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zopfli

This repository is a derivative of the google/zopfli source repository and includes modifications of the upstream source files in the master branch.

Zopfli Compression Algorithm is a compression library programmed in C to perform very good, but slow, deflate or zlib compression.

The basic function to compress data is ZopfliCompress in zopfli.h. Use the ZopfliOptions object to set parameters that affect the speed and compression. Use the ZopfliInitOptions function to place the default values in the ZopfliOptions first.

ZopfliCompress supports deflate, gzip and zlib output format with a parameter. To support only one individual format, you can instead use ZopfliDeflate in deflate.h, ZopfliZlibCompress in zlib_container.h or ZopfliGzipCompress in gzip_container.h.

ZopfliDeflate creates a valid deflate stream in memory, see: http://www.ietf.org/rfc/rfc1951.txt ZopfliZlibCompress creates a valid zlib stream in memory, see: http://www.ietf.org/rfc/rfc1950.txt ZopfliGzipCompress creates a valid gzip stream in memory, see: http://www.ietf.org/rfc/rfc1952.txt

This library can only compress, not decompress. Existing zlib or deflate libraries can decompress the data.

zopfli_bin.c is separate from the library and contains an example program to create very well compressed gzip files. Currently the makefile builds this program with the library statically linked in.

The source code of Zopfli is under src/zopfli. Build instructions:

To build zopfli, compile all .c source files under src/zopfli to a single binary with C, and link to the standard C math library, e.g.: gcc src/zopfli/*.c -O2 -W -Wall -Wextra -Wno-unused-function -ansi -pedantic -lm -o zopfli

A makefile is provided as well, but only for linux. Use make to build the binary, make libzopfli to build it as a shared library. For other platforms, please use the build instructions above instead.

Zopfli Compression Algorithm was created by Lode Vandevenne and Jyrki Alakuijala, based on an algorithm by Jyrki Alakuijala.

zopflipng

ZopfliPNG is a command line program to optimize the Portable Network Graphics (PNG) images. This version has the following features:

• uses Zopfli compression for 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.

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.