Alpha mask

Can blender create an alpha mask of a model? If it can, how?

There are many tutorials for this sort of thing but you may want to create an ID pass and save it through the compositor.

ID pass? ok I guess I’ll dig deeper into the tutorials…

When you use an output image-file format which supports Alpha (e.g. .JPG does not), then your pixels will be "(R,G,B,A) where “A=Alpha” indicates transparency. There’s your “alpha mask.”

There are other ways to identify (and thus, to separate out) the elements of your image. The id-pass is one, which tags the pixels with an arbitrary id-number.

I routinely suggest that you use MultiLayer OpenEXR as the file-format for your entire process, up to and including the final “answer print,” which you then use as a source for creating whatever “deliverable file or files” (.MOV, .QT, or what-have-you) from it. This will keep all of the data that you have generated, neatly separated by layer and stored in high-resolution lossless numeric format, from start to finish. (When you produce the “prints,” yes, at that point you’re compressing and so-forth, but only at that point, at which point of course you have “lost” nothing at all.)