Rendering the front of an object with alpha

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Old 02 February 2006   #1
Rendering the front of an object with alpha

I have a snake coiling around a pillar. I want to output only the snake with an alpha channel. I don't want to see the parts of the snake as it goes behind the pillar. I want to put both pillar and snake into aftereffects and have them both on seperate layers. I tried using a plane with a black material on it and a composite tag to cover where the pillar would be, which I thought would work, but no luck.
Any help would be great. Thanks
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Old 02 February 2006   #2
As a quick testI rendered out 3 passes. One with the snake only, one with the pillar only and one with both, the trick being that in the final pass, the snake's texture was 100% white in lum channel and the pillar was black with no specularity. I comped them in AE using "effects/set matte" in the snake only layer using the final pass as the matte, in effect using it as an alpha channel.

Don't know if this will work in your sit, but maybe it helps?

(I posted the AE & .mov files at C4Dcafe.com in your thread in the texturing forum there. The zipped file was too big to post here.)

Dave W.
 
Old 02 February 2006   #3
Does it not work with a compositing tag? And then assign a object ID and render out a Multipass with this object ID??

Peace,
Rich-Art.
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Old 02 February 2006   #4
The Object masks would then cut out the part of the snake that is behind the pillow.
Think of what would happen if the opacity of the pillow is turned down.

There is a AE plugin by Buena that can use the depth map to mask layers.
It doesn't, as it seems, combine layer depths , that would solve this totaly
doing only two renders.
It also use RLA/RPF, so NET rendering won't work with it.
davesinks solution is very clever.

Cheers
Lennart
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Old 02 February 2006   #5
Thanks davesink. I figured a similar way to that. I wanted to find a way to do it straight out of C4D so that I would have less hassle in AFX. I may well use this if there's not an easier answer

Rich-Art. How do I assign an object ID and to what object or objects do I assign it to. I'm using 9.5XL
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Last edited by jimzepellin : 02 February 2006 at 11:27 AM.
 
Old 02 February 2006   #6
Like Lennart wrote, that won't work the way you want.

You must put a Compositing tag to the object (S) you want. and in the AM you can set an unic ID to the objects.
In the rendersettings under Multi-Pass you choose the Oject ID Pass and set the correct nr.
You can use multiple object ID passes.
If you open this output in PhotoShop you will find a Correct alpha cannel in the channels tab.

Peace,
Rich-Art.
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Old 02 February 2006   #7
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