I can't understand the lut-baking process in FColorGrading initializing functon

[dandingol03]

Hi, I have see the lut-baking implementation code listed below, so there are some step in lut-baking:
1.do logC encoding, change value fro logc to linear
2.kill negative
3.colorGradingIn depend on the tonemapping choose

Let's say if i choose ACES_LEGACY tone mapping, so colorGradingIn will move the value in srgb color space to REC2020. But my question is the value after LogC_to_linear isn't linear? Why assume it is in srgb color space, i can't understand.

  for (size_t g = 0; g < config.lutDimension; g++) {
      for (size_t r = 0; r < config.lutDimension; r++) {
          float3 v = float3{r, g, b} * (1.0f / float(config.lutDimension - 1u));

          // LogC encoding
          v = LogC_to_linear(v);

          // Kill negative values near 0.0f due to imprecision in the log conversion
          v = max(v, 0.0f);

          if (builder->hasAdjustments) {
              // Exposure
              v = adjustExposure(v, builder->exposure);

              // Purkinje shift ("low-light" vision)
              v = scotopicAdaptation(v, builder->nightAdaptation);
          }

          // Move to color grading color space
          v = c.colorGradingIn * v;
     }
  }
               

[dandingol03]

@romainguy give me some help if it is convinient for you

[romainguy]

This has nothing to do with the current color space, it's just compression. Why do you need to know about those details?

[dandingol03]

@romainguy Because our artist meet the chromatism difference between blender and filament, and this difference is so obvious that we can not ignore it . So i read the color management code in blender and filament, and try to figure out why there is some chromatism when we both choose 'filmic' tone mappiing.
So i just need the answer of this question: the colorGradingIn function suppose the current colorspace is srgb. Does this srgb is the same thing with linear rgb? Because i always treat srgb as the srgb color space which gamma = 0.45, i can't understand how the colorspace with gamma which is not 1 can be called linear.

[romainguy]

LogC has nothing to do with the final colors, is just a way to encode the values in the LUT. The opposite is done when reading from the LUT.

Filament's "filmic" curve is not the same as Blender's "filmic" curve, it's not more complicated than that :) I'm planning on adding the AgX tone mapping curve to Filament, which is also available in Blender.

And to answer your question, sRGB is a confusing name. You can have "linear sRGB" which really means the Rec709 gamut with a linear transfer function (and you'll see that in the code we use more precise definitions like this: ColorSpace outputColorSpace = Rec709-sRGB-D65;). But again the LogC_to_linear function has nothing to do with this.

[dandingol03]

Thanks for your kindly reply.

[romainguy]

See #7236

This will match Blender 4.0.