Tk_FindPhoto(3tk) Tk Library Procedures Tk_FindPhoto(3tk)
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NAME
Tk_FindPhoto, Tk_PhotoPutBlock, Tk_PhotoPutZoomedBlock, Tk_PhotoGetIm-
age, 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 af-
fected.
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 exist-
ing photo image contents
in the rectangle covered
by the data block.)
int height (in) Specifies the height of
the image area to be af-
fected (for Tk_PhotoPut-
Block) or the desired im-
age height (for Tk_Photo-
Expand and Tk_PhotoSet-
Size).
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 di-
rection to pixels being
written to the photo im-
age.
int zoomY (in) Specifies the zoom factor
to be applied in the Y di-
rection to pixels being
written to the photo im-
age.
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DESCRIPTION
Tk_FindPhoto returns an opaque handle that is used to identify a par-
ticular 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 ima-
geName 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 dis-
played. 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 out-
side the current bounds of the image, the image will be expanded to in-
clude 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, de-
fined 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 dif-
ference 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 cor-
rectly 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 ad-
dress 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 pro-
vide image data without an alpha channel by setting the offset for al-
pha 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_COM-
POSITE_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 inde-
pendently 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 im-
age can be reduced or enlarged for display. The subsampleX and subsam-
pleY parameters allow the size of the image to be reduced by subsam-
pling. Tk_PhotoPutZoomedBlock will use only pixels from the input im-
age whose X coordinates are multiples of subsampleX, and whose Y coor-
dinates 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 spe-
cial 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 pa-
rameter 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_PhotoPut-
ZoomedBlock.
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 un-
changed 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 ef-
ficient 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_PhotoPutZoomed-
Block 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 in-
terface 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_Pho-
toPutBlock promises great flexibility in the layout of the data (e.g.
separate planes for the red, green, blue and alpha channels). Unfortu-
nately, 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 pixel-
Size 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
Tk 8.0 Tk_FindPhoto(3tk)
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