automatic pixel format for png export makes the wrong choice if the image has only gray pixels
GIMP version: 2.10.18
Note: bug reporters are expected to have verified the bug still exists either in the last stable version of GIMP or on updated development code (master branch).
Operating System: Linux
Package: archlinux (gimp 2.10.18-4)
Description of the bug
If the image contains only greyscale pixels, exporting to PNG with GIMP produces a file which is considered invalid by imagemagick:
~ $ magick test.png /dev/null magick: profile 'icc': 'RGB ': RGB color space not permitted on grayscale PNG `/dev/null' @ warning/png.c/MagickPNGWarningHandler/1748.
Is the bug reproducible? Always.
- Open GIMP on a brand new profile (full defaults)
- File → New, OK
- Select paintbrush tool and draw something (it will be in black on white)
- File → Export…, choose the filename, press Export
- The Export image as PNG dialog appears, press Export
You have an image that imagemagick complains is invalid (see above).
The exported PNG should not raise errors or warnings when feeding it to imagemagick.
Imagemagick complains that RGB color space is not permitted on grayscale PNG
This never happens if there is any color which is not a gray within the image, for example if I add some red before exporting.
It is possible to fix this by selecting "8pbc GRAY" manually as the pixel format in the PNG export dialog in this case.
There is a discussion at imagemagick: https://github.com/ImageMagick/ImageMagick/issues/884 Nothing is "fixable" on magick's side - it is indeed an inconsistency, and as such it is reported in a warning.
Here is an example image produced using the reproduction steps: