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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.
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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 at 11:53

3 Answers 3

up vote 3 down vote accepted

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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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.

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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 \
OUTFILE.png

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:

49444154081d013400cbff00
11223311223311223311223300
44556644556644556644556600
77889977889977889977889900
aabbccaabbccaabbccaabbcc
7d6f14b9...

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

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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. –  Selenium 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. –  Selenium Sep 17 '12 at 13:05

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