libjxl produces too-dark HDR images when sRGB is requested

[Traneptora]

Describe the bug HDR lossy XYB-encoded JXL images are too dark when sRGB is requested from libjxl.

To Reproduce Take an HDR image, XYB-encoded (example attached). Decode it to sRGB with:

$ djxl --color_space=RGB_D65_SRG_Rel_SRG hdr.jxl sdr.png

and view the resulting sRGB PNG with any PNG viewer. The image will be too dark.

Then, decode it to a PQ PNG with:

$ djxl hdr.jxl hdr.png

(this image is tagged as BT.2100/PQ so the explicit color space does not need to be requested). View that PQ png in Chromium, mpv, Preview, or any ICC-aware viewer that tonemaps correctly. The colors will be much more accurately bright and not dark.

Expected behavior The SDR should look like an SDR approximation of the HDR image. Some tradeoffs need to be made, but darkening the whole image is definitely undesirable.

Changing the rendering intent from Rel to Abs, Sat, or Per didn't fix the issue.

Screenshots ^ the above .jpg is a JXL file sdr.png is 34 MiB but you can view it here: https://files.catbox.moe/9hjw4u.png

Environment djxl 60ab29d7

 - OS: Arch Linux x86_64
 - Compiler version: gcc 12.2.1
 - cjxl/djxl version string: djxl v0.9.0 60ab29d7 [AVX2]
``

[Traneptora]

Extra information: somehow libjxl is only using ~half of the 0-65535 range for this 16-bit PNG. Here's part of the output of identify -verbose sdr.png:

Channel depth:
    Red: 16-bit
    Green: 16-bit
    Blue: 16-bit
    Alpha: 1-bit
  Channel statistics:
    Pixels: 8294400
    Red:
      min: 0  (0)
      max: 32740 (0.49958)
      mean: 6524.4 (0.099556)
      median: 5916 (0.0902724)
      standard deviation: 5019.01 (0.0765851)
      kurtosis: 0.54006
      skewness: 0.894579
      entropy: 0.946897
    Green:
      min: 0  (0)
      max: 30019 (0.458061)
      mean: 7113.79 (0.10855)
      median: 6916 (0.105531)
      standard deviation: 5159.37 (0.078727)
      kurtosis: -0.27073
      skewness: 0.513194
      entropy: 0.955864
    Blue:
      min: 0  (0)
      max: 29593 (0.45156)
      mean: 7540.07 (0.115054)
      median: 3124 (0.0476692)
      standard deviation: 8041.49 (0.122705)
      kurtosis: -1.54838
      skewness: 0.493819
      entropy: 0.800535
    Alpha:
      min: 65535  (1)
      max: 65535 (1)
      mean: 65535 (1)
      median: 65535 (1)
      standard deviation: -nan (-nan)
      kurtosis: -2.76473e+58
      skewness: 1.62719e+41
      entropy: 0

GIMP reports only partial levels used as well:

[Traneptora]

Second update: This hdr.jxl has intensity_target tagged at 10000 nits, but since SDR is display-referred we should be using 255 nits for SDR color spaces.

[qbnu]

I think this is fixed in master