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+ZopfliPNG is a command line program to optimize the Portable Network Graphics
+(PNG) images. This version has the following features:
+- uses Zopfli compression for the Deflate compression,
+- compares several strategies for choosing scanline filter codes,
+- chooses a suitable color type to losslessly encode the image,
+- removes all chunks that are unimportant for the typical web use (metadata,
+  text, etc...),
+- optionally alters the hidden colors of fully transparent pixels for more
+  compression, and,
+- optionally converts 16-bit color channels to 8-bit.
+
+This is an alpha-release for testing while improvements, particularly to add
+palette selection, are still being made. Feedback and bug reports are welcome.
+
+Important:
+
+This PNG optimizer removes ancillary chunks (pieces of metadata) from the
+PNG image that normally do not affect rendering. However in special
+circumstances you may wish to keep some. For example for a design using
+custom gamma correction, keeping it may be desired. Visually check in the
+target renderer after using ZopfliPNG. Use --keepchunks to keep chunks, e.g.
+--keepchunks=gAMA,pHYs to keep gamma and DPI information. This will increase
+file size. The following page contains a list of ancillary PNG chunks:
+http://www.libpng.org/pub/png/spec/1.2/PNG-Chunks.html
+
+Build instructions:
+
+To build ZopfliPNG, compile all .c, .cc and .cpp files from src/zopfli,
+src/zopflipng and src/zopflipng/lodepng, except src/zopfli/zopfli_bin.c, to a
+single binary with C++, e.g.:
+g++ src/zopfli/{blocksplitter,cache,deflate,gzip_container,hash,katajainen,lz77,squeeze,tree,util,zlib_container,zopfli_lib}.c src/zopflipng/*.cc src/zopflipng/lodepng/*.cpp -O2 -W -Wall -Wextra -Wno-unused-function -ansi -pedantic -o zopflipng
+
+A makefile is provided as well, but only for linux: use "make zopflipng" with
+the Zopfli makefile. For other platforms, please use the build instructions
+above instead.
+
+The main compression algorithm in ZopfliPNG is ported from WebP lossless, but
+naturally cannot give as much compression gain for PNGs as it does for a more
+modern compression codec like WebP
+https://developers.google.com/speed/webp/docs/webp_lossless_bitstream_specification.
+
+Compared to libpng -- an often used PNG encoder implementation -- ZopfliPNG uses
+2-3 orders of magnitude more CPU time for compression. Initial testing using a
+corpus of 1000 PNGs with translucency, randomly selected from the internet,
+gives a compression improvement of 12% compared to convert -q 95, but only 0.5%
+compared to pngout (from better of /f0 and /f5 runs).
+
+By releasing this software we hope to make images on the web load faster without
+a new image format, but the opportunities for optimization within PNG are
+limited. When targeting Android, Chrome, Opera, and Yandex browsers, or by using
+suitable plugins for other browsers, it is good to note that WebP lossless
+images are still 26 % smaller than images recompressed with ZopfliPNG.
+
+2013-05-07, Lode Vandevenne and Jyrki Alakuijala