Reduces number of distinct color/alpha intensities in the image. Unlike typical posterization, which distributes levels evenly, this one tries to pick levels intelligently using varaince-based Median Cut and Voronoi iteration.

The goal of this tool is to make RGB/RGBA PNG images more compressible, assuming that lower number of unique byte values increses chance of finding repetition and improves efficiency of Huffman coding.

With -b argument it reduces PNG file sizes by selectively blurring the image. It makes image more compressible by making it more predictable for PNG's diagonal average prediction filter.

posterize [ -v ] [ -b ] [ -d ] [ -Q <quality> ] [ levels ] -- [ input.png ] [ output.png ]

• -b — Use blurring instead of posterization (recommended). Without this argument posterization is used.
• -Q num — Picks minimum number of levels needed to achieve given quality. num is quality 0-100 (100 is best, similar to JPEG). Number of levels is optional if quality is specified.
• levels — Number of levels to use (2-255). Lower number gives worse quality, but smaller file.
• -d — Enables simple ordered dithering in posterization mode.
• -v — Verbose output. Prints mean square error (MSE) caused by posterization.

If input/output files are not specified then stdin/stdout is used respectively.

Posterized images can be further compressed using PNGOUT or similar. Try ImageOptim.

Integrated in ImageAlpha.app.

© 2011-2014 Kornel Lesiński.

This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

© 1997-2002 by Greg Roelofs. © 2009-2014 by Kornel Lesiński.

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