TkPNG - PNG Photo Image extension for Tcl/Tk

Copyright (c) 2005 Michael Kirkham <mikek@muonics.com> & Muonics

See the file "license.terms" for information on usage and redistribution
of this file, and for a DISCLAIMER OF ALL WARRANTIES.

This package implements support for loading and using PNG images with
Tcl/Tk.  Although other extensions such as Img also add support for PNG
images, I wanted something that was lightweight, did not depend on libpng,
and which would be suitable for inclusion in the Tk core, as Tk does not
currently support any image formats natively that take advantage of its
internal support for alpha blending, and alpha antialiasing and drop shadows
really go a long way toward beautifying Tk applications.

At this time, the package supports reading images from files or
binary data.  Base64 decoding is not supported yet*, partly because I'm
of the opinion this should be handled by the Tk core photo code which
calls the image handlers.  Exporting images to PNG format is also not
supported yet.

* For now, if you base64 encode your images for inclusion in Tcl scripts,
you will need to base64 decode the image data in order to generate the
images.  e.g.:

	package require base64

    image create photo [base64::decode {
    iVBORw0KGgoAAAANSUhEUgAAAAEAAAABAQMAAAAl21bKAAAABGdBTUEAAYag
    MeiWXwAAAANzQklUBAQEd/i1owAAAANQTFRFAAD/injSVwAAAApJREFUeJxj
    YAAAAAIAAUivpHEAAAAASUVORK5CYII=
    }]

The package supports the full range of color types, channels and bit
depths from 1 bit black & white to 16 bit per channel full color
with alpha (64 bit RGBA) and interlacing.  Ancillary "chunks" such
as gamma, color profile, and text fields are ignored, although they
are checked at a minimum for correct CRC.

This extension is provided under the Tcl license (see the file
"license.terms" for details), though acknowledgements and/or
donations are of course accepted and appreciated. :)

Special thanks to Willem van Schaik's suite for his suite of PNG test
images, which are available from:

http://www.schaik.com/pngsuite/pngsuite.html