How to render objects in separate passes when using Ambient Occlusion?

[magneto]

Hi,

I have some objects in a scene with ground and other geometry in the environment that contributes to the final render as I am using a skylight and rendering with PBR.

But how can I render only a single object while still accounting for all objects in the scene? Basically I want to only have this object and its alpha in the final render, but I want this object still look the same as if I render all objects in the scene.

If I just put this object in Candidate Objects, then I don't get other objects' contribution. Is there a way around this?

Thanks

[anim]

put other objects into Forced Matte so they will cast AO but not be visible, on top of that they will mask out your object so it really looks the same as with them

if you don't want it to be masked you can use Forced Phantom, but that's probably not what you want for AO

and instead of Candidate Objects, try using Force Objects to avoid surprises when playing with display flags in the scene especially if you need to render different mantra nodes with different objects, this way you can avoid takes for a lot of cases

EDIT: one more thing, when using matte, don't do that mutually for 2 objects

so if you have a ground and let's say a sphere partially in the ground then render

force object: ground

phantom object: sphere

so the ground is not cut by the sphere but receives AO from it

then render:

force object: sphere

forced matte: ground

so that sphere is masked by the invisible ground and therefore can be comped over the first render

Edited June 6, 2013 by anim

[magneto]

Thanks anim, that's great. I don't have Houdini in front of me right now but will the objects in Forced Matte contribute to AO? I might have used Forced Phantom I guess, but when I tried rendering only a single object, the lighting on the object was different.

[anim]

yes, both Phantom and Matte contribute to AO as well as other secondary rays like reflection, refraction, diffuse...

[magneto]

Thanks anim, appreciate it

[substep]

apologies if this is a stupid question, but is there anything like contribution maps in mentalray/maya inside mantra/houdini?

While I'll admit they're a little strange, once you get the hang of them, they drastically reduce the need for lot's of render layers.