*Now, silly question I know, but you do have to have shadows on for the IBL light don't you?  *

No I don't. I'm using no shadows on the IBL - I always use material based AO - more control and better quality. Fewer odd behaviors, less grainy, and faster usually.

*And then you had an infinite light as well, also with shadows on?  *

Yes. But the shadow catcher I showed you is not doing directional shadows at all, so it doesn't matter.

*And you had a diffuse light too?  

Just the IBL and the infinite. That's all. Sometimes you want to make the infinite only do specular. This is one of those times.

*Do these all have shadows on or off.  

Shadows on for the infinite - this is so you don't get highlights on the 3D in wrong places. However, the shadow catcher using only AO will not catch directional shadows. My super duper new shadow catcher can do directional only, AO only, or both.

*If I turn off all shadows I get no shadows, even using that ground material setting for shadow catching that you showed me, and shadows on for render settings.

Probably because of the Irradiance Caching.

Material-based AO never needs a shadow on the light - but you do have to enable ray-tracing and enable shadows in render options.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)