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pngquant version 0.95 (DARNED CLOSE TO FINAL BETA) of 29 December 2000
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pngquant is a simple tool with one purpose:  convert 32-bit RGBA PNGs into
8-bit RGBA-palette PNGs (or fewer than 8 bits, if you want), via quantization
and either ordered or diffusion (Floyd-Steinberg) dithering.  The fact that
you can also use it on RGB or even palette images (e.g., to further color-
reduce them to 16 colors or whatever) is just a nice little bonus.

The current version is fully functional in the sense that it can do:

 - nice reduction of all PNG image types to 256-color (or smaller) palette
 - automatic optimization of tRNS chunks
 - batch conversion of multiple files (e.g., "pngquant 256 *.png")
 - Unix-style command-line filtering (e.g., "... | pngquant 16 | ...")

...on at least Unix and Win32.  It should be pretty easy to port to almost
any other platform with a command-line interface and unstructured (stream-
oriented) files, including VMS but probably not VM or MVS.

It does still lack a few features:

 - no ancillary chunk preservation (except gAMA)
 - no preservation of significant-bits info after rescaling (sBIT chunk)
 - no mapfile support
 - no "native" handling of 16-bit-per-sample files or gray+alpha files
    (i.e., all samples are truncated to 8 bits and all images are promoted
    to RGBA before quantization)

These issues will be addressed in a post-1.0 release.

By the way, be sure to check "before" and "after" file sizes, preferably with
pngcrush (http://pmt.sourceforge.net/pngcrush/); dithered palette images may
be four times smaller to begin with, but they don't compress nearly as well
as grayscale and truecolor images.  Some images, such as Henri Sivonen's alpha
button (http://www.pp.htv.fi/hsivone1/css-test/bitmapstyle.html), can be made
smaller as full 32-bit RGBA images (4076 bytes in this case) than as either
FS-dithered palette (4550 bytes) or ordered-dither palette (4482 bytes) images.

See http://www.libpng.org/pub/png/apps/pngquant.html for updates and links
to compiled binaries for various platforms.

Greg Roelofs
newt@pobox.com



Copyright and license info:
--------------------------

The quantization and dithering code is lifted from Jef Poskanzer's ppmquant,
part of his wonderful PBMPLUS tool suite.  I hacked it into a slightly cheesy
"pamquant" back in 1997 (http://pobox.com/~newt/greg_rgba.html) and finally
ripped out the cheesy file-I/O parts and replaced them with nice PNG code in
December 2000.  The PNG reading and writing code is a merged and slightly
simplified version of readpng, readpng2, and writepng from my book, "PNG:
The Definitive Guide."  pngquant therefore inherits both licenses, one for
each source file.  (Note that both licenses are basically BSD-like; that is,
use the code however you like, as long as you acknowledge its origins.)

pngquant.c:

   Copyright (C) 1989, 1991 by Jef Poskanzer.
   Copyright (C) 1997, 2000 by Greg Roelofs; based on an idea by
                            Stefan Schneider.
  
   Permission to use, copy, modify, and distribute this software and its
   documentation for any purpose and without fee is hereby granted, provided
   that the above copyright notice appear in all copies and that both that
   copyright notice and this permission notice appear in supporting
   documentation.  This software is provided "as is" without express or
   implied warranty.

rwpng.c (and rwpng.h):

   Copyright (c) 1998-2000 Greg Roelofs.  All rights reserved.

   This software is provided "as is," without warranty of any kind,
   express or implied.  In no event shall the author or contributors
   be held liable for any damages arising in any way from the use of
   this software.

   Permission is granted to anyone to use this software for any purpose,
   including commercial applications, and to alter it and redistribute
   it freely, subject to the following restrictions:

   1. Redistributions of source code must retain the above copyright
      notice, disclaimer, and this list of conditions.
   2. Redistributions in binary form must reproduce the above copyright
      notice, disclaimer, and this list of conditions in the documenta-
      tion and/or other materials provided with the distribution.
   3. All advertising materials mentioning features or use of this
      software must display the following acknowledgment:

         This product includes software developed by Greg Roelofs
         and contributors for the book, "PNG: The Definitive Guide,"
         published by O'Reilly and Associates.
