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linux - Convert image to uncompressed PNG from the command-line


I have a compressed PNG image compressed.png. I can convert it to an uncompressed PNG decompressed.png using GIMP (saving as PNG and setting compression level to 0). How can this be done on the command line (Linux)?


I recall doing this in the past using Imagemagick's convert, but I forgot how. I tried some things that I thought should work based on the documentation:



  • convert compressed.png -compress None decompressed.png

  • convert compressed.png +compress decompressed.png

  • convert compressed.png -quality 0 decompressed.png

  • convert compressed.png -quality 00 decompressed.png


just wrote an ordinary compressed PNG.


Aside: why would you want an uncompressed PNG?


Some cases:



  • You want to support efficient (binary) diffs of the image data, while still using other features of the PNG format (as opposed to storing raw image data or BMP).

  • You want to compress several PNGs together in a tarball or 7z archive, but want to keep using PNG features. If the images are sufficiently similar this can give a better compression ratio than compressing individually.

  • Useful as a baseline size for testing PNG optimizers.



Answer



ImageMagick will always compress PNG files. The worst compression you can get is using:


convert -verbose -quality 01 input.png output.png

but it depends on the image content (the 0 will use Huffman compression which sometimes compress better than zlib).


You can try other tools like pngcrush (http://pmt.sourceforge.net/pngcrush/) to disable the compression:


pngcrush -force -m 1 -l 0 input.png output.png

which create a file the same size GIMP create when using Compression Level 0 (few bytes more or less).


Some example sizes (for a photographic PNG, 1600x1200):



  • original: 1,693,848 bytes.

  • after IM: 2,435,983 bytes.

  • after GIMP: 5,770,587 bytes.

  • after pngcrush: 5,802,254 bytes.


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