View Full Version : compositing question with overlapping geometry

04 April 2009, 11:30 AM
Hello there,

I am new to compositing and I am trying to figure out in photoshop how I can get the following problem solved:
I have an object that sits on a ground plane. But it does not just sit on top; it is intersected by the ground plane.
Now I rendered several passes (diffuse, matte, etc.) for the object and the ground plane.
If I mask the object out by its matte and have the ground plane in the background the whole object will be in front of the ground plane.
but as it intersects I need to have it clipped by the ground plane.
I already tried to get that fixed using a z depth pass but I canŽt get the hang of it.
Can anyone help me on this one?


04 April 2009, 03:53 PM
i dont know which 3d app you are using - in Cinema 4d is the Option to define a object as Matte-object which will then influence other underlying or intersecting objects alpha channels.

04 April 2009, 06:17 AM

I am using Maya 2008.
And IŽm not sure if matte renders will do the trick as the objects are moving and one object is in the background at first an will come eventually to the foreground, overlapping the other object. So the compositing app needs to know which object is the foreground one and which one is the background object on every frame.
If I render the objects separately in object passes there will be no occlusion. But maybe it will help if I do a matte pass for both objects separately using a black hole shader for the respective foreground object. IŽll give it a try.

Thanks for the reply.


04 April 2009, 04:21 PM
Another solution: disable all lights, and use for everything excepct the designated object a 100% black material. For the intersecting object use a complete white (100% luminance) material - this should give you a clean luma matte for the intersections.

04 April 2009, 05:26 AM
Hi LordSaddler,

this sounds really clever. IŽll give it a shot.
Thanks a lot.

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04 April 2009, 05:26 AM
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