Creating GI Multiplier: useful custom output material node

Our team was creating cutscenes for the game and they had problems setting up proper lighting. We generally use stationary lights, because we want precomputed GI and shadows for the game. Unfortunately the cutscene team was unable to get rid of GI on characters, without modifying lighting on the environment. Custom code was needed. At first I thought I would have to add another output to the main material node, but while looking inside the code I realised that I can create a custom output node. I used UMaterialExpressionBentNormalCustomOutput as my template. Work on C++ side consisted of creating the node and registering it in FMaterialAttributeDefinitionMap::InitializeAttributeMap.

AddCustomAttribute(FGuid(0x2E5E2BF3, 0x646F4531, 0x8E36B5C0, 0xA8055AFB), "GIMultiplier", "GIMultiplier", MCT_Float1, FVector4(1, 0, 0, 0));

The engine created a shader define and function for me. On HLSL side I just used them in GetPrecomputedIndirectLightingAndSkyLight function in BasePassPixelShader.ush

  float GIMultiplier = saturate(GIMultiplier0(MaterialParameters));
  OutDiffuseLighting *= GIMultiplier;
  OutSubsurfaceLighting *= GIMultiplier;

When my node was ready I placed it together with a parameter from Material Parameter Collection in base materials for characters. Now our cutscene lighting artist is able to use it inside a sequencer to scale GI to his wish.