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.
  • What are you really trying to do? I can't think of any reason in practice to want a larger PNG file. – Michael Hampton Aug 1 '12 at 16:18
  • 1
    @MichaelHampton Edited with examples – Mechanical snail Aug 1 '12 at 16:33
  • I'm using this to add a huge png file to my app's test suite to validate upload restrictions. In git though, this file is easily compressed (26MB -> 300kB) and doesn't blow up the whole repo. – Pascal Oct 17 '14 at 11:53

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.

To quote the ImageMagick doc: Not all combinations of compression level, strategy, and PNG filter type can be obtained using the -quality option. For more precise control, you can use the -define option.

For example, this command should create an uncompressed (RGB) PNG file:

convert INFILE \
-define png:compression-level=0 \
-define png:compression-filter=0 \
-define png:color-type=2 \

You might get a warning, which is a harmless bug. It will still produce a correct png. http://www.imagemagick.org/discourse-server/viewtopic.php?f=1&t=20257

Sample image:

112233 112233 112233 112233
445566 445566 445566 445566
778899 778899 778899 778899
aabbcc aabbcc aabbcc aabbcc

IDAT chunk of the PNG file created with IM:


However, for certain cases (e.g. group compression) it could be worthwhile testing a format with less overhead, for example TARGA (.tga).

  • Didn't seem to work for me (ImageMagick 6.6.9-7 on Linux)—output is still compressed. – Mechanical snail Sep 16 '12 at 22:42
  • I've tested it with ImageMagick 6.7.2-7. It seems that this key was added recently, so you will need to use a more up-to-date version of IM. – Meyer Sep 16 '12 at 23:17
  • imagemagick.org/script/command-line-options.php says "png:compression-level=value: valid values are 0 through 9, with 0 providing the least but fastest compression and 9 usually providing the best and always the slowest.". – Mechanical snail Sep 17 '12 at 7:07
  • Sorry, I think I misunderstood your first comment. Looking at the actual bits, you are right, the image was still not a clean bitmap. Importantly, this was also the case with the GIMP generated version. However, further experimenting with the IM define option indicates that it still is possible. I edited the answer accordingly. – Meyer Sep 17 '12 at 13:05

I like the netpbm suite of tools, it is designed in the UNIX "software tools" tradition: "write programs that do one thing and do it well".

You can do what you asked like this:

< in.png pngtopnm > image.pnm
< in.png pngtopnm -alpha > alpha.pnm
<image.pnm pnmtopng -alpha alpha.pnm -compression 0 > out.png
rm image.pnm alpha.pnm

You might lose certain metadata from the original png file.

It seems a bit complicated, due to the way we handle the alpha channel.

Netpbm is a toolkit for manipulation of graphic images, including conversion of images between a variety of different formats. There are over 220 separate tools in the package including converters for more than 80 graphics formats.

Your Answer

By clicking "Post Your Answer", you acknowledge that you have read our updated terms of service, privacy policy and cookie policy, and that your continued use of the website is subject to these policies.

Not the answer you're looking for? Browse other questions tagged or ask your own question.