PNG or Portable Network Graphics is a raster image file format meant to replace GIF. PNG images use lossless image compression, producing high-quality images at a higher cost for file size.
Some popular PNG compressors for Linux are optipng, pngquant, and pngng. These tools can help reduce the size of a PNG image by performing lossy and lossless compression along with other optimizations. pngquant is usually the best option as it seems to optimize the file size the most without sacrificing much quality.
$ sudo apt update && sudo apt install --assume-yes pngquant #Ubuntu and variants > sudo zypper refresh && sudo zypper install --non-interactive pngquant # SUSE and variants $ sudo dnf install --assumeyes pngquant # Red Hat and variants
$ ls -lh filename.png -rwxr--r-- 1 user user 295K Oct 11 09:40 filename.png
$ pngquant --verbose filename.png filename.png: read 295KB file made histogram...3507 colors found selecting colors...5% selecting colors...35% selecting colors...70% selecting colors...100% moving colormap towards local minimum eliminated opaque tRNS-chunk entries...6 entries transparent mapped image to new colors...MSE=0.064 (Q=99) writing 256-color image as filename-fs8.png copied 1KB of additional PNG metadata Quantized 1 image.
$ ls -lh filename*.png -rw-rw-r-- 1 user user 73K Oct 11 09:42 filename-fs8.png -rwxr--r-- 1 user user 295K Oct 11 09:40 filename.png
-fs8 is appended to the compressed filename by default.
$ pngquant --help pngquant, 2.12.2 (July 2019), by Kornel Lesinski, Greg Roelofs. Compiled with no support for color profiles. Using libpng 1.6.37. usage: pngquant [options] [ncolors] -- pngfile [pngfile ...] pngquant [options] [ncolors] - >stdout <stdin options: --force overwrite existing output files (synonym: -f) --skip-if-larger only save converted files if they're smaller than original --output file destination file path to use instead of --ext (synonym: -o) --ext new.png set custom suffix/extension for output filenames --quality min-max don't save below min, use fewer colors below max (0-100) --speed N speed/quality trade-off. 1=slow, 4=default, 11=fast & rough --nofs disable Floyd-Steinberg dithering --posterize N output lower-precision color (e.g. for ARGB4444 output) --strip remove optional metadata (default on Mac) --verbose print status messages (synonym: -v) Quantizes one or more 32-bit RGBA PNGs to 8-bit (or smaller) RGBA-palette. The output filename is the same as the input name except that it ends in "-fs8.png", "-or8.png" or your custom extension (unless the input is stdin, in which case the quantized image will go to stdout). If you pass the special output path "-" and a single input file, that file will be processed and the quantized image will go to stdout. The default behavior if the output file exists is to skip the conversion; use --force to overwrite. See man page for full list of options.
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