[Christian Reid]

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.