ZopfliPNG – high-compression PNG optimizer

ZopfliPNG is a command-line PNG optimiser built around Google’s Zopfli compression algorithm. Its aim is to produce smaller PNG files than faster general-purpose compressors by spending substantially more CPU time searching for better DEFLATE output.

The program does more than run Zopfli over the existing IDAT stream. It can try alternative PNG filter strategies and colour representations, then keep the smallest result. That makes it best suited to assets where final file size matters more than processing speed, such as web images prepared once and served many times.

ZopfliPNG can process individual files or batches and can be run without writing output when you want to compare compression results first.

This is free and open source software.

Key Features

• Uses Zopfli compression to pursue smaller DEFLATE streams at the expense of much longer processing time.
• Provides a higher-compression mode that increases the number of optimisation iterations.
• Lets users set the number of Zopfli iterations directly to trade processing time against final file size.
• Tests different PNG filtering strategies, including standard per-scanline filters, entropy-based selection and brute-force filtering.
• Can preserve the original PNG colour type and bit depth when compatibility is more important than maximum compression.
• Can remove hidden RGB colour information from fully transparent pixels for smaller output, with no visible difference.
• Can reduce 16-bit-per-channel PNG images to 8-bit when the user explicitly accepts that lossy transformation.
• Strips non-essential metadata by default while allowing selected PNG chunks to be retained.
• Supports a quick mode for rapidly testing filters and colour types before committing to slower compression.
• Can use filename prefixes to optimise multiple PNG files in one invocation.
• Avoids replacing an existing result when that file is already smaller.
• Provides a dry-run mode for measuring compression without writing files.
• Produces standards-compliant PNG files that can be decoded by ordinary PNG software.

Website: github.com/google/zopfli
Support:
Developer: Lode Vandevenne and Jyrki Alakuijala
License: Apache License 2.0

zopflipng is written in C++ and C. Learn C++ with our recommended free books and free tutorials. Learn C with our recommended free books and free tutorials.

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