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Can you render alpha of specific objects only

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Joseph Herman
Can you render alpha of specific objects only
on Dec 11, 2011 at 7:34:20 pm

Hello all,

Is there a way to render only the alpha of selected objects.

Here's what I mean: Say I put an object on the floor and I want to render out its alpha, but I don't want to render the floor's alpha.

I know I can just hide the floor and render it out, but that will affect the GI, so I don't want to do that. I just want to render the alpha of the object.

Thank you.


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Brian Jones
Re: Can you render alpha of specific objects only
on Dec 11, 2011 at 8:22:11 pm

Yes, add a compositing tag to your object and enable one of the Object Buffers there.
Enable MultiPass rendering and make sure Object Buffer is checked for that, the buffer numbers have to match - if you enabled buffer 1 in the Compositing tag you have to use 1 in the multi pass object buffer because you can have lots of object buffers if you want so they keep it straight by numbering them.


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Joseph Herman
Re: Can you render alpha of specific objects only
on Dec 11, 2011 at 8:29:06 pm

Thanks, I am going to try this.


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Joseph Herman
Re: Can you render alpha of specific objects only
on Dec 12, 2011 at 4:59:13 pm

Hi.

Thanks again for your help.

Although the explanation sounds straightforward enough, for some reason this is not working. Maybe I am doing something wrong.

Here's what i did to test it. I opened a new scene, dropped in a floor, then made a sphere. I set the compositing tag on the sphere and in the object buffer tab, enabled buffer 1.

In the render settings, Multi-Pass is enabled. I pressed the "Multi-Pass..." button below and chose Object Buffer from the list which then appears under the Multi-Pass item in the render settings.

After clicking on that item it says "Group ID: 1" to the right.

After rendering and bringing the object into Photoshop, the Alpha is wrong (it still includes the floor). Should there be another file for the buffer?

Perhaps I am doing something wrong?


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Joseph Herman
Re: Can you render alpha of specific objects only
on Dec 12, 2011 at 5:16:42 pm

Okay, got it. You need to tick the Multi Pass option in the Save section of the render settings. Thanks.


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