TIP #244: PNG Photo Image Support for Tk
Michael Kirkham
TIP #244: PNG PHOTO IMAGE SUPPORT FOR TK
==========================================
Version: $Revision: 1.1 $
Author: Michael Kirkham <mikek_at_muonics.com>
State: Draft
Type: Project
Tcl-Version: 8.5
Vote: Pending
Created: Tuesday, 22 March 2005
URL: http://purl.org/tcl/tip/244.html
WebEdit: http://purl.org/tcl/tip/edit/244
Discussions-To: news:comp.lang.tcl
Post-History:
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ABSTRACT
==========
Tk's photo image type has had support for alpha channels internally
since version 8.3. However, to date there is no photo format in the
core that exposes this functionality. This TIP proposes adding support
for the PNG (Portable Network Graphics) format.
RATIONALE
===========
There is a current push for "beautifying" Tcl/Tk with more modern
appearing widgets and default configuration options. One method that
can provide significant improvement in appearance is the use of images
with alpha channels for antialiasing and smooth blending into the
background of the GUI. While the Tk core supports alpha channels, this
support is neither exposed through the Tk_GetColor() format used by
*image put* and similar interfaces, nor by the only photo image format,
GIF, which allows only fully-opaque or fully-transparent pixels.
Instead, a third party package is currently required to add a format
supporting alpha channels, which may not have a suitable license or
increase the number of external dependencies unreasonably.
The PNG image format is an open standard supporting a wide range of
colors, from 1 bit per pixel black and white to 16 bit per pixel RGBA,
with loss-less compression. It's becoming increasingly widely used as
the image format of choice in many areas including GUI and web page
design for its color range, alpha support and small image size. As such
it is the obvious choice for adding alpha support to Tk. An
implementation can be light-weight and not add any new dependencies
(such as on libpng) to Tk: while the format also depends on the zlib
format for compression, [TIP #234] proposes zlib support for the Tcl
core.
SPECIFICATION
===============
The implementation shall use the /Tk_CreatePhotoImageFormat()/
mechanisms to supply handlers to Tk for PNG support. The implementation
must:
1. Support the full range of color types and bit depths allowed and
required by the PNG standard, including alpha channels and
boolean transparency.
2. Support reading of interlaced images.
3. Support base64 encoded data through *image create photo -data*
4. Otherwise conform to the requirements of the PNG specification,
such as the handling of unsupported ancillary and critical chunks
and support for all required filter types.
In addition, the implementation should:
5. Support exporting to the PNG format through *image data*.
6. Allow modification of the overall alpha transparency of imported
images through *-format "png -alpha value"*
7. Allow application of display gamma (if known) in conjunction with
the "gAMA" chunk of imported images, if present, through *-format
"png -gamma value"*.
REFERENCE IMPLEMENTATION
==========================
A reference implementation is
available[<URL:http://www.muonics.com/FreeStuff/TkPNG/>]. This
reference implementation has been done as a loadable extension, but
requires only the file /tkImgPNG.c/ and the
/Tk_CreatePhototoImageFormat()/ invocation from /tkImgPNGInit.c/ to be
added to /CreateTopLevelWindow()/ in /tkWindow.c/ to be built into the
core.
The only new dependency for this implementation is on zlib. It
currently meets requirements 1 - 4 (the "musts"); it does not yet
implement 5 - 6 (the "shoulds").
NOTES
=======
1. It is understood that there will likely be some reformatting
required to conform to the Tcl Style Guide if this TIP is
accepted.
2. Performance is currently comparable to Img for small (i.e.,
toolbar button) sized images - a little faster or slower
depending on the image. It seems to be somewhat slower for larger
images, but there may be room for additional optimizations.
3. At this time there is a bug (1155596) in Tk Aqua bevel buttons
that will crash Tk if images with partially transparent pixels
are used. This is not the fault of the extension, but occurs also
with Img. The author is attempting to find a fix, however.
4. The implementation has been tested with PngSuite, which is fairly
exhaustive, but more tests would probably be useful.
COPYRIGHT
===========
This document has been placed in the public domain.
REFERENCES
============
PngSuite [<URL:http://www.schaik.com/pngsuite/pngsuite.html>]
Portable Network Graphics (PNG) Specification and Extensions
[<URL:http://www.libpng.org/pub/png/spec/>]
zlib Compression Library [<URL:http://www.gzip.org/zlib/>]
zlib Specifications [<URL:http://www.gzip.org/zlib/zlib_docs.html>]
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