was created and in
which error reporting is to be done.
const char *imageName (in) Name of the photo image.
Tk_PhotoHandle handle (in) Opaque handle identifying the photo image to
be affected.
Tk_PhotoImageBlock *blockPtr (in) Specifies the address and storage layout of
image data.
int x (in) Specifies the X coordinate where the top-left
corner of the block is to be placed within
the image.
int y (in) Specifies the Y coordinate where the top-left
corner of the block is to be placed within
the image.
int width (in) Specifies the width of the image area to be
affected (for Tk_PhotoPutBlock) or the
desired image width (for Tk_PhotoExpand and
Tk_PhotoSetSize).
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
TK_PHOTO_COMPOSITE_OVERLAY (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
TK_PHOTO_COMPOSITE_SET (which discards the
existing photo image contents in the
rectangle covered by the data block.)
int height (in) Specifies the height of the image area to be
affected (for Tk_PhotoPutBlock) or the
desired image height (for Tk_PhotoExpand and
Tk_PhotoSetSize).
int *widthPtr (out) Pointer to location in which to store the
image width.
int *heightPtr (out) Pointer to location in which to store the
image height.
int subsampleX (in) Specifies the subsampling factor in the X
direction for input image data.
int subsampleY (in) Specifies the subsampling factor in the Y
direction for input image data.
int zoomX (in) Specifies the zoom factor to be applied in
the X direction to pixels being written to
the photo image.
int zoomY (in) Specifies the zoom factor to be applied in
the Y direction to pixels being written to
the photo image.
_________________________________________________________________________________________________
DESCRIPTION
Tk_FindPhoto 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 image create photo command, or assigned by that command if no name was specified.
If imageName does not exist or is not a photo image, Tk_FindPhoto returns NULL.
Tk_PhotoPutBlock is used to supply blocks of image data to be displayed. The call affects
an area of the image of size width x height pixels, with its top-left corner at
coordinates (x,y). All of width, height, x, and y 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 -width and/or
-height widget configuration options (see photo(3tk)); in that case the area is silently
clipped to the image boundaries.
The block parameter is a pointer to a Tk_PhotoImageBlock structure, defined as follows:
typedef struct {
unsigned char *pixelPtr;
int width;
int height;
int pitch;
int pixelSize;
int offset[4];
} Tk_PhotoImageBlock;
The pixelPtr field points to the first pixel, that is, the top-left pixel in the block.
The width and height fields specify the dimensions of the block of pixels. The pixelSize
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 offset 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 BUGS section
below). The pitch field specifies the address difference between two vertically adjacent
pixels. The offset 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
pixelSize field accordingly. This use is discouraged, though (see the BUGS section below).
The compRule parameter to Tk_PhotoPutBlock specifies a compositing rule that says what to
do with transparent pixels. The value TK_PHOTO_COMPOSITE_OVERLAY says that the previous
contents of the photo image should show through, and the value TK_PHOTO_COMPOSITE_SET 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 TK_PHOTO_COMPOSITE_OVERLAY as a compositing rule.
The value given for the width and height parameters to Tk_PhotoPutBlock do not have to
correspond to the values specified in block. If they are smaller, Tk_PhotoPutBlock
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.
Tk_PhotoPutBlock normally returns TCL_OK, though if it cannot allocate sufficient memory
to hold the resulting image, TCL_ERROR is returned instead and, if the interp argument is
non-NULL, an error message is placed in the interpreter's result.
Tk_PhotoPutZoomedBlock works like Tk_PhotoPutBlock except that the image can be reduced or
enlarged for display. The subsampleX and subsampleY parameters allow the size of the
image to be reduced by subsampling. Tk_PhotoPutZoomedBlock will use only pixels from the
input image whose X coordinates are multiples of subsampleX, and whose Y coordinates are
multiples of subsampleY. For example, an image of 512x512 pixels can be reduced to
256x256 by setting subsampleX and subsampleY to 2.
The zoomX and zoomY parameters allow the image to be enlarged by pixel replication. Each
pixel of the (possibly subsampled) input image will be written to a block zoomX pixels
wide and zoomY pixels high of the displayed image. Subsampling and zooming can be used
together for special effects.
Tk_PhotoGetImage can be used to retrieve image data from a photo image. Tk_PhotoGetImage
fills in the structure pointed to by the blockPtr 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.
It is possible to modify an image by writing directly to the data the pixelPtr field
points to. The size of the image cannot be changed this way, though. Also, changes made
by writing directly to pixelPtr will not be immediately visible, but only after a call to
Tk_ImageChanged 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 Tk_PhotoPutBlock or Tk_PhotoPutZoomedBlock.
Tk_PhotoGetImage returns 1 for compatibility with the corresponding procedure in the old
photo widget.
Tk_PhotoBlank blanks the entire area of the photo image. Blank areas of a photo image are
transparent.
Tk_PhotoExpand requests that the widget's image be expanded to be at least width x height
pixels in size. The width and/or height are unchanged if the user has specified an
explicit image width or height with the -width and/or -height configuration options,
respectively. If the image data are being supplied in many small blocks, it is more
efficient to use Tk_PhotoExpand or Tk_PhotoSetSize at the beginning rather than allowing
the image to expand in many small increments as image blocks are supplied.
Tk_PhotoExpand normally returns TCL_OK, though if it cannot allocate sufficient memory to
hold the resulting image, TCL_ERROR is returned instead and, if the interp argument is
non-NULL, an error message is placed in the interpreter's result.
Tk_PhotoSetSize specifies the size of the image, as if the user had specified the given
width and height values to the -width and -height configuration options. A value of zero
for width or height does not change the image's width or height, but allows the width or
height to be changed by subsequent calls to Tk_PhotoPutBlock, Tk_PhotoPutZoomedBlock or
Tk_PhotoExpand.
Tk_PhotoSetSize normally returns TCL_OK, though if it cannot allocate sufficient memory to
hold the resulting image, TCL_ERROR is returned instead and, if the interp argument is
non-NULL, an error message is placed in the interpreter's result.
Tk_PhotoGetSize returns the dimensions of the image in *widthPtr and *heightPtr.
PORTABILITY
In Tk 8.3 and earlier, Tk_PhotoPutBlock and Tk_PhotoPutZoomedBlock 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.
In Tk 8.4, Tk_PhotoPutBlock, Tk_PhotoPutZoomedBlock, Tk_PhotoExpand and Tk_PhotoSetSize
did not take an interp 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.
BUGS
The Tk_PhotoImageBlock structure used to provide image data to Tk_PhotoPutBlock 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 pixelSize field is (incorrectly) used to determine whether the image
has an alpha channel. Currently, if the offset for the alpha channel is greater or equal
than pixelSize, tk_PhotoPutblock 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 pixelSize 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
pixelSize of 2 for grayscale and 4 for RGB data with offsets of 0, 0, 0, 1 or 0, 1, 2, 3
respectively.
CREDITS
The code for the photo image type was developed by Paul Mackerras, based on his earlier
photo widget code.
KEYWORDS
photo, image