Convert image to uncompressed PNG from the command-line
Solution 1
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.
Solution 2
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).
Solution 3
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.
Mechanical snail
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Updated on September 18, 2022Comments
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Mechanical snail over 1 year
I have a compressed PNG image
compressed.png
. I can convert it to an uncompressed PNGdecompressed.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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Mechanical snail almost 12 years@MichaelHampton Edited with examples
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Mechanical snail over 11 yearsDidn't seem to work for me (ImageMagick 6.6.9-7 on Linux)—output is still compressed.
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Meyer over 11 yearsI'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.
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Mechanical snail over 11 yearsimagemagick.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.". -
Meyer over 11 yearsSorry, 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.
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Ruslan almost 3 yearsI still get compressed output (although the size is very similar to uncompressed) with ImageMagick 7.0.5-5. The
pngcrush
-based answer works well instead (though it requires a PNG input, which is not good when one wants fast conversion between e.g. BMP and PNG).