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Rendering ONLY an Alpha Channel

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Chris Arnold
Rendering ONLY an Alpha Channel
on Mar 25, 2008 at 8:58:47 pm

I already rendered a scene in Cinema 4D, but now I want to render an alpha channel.

Is there a way to ONLY render the alpha channel without rendering the actual video? It's just a timing issue. I need this render done quick and to render the alpha with the video all over again will take TOO long.


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Lennart Wåhlin
Re: Rendering ONLY an Alpha Channel
on Mar 26, 2008 at 12:45:20 am

To the best of my knowledge, no. Cinema always render "everything".

Try copy your scene , delete all lights and replace all mats with a pure luminance mat.
You'll have to keep any semitransparant mats thou.


Cheers
Lennart

aka tcastudios


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Chris Arnold
Re: Rendering ONLY an Alpha Channel
on Mar 26, 2008 at 1:22:45 pm

Thanks,

That seems like the quickest way to do it for me as well. It's to bad Cinema doesn't have that functionality though.



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Christian Reid
Re: Rendering ONLY an Alpha Channel
on Aug 15, 2017 at 10:37:16 pm

I know this question has been asked almost a decade ago, but I was searching for a solution for this matter and it is not as easy to find a more recent post. So, here it goes.

C4D R17 (not sure about 16 or earlier) has a "Material Override" option in the render settings.

The steps to get an Alpha Channel are simple, but not straightforward:
1. Create a simple material with a pure black luminance channel, and apply it to the Material Override window.
The name is self explanatory, it applies this selected material OVER all of the other materials in your project, saving you the trouble of doing that manually.

- Now, you hide everything you don't want the alpha to see from the scene (backgrounds, skies, etc), and do a normal render with Alpha. To make it easier, just make sure you choose "Separate Alpha" before rendering.

- In my case, an animation, I got a pure black PNG sequence, and the Alpha Channel on a separate sequence.
Dump the black stuff, and keep the Alpha. Done.


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Steve Bentley
Re: Rendering ONLY an Alpha Channel
on Aug 16, 2017 at 2:27:28 am

Turn off ambient occlusion - that's not doing anything to the edges.
You can also sometimes turn down the anti aliasing or the quality level in the physical render depending on what you need the alpha for.
Both of these speed things up immensely.
You can also turn off transparency, shadows reflections and refractions if you still need some materials for the render but be careful if some of your materials create "holes" in things - you might still need transparency.
Motion blur quality can often be turned down as well depending on the use of the alpha.
\
If there is a way to not use physical (i've been able to do this a few times for just an alpha) then use another renderer that can use more cores. You might have a kick'n box, but physical can only see two of those cores and sometimes only one.



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