Writing PNG with pure Python

[Johann C. Rocholl]

Just in case anybody has the same problem, here's my first attempt at
implementing a subset of the PNG spec in pure Python. I license it to
you under the terms of the GNU GPL.

http://trac.browsershots.org/browser/trunk/shotfactory/lib/image/png.py

It encodes RGB images with 24 bits per pixel into PNG, using only the
modules sys, zlib and struct. These are all included in the base
distribution of Python. You don't need gd or imlib.

I have done a little testing, and my implementation processes 8 megs of
RGB input in 0.6 seconds. With Adam7 interlacing enabled, it takes 10
times longer.

I would really appreciate any feedback and suggestions for improvement.

Cheers,
Johann