Combining LUTs and shaders may result in broken rendering

[MoSal]

I've seen weirdly broken rendering with a real-world LUT, but this should be an easy reproducer.

LUT

A simple LUT generated with:

#include <stdio.h>

int main() {
  int size = 13;
  float step = 1.0f / (size - 1);
  printf("LUT_3D_SIZE %d\n\n", size);
  for (int b_c=size-1; b_c >= 0; b_c--) {
    for (int g_c=size-1; g_c >= 0; g_c--) {
      for (int r_c=size-1; r_c >=0; r_c--) {
        printf("%g %g %g\n", (step*r_c - 0.1)*11.0/9.0, (step*g_c - 0.1)*11.0/9.0, (step*b_c - 0.1)*11.0/9.0);
      }
    }
  }
}

Shader

invert_custom.glsl.zip

Reproduction

mpv  --no-config --pause -image-lut file.lut -glsl-shader file.glsl av://lavfi:testsrc=size=1280x720
mpv  --no-config --pause -lut file.lut -glsl-shader file.glsl av://lavfi:testsrc=size=1280x720
mpv  --no-config --pause -target-lut file.lut -glsl-shader file.glsl av://lavfi:testsrc=size=1280x720
# or without mpv
ffplay -vf libplacebo=lut=file.lut:custom_shader_path=file.glsl -f lavfi testsrc

Results

Broken

With image-lut + unscaled

Working

With image-lut + upscaled (lanczos)

---

It's broken in all other cases: downscaling, lut, target-lut, or upscaling without a libplacebo shader (bilinear).

And even when the rendering is okay, non-window screenshots would still look broken.

[kasper93]

Do you have similar problem with older libplacebo releases?

[MoSal]

Do you have similar problem with older libplacebo releases?

How old? I just reproduced with 7.349.0 using old Arch packages (grabbed with agetpkg):

x265-4.0-1-x86_64.pkg.tar.zst
ffmpeg-2:7.1.1-2-x86_64.pkg.tar.zst
libplacebo-7.349.0-7-x86_64.pkg.tar.zst
mpv-1:0.40.0-2-x86_64.pkg.tar.zst

[kasper93]

How old?

7.349.0 is ok, I though it might be regression from more recent changes.