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36 lines
1.8 KiB
36 lines
1.8 KiB
4 months ago
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ZopfliPNG is a command line program to optimize the Portable Network Graphics
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(PNG) images. This version has the following features:
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- uses Zopfli compression for the Deflate compression,
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- compares several strategies for choosing scanline filter codes,
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- chooses a suitable color type to losslessly encode the image,
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- removes all chunks that are unimportant for the typical web use (metadata,
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text, etc...),
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- optionally alters the hidden colors of fully transparent pixels for more
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compression, and,
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- optionally converts 16-bit color channels to 8-bit.
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This is an alpha-release for testing while improvements, particularly to add
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palette selection, are still being made. Feedback and bug reports are welcome.
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To build ZopfliPNG, use "make zopflipng", or compile all the sources except
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zopfli_bin.c.
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The main compression algorithm in ZopfliPNG is ported from WebP lossless, but
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naturally cannot give as much compression gain for PNGs as it does for a more
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modern compression codec like WebP
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https://developers.google.com/speed/webp/docs/webp_lossless_bitstream_specification.
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Compared to libpng -- an often used PNG encoder implementation -- ZopfliPNG uses
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2-3 orders of magnitude more CPU time for compression. Initial testing using a
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corpus of 1000 PNGs with translucency, randomly selected from the internet,
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gives a compression improvement of 12% compared to convert -q 95, but only 0.5%
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compared to pngout (from better of /f0 and /f5 runs).
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By releasing this software we hope to make images on the web load faster without
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a new image format, but the opportunities for optimization within PNG are
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limited. When targeting Android, Chrome, Opera, and Yandex browsers, or by using
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suitable plugins for other browsers, it is good to note that WebP lossless
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images are still 26 % smaller than images recompressed with ZopfliPNG.
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2013-05-07, Lode Vandevenne and Jyrki Alakuijala
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