Multimatte from invisible object

[jonahhawk]

Is there a way to pull a matte from an object that is both Invisible to Camera yet still casts shadows? We do this now with two passes but it seems this could be done in one.

We use animated people to create a black silhouette in our architectural scenes. Camera pass as usual. Then a people pass with everything else as Matte objects, shadows, Alpha -1. Then the people pass becomes a matte for the silhouettes.

It would be nice to accomplish this in a single pass. If in the object properties "Visible to Camera" is unchecked, the people cast shadows but don't show up in the MultiMatte.

The trick I'm looking for is to is to have a single rendered pass that has the shadows of the people but not the people and the multimatte as well.

This would save us a render pass on the farm and would skim a little more time off of tight deadlines.

any thoughts?

[instinct]

That would be possible by giving the ppl a 100% refracting material with IOR 1. BUT you will face sampling issues in any case as multimatte has to use the beauty pass's sampling. And as the object is supposed to be visible it will be undersampled in many cases and cause artifacts in the matte pass i'm afraid. With invisible planes a workaround is using Adaptive AA and checking Object Outline and Normal Threshold to force the edges to be sampled properly.

Why not make them perfectly black or matte objects if you overlay them in post anyways ? that way you wont hit sampling issues and still get rid of the additional pass, no ?

Regards,
Thorsten

[jonahhawk]

Thanks for the reply.

To clarify, we would use 100% refraction without the Affect Shadows box checked. That might just be the trick. I'll have to experiment a bit with the sampling.

We don't want to make them matte objects because we want to keep the alpha intact for the buildings. Is that what you mean? In the final comp we like to control the opacity of the people and don't want to render them into the beauty pass. Does that make sense? Sometimes we just drop them out all together.