Tk_FindPhoto(3) Tk Library Procedures Tk_FindPhoto(3)

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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.
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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(n)); 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

Tk 8.0 Tk_FindPhoto(3)