Provided by: tk8.6-doc_8.6.14-1build1_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 than or equal to 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