Possible bug in optimizer with colorspace transform with mixed scene referred/display colorspaces

[SRHMorris]

I'm unsure if this is really a bug or not, as I'm unsure if using display colorspaces with scene referred colorspaces in a OCIO::ColorSpaceTransform is valid (i.e. not a DisplayViewTransform).

Here's my code, with OCIO v2.4.2.

auto config = OCIO::Config::CreateFromFile("ocio://default");

auto cst = OCIO::ColorSpaceTransform::Create();
cst->setDst("Rec.1886 Rec.709 - Display");
// cst->setSrc("Linear Rec.709 (sRGB)");
cst->setSrc("Gamma 2.4 Encoded Rec.709"); // <- this causes issue

auto group = OCIO::GroupTransform::Create();
group->appendTransform(cst);

auto proc = config->getProcessor(group)->getOptimizedGPUProcessor(OCIO::OptimizationFlags::OPTIMIZATION_LOSSLESS); // <- setting to NONE fixes it

auto gpuShaderInfo = OCIO::GpuShaderDesc::CreateShaderDesc();
gpuShaderInfo->setLanguage(OCIO::GpuLanguage::GPU_LANGUAGE_HLSL_SM_5_0);
proc->extractGpuShaderInfo(gpuShaderInfo);

const char *text = gpuShaderInfo->getShaderText();
std::cout << text << std::endl;

With the linear Rec709 space set as the source, I get the following shader text which is what I'd expect, e.g. just a 2.4 gamma transform:

// Declaration of the OCIO shader function

float4 OCIOMain(float4 inPixel)
{
  float4 outColor = inPixel;
  
  // Add Gamma 'basicRev' processing
  
  {
    float4 gamma = float4(0.41666666666666669, 0.41666666666666669, 0.41666666666666669, 1.);
    float4 res = pow( max( float4(0., 0., 0., 0.), outColor ), gamma );
    outColor.rgb = float3(res.x, res.y, res.z);
    outColor.a = res.w;
  }

  return outColor;
}

However with the 2.4 gamma space set as the source, I'd expect a no-op transform. Instead I get this (a gamma of 5.76?):

// Declaration of the OCIO shader function

float4 OCIOMain(float4 inPixel)
{
  float4 outColor = inPixel;
  
  // Add Gamma 'basicRev' processing
  
  {
    float4 gamma = float4(0.17361111111111113, 0.17361111111111113, 0.17361111111111113, 1.);
    float4 res = pow( max( float4(0., 0., 0., 0.), outColor ), gamma );
    outColor.rgb = float3(res.x, res.y, res.z);
    outColor.a = res.w;
  }

  return outColor;
}

With OPTIMIZATION_NONE it seems reasonable:

// Declaration of the OCIO shader function

float4 OCIOMain(float4 inPixel)
{
  float4 outColor = inPixel;
  
  // Add Gamma 'basicPassThruFwd' processing
  
  {
    float4 gamma = float4(2.3999999999999999, 2.3999999999999999, 2.3999999999999999, 1.);
    float4 breakPnt = float4(0., 0., 0., 0.);
    float4 isAboveBreak = float4((outColor[0] > breakPnt[0]) ? 1.0 : 0.0, (outColor[1] > breakPnt[1]) ? 1.0 : 0.0, (outColor[2] > breakPnt[2]) ? 1.0 : 0.0, (outColor[3] > breakPnt[3]) ? 1.0 : 0.0);
    float4 powSeg = pow(max( float4(0., 0., 0., 0.), outColor ), gamma);
    float4 res = isAboveBreak * powSeg + ( float4(1., 1., 1., 1.) - isAboveBreak ) * outColor;
    outColor.rgb = float3(res.x, res.y, res.z);
    outColor.a = res.w;
  }
  
  // Add Matrix processing
  
  {
    float4 res = float4(outColor.rgb.r, outColor.rgb.g, outColor.rgb.b, outColor.a);
    float4 tmp = res;
    res = mul(tmp, float4x4(0.43963298191949191, 0.089776442958842226, 0.017541170383172787, 0., 0.38298869815155351, 0.81343942874897801, 0.11154655330238722, 0., 0.1773783199289555, 0.096784128292177099, 0.87091227631444246, 0., 0., 0., 0., 1.));
    outColor.rgb = float3(res.x, res.y, res.z);
    outColor.a = res.w;
  }
  
  // Add Matrix processing
  
  {
    float4 res = float4(outColor.rgb.r, outColor.rgb.g, outColor.rgb.b, outColor.a);
    float4 tmp = res;
    res = mul(tmp, float4x4(0.93827984927725538, 0.33736889078783672, 0.0011739508496858789, 0., -0.0044514458123613987, 0.72952156669026536, -0.0037107064020525625, 0., 0.016627523586776168, -0.066890457478102336, 1.0915945063122465, 0., 0., 0., 0., 1.));
    outColor.rgb = float3(res.x, res.y, res.z);
    outColor.a = res.w;
  }
  
  // Add Matrix processing
  
  {
    float4 res = float4(outColor.rgb.r, outColor.rgb.g, outColor.rgb.b, outColor.a);
    float4 tmp = res;
    res = mul(tmp, float4x4(3.2409699419045213, -0.96924363628087973, 0.055630079696993649, 0., -1.5373831775700932, 1.8759675015077204, -0.20397695888897652, 0., -0.4986107602930035, 0.04155505740717564, 1.0569715142428788, 0., 0., 0., 0., 1.));
    outColor.rgb = float3(res.x, res.y, res.z);
    outColor.a = res.w;
  }
  
  // Add Gamma 'basicRev' processing
  
  {
    float4 gamma = float4(0.41666666666666669, 0.41666666666666669, 0.41666666666666669, 1.);
    float4 res = pow( max( float4(0., 0., 0., 0.), outColor ), gamma );
    outColor.rgb = float3(res.x, res.y, res.z);
    outColor.a = res.w;
  }

  return outColor;
}

[doug-walker]

@SRHMorris , I think you may be running into this bug we fixed back in May. It's in the main branch, so will release with 2.5.0, but was not in 2.4.2.

Are you able to test fixes in the main branch?

[SRHMorris]

@doug-walker Yeah you're right, sorry for the noise! Seems to be fixed on main.