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If you are using hierachical instancing or instance arrays, make sure that the mesh light is below the instance source location so that RfK will maintain the material properties and light filters associated with the light location.
Mesh lights can also be used in light arrays. In this case, RfK uses the material from the light array location rather than the material on the geometry location.
Additional Notes
The geometry can be rendered with or without a Bxdf. RfK assigns PxrBlack to mesh lights if no other Bxdf is assigned (does not apply to Method 3). If a Bxdf is assigned to the geometry then the geometry will be rendered according to the characteristics of that shader.
It is recommended that you increase the micropolygonlength on the mesh light for improved performance. This attribute is available on the PxrMeshLight GafferThree package in the Object Tab.
If you want to use the Texture Color parameter of PxrMeshLight, you can create a shading network with PrmanShadingNodes and a NetworkMaterial node. Then assign the NetworkMaterial to the light location with a MaterialAssign node, downstream of the GafferThree where the light was created.