I have a basic test scene with two meshlights and an area light, each has it's own light group (pink,green & yellow). Emission and diffuse direct AOV's are enabled with 'All Light Groups' selected.
On batch rendering a test frame the direct_diffuse gives me correct separation for each light group but the emission channels are all black. It does, however, make a new 'emission_default' channel with all the emissive meshlights in despite the fact that the 'default' light group doesn't exist in the scene.
Light path expressions for separating emission per light group also appears to broken (although I haven't tested this as thoroughly).
Is emission light group separation not possible?
MtoA 2.1.0.2
Arnold Core 5.0.2.3
Maya 2017 Update 3
Emission and light groups don't work for mesh lights. Probably because a visible mesh light is not a light, but a mesh with an emissive surface shader.
Can this be added to the list for fixing at some point in the future please? It's pretty important for compositing.
At the very least can this be flagged in the docs? (Somewhere on the light groups / light path expressions pages).
Maybe the AOV light group text box should be removed from mesh lights too if it doesn't work.
Same problem here , and docs doesn't mention「light groups don't work for visible mesh light 」anywhere at all.
You can force a meshlight as lightgroup by manually adding the string in the Light Groups List (aiAOV), by selecting the AOV and edit the list Light Groups List string but still it doesn't work for Emission AOV. It's also a big issue that the emissive mesh is not added to RGBA_YourLightgroup AOV. This problem is with visible mesh and area Lights and should be fixed by the developers.