According to the docs on shadow matte (it seems in all of the documentation, MtoA, HtoA, MAXtoA, C4DtoA, etc.) there are two defined "Background Types", 'image_plane' and 'sky'.
What I don't understand and the docs don't seem to explain very well, is what is the difference between the two exactly?
Most times I've needed the shadow_matte, I don't have one or the other, it's typically an HDRI in an arnold sky (not visible by camera) and a backplate/background (image_plane).
When setting the background type to "image_plane" I get an accurate indirect specular on any objects on the shadow matte, but there is the issue of offscreen color, and adding in an image texture to "offscreen color" just never looks very good/natural.
As far as setting the background type to 'sky', I have yet to discover any situation/scenario where this looks even close to natural/accurate.
Some help understanding these two options would be great.
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