Saving image with non-sRGB color profile as .JXL results in weird saturation

[rickbrew]

This uses Facebook's "c2" color profile, but it doesn't seem to matter which color profile I convert it to unless it's sRGB. Both DP3 and ProPhoto exhibit similar issues.

When loading the image, if I break into the debugger at the point where the plugin is writing pixel values to the document, the values are wrong. Instead of R=128,G=150,B=147 I see R=56,G=76,B=73 for the pixel at x=0,y=0. This makes me think maybe there's a busted linear/companded conversion somewhere.

I thought I'd made a mistake in some of the new PDN 5.2 code, or in my code for updating pdn-jpegxl for 5.2, but this also happens in PDN 5.1.11

Strangely, if I open the .JXL in Krita, it looks correct but the color values are off-by-1, e.g. R=129 instead of R=128.

tiled--ali-pazani-by6kqgS6DOI-unsplash.jpg.zip

[rickbrew]

This happens in PDN 5.1.11 w/ libjxl 0.11.1, but also in PDN 5.2 w/ libjxl 0.11.2

[rickbrew]

If I enable the "Lossless" checkbox, it looks correct

[0xC0000054]

No idea what is going on here, perhaps a bug in libjxl's ICC profile conversion to whatever its internal color representation is. IIRC Facebook's c2 is just a tiny version of sRGB, so I don't know why libjxl isn't converting it to the internal sRGB encoding.

Krita appears to be telling libjxl to use the original profile for lossy RGB encoding, which I also did before being told that it is an unsupported configuration.

Here is the jxlinfo output for cjxl, PDN and Krita:

tiled--ali-pazani-by6kqgS6DOI-unsplash-cjxl.jxl
JPEG XL file format container (ISO/IEC 18181-2)
JPEG XL image, 6720x4480, lossy, 8-bit RGB
Color space: RGB, Custom, white_point(x=0.345703,y=0.358538), Custom primaries: red(x=0.648454,y=0.330852),  green(x=0.321198,y=0.597844),  blue(x=0.155885,y=0.066038)sRGB transfer function, rendering intent: Perceptual
Brotli-compressed Exif metadata: 92 compressed bytes

tiled--ali-pazani-by6kqgS6DOI-unsplash-pdn.jxl
JPEG XL file format container (ISO/IEC 18181-2)
JPEG XL image, 6720x4480, lossy, 8-bit RGB
Color space: 524-byte ICC profile, CMM type: "lcms", color space: "RGB ", rendering intent: 0
Uncompressed Exif metadata: 186 bytes

tiled--ali-pazani-by6kqgS6DOI-unsplash-krita.jxl
JPEG XL file format container (ISO/IEC 18181-2)
JPEG XL image, 6720x4480, (possibly) lossless, 8-bit RGB+Alpha
Color space: 524-byte ICC profile, CMM type: "lcms", color space: "RGB ", rendering intent: 0