Convert image to uncompressed PNG from the command-line

Question

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 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.

Answer 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).

Answer 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.