Anime4K_Clamp_Highlights.glsl causes darker colors when upscaling with gpu-next

mpv Information

mpv v0.39.0-26-gc3d9243a3e Copyright © 2000-2024 mpv/MPlayer/mplayer2 projects
 built on Sep 28 2024 16:19:34
libplacebo version: v7.349.0 (v7.349.0-13-ga018ab04-dirty)
FFmpeg version: n7.0.2
FFmpeg library versions:
   libavcodec      61.3.100
   libavdevice     61.1.100
   libavfilter     10.1.100
   libavformat     61.1.100
   libavutil       59.8.100
   libswresample   5.1.100
   libswscale      8.1.100

Important Information

- Platform version: Latest Arch Linux
- GPU model, driver and version: can be reproduced on both AMD+Mesa and Nvidia proprietary drivers

Reproduction Steps

Download https://raw.githubusercontent.com/bloc97/Anime4K/7684e9586f8dcc738af08a1cdceb024cc184f426/glsl/Restore/Anime4K_Clamp_Highlights.glsl
mpv --no-config --vo=gpu-next --glsl-shaders=Anime4K_Clamp_Highlights.glsl with any image or video
Upscale the image or video

Expected Behavior

Colors stay the same

Actual Behavior

Colors become darker. In some images they even blend with the desktop wallpaper if the platform supports background transparency. This issue doesn't occur on vo_gpu or if upscaling with a shader.

Possibly related to https://github.com/mpv-player/mpv/issues/9524

Log File

mpv.log

Sample Files

Original screenshot:

Upscaled with no shader:

Upscaled with Anime4K_Clamp_Highlights.glsl:

I carefully read all instruction and confirm that I did the following:

• I tested with the latest mpv version to validate that the issue is not already fixed.
• I provided all required information including system and mpv version.
• I produced the log file with the exact same set of files, parameters, and conditions used in "Reproduction Steps", with the addition of --log-file=output.txt.
• I produced the log file while the behaviors described in "Actual Behavior" were actively observed.
• I attached the full, untruncated log file.
• I attached the backtrace in the case of a crash.

Start by reproducing the supplied command with Anime4K_Clamp_Highlights.glsl and gpu-next, using the attached log and screenshots as the baseline. Compare the result with vo_gpu and shader-free upscaling; the issue is resolved when colors remain unchanged during gpu-next upscaling.