GafferRenderMan : Display and Sample Filter support

[boberfly]

Generally describe what this PR will do, and why it is needed

• GafferRenderMan : Display and Sample filter support.
  https://rmanwiki-26.pixar.com/display/REN/Sample+and+Display+Filters

Related issues

• N/A

Dependencies

• N/A

Breaking changes

• Maybe, but we are WIP

Checklist

• I have read the contribution guidelines.
• I have updated the documentation, if applicable.
• I have tested my change(s) in the test suite, and added new test cases where necessary.
• My code follows the Gaffer project's prevailing coding style and conventions.

[johnhaddon]

Before I get into reviewing this, my main question is about RenderMan itself. The Riley API lets us pass a list of display filters, so why does PxrDisplayFilterCombiner exist? What happens if we pass a list of display filters? Are they automatically combined?

[boberfly]

Good question I could've sworn that I read somewhere that it was for a future update to Riley but I can't remember where now. I used the hdPrman delegate as reference and this is how they approached it.

When I originally approached this I used an ObjectVector of ShaderNetwork and was making the combiner in IERenderMan backend but it wasn't too elegant in the UI seeing one of those. Is there a dummy array shader node I can use in a ShaderNetwork to replace the combiner?

[johnhaddon]

When I originally approached this I used an ObjectVector of ShaderNetwork and was making the combiner in IERenderMan backend but it wasn't too elegant in the UI seeing one of those.

Yeah, this is what I was wondering about doing. It seems potentially less faffy than dealing with PxrDisplayFilterCombiner in our RenderManDisplayFilter node. But I suppose there are other benefits to PxrDisplayFilterCombiner - we could use ShaderTweaks on one directly for instance (well, a hypothetical GlobalShaderTweaks variant at least).