I'm having some trouble getting my head around this. I want to use noise as a mask, with another image only showing up through the white parts. I've tried using [pix_mask], but I'm a little lost at this point.

Here's an image to describe what I'm looking for, and a simple patch I've been working trying to get it going. sphere_noise.pd

The image is a fairly rough example. The plan is for the output to be in the GEM window. Thanks, guys.

If I'm not mistaken, [pix_mask] works with images, not primitives (like shpere). Try using an image (.jpeg for example) instead, both for the shpere and the noise.
If you want it to evolve, you could maybe make a small movie file with different frames of the noise (just guessing).
About translating and rotating, I don't really know, but the movie technique might work there too.

Bummer. I've recently discovered [pix_composite] and other stuff (Still trying to get to grips with GEM). Is there a way to create an alpha within GEM based on colour or lightness data? Say, turn all white values into alpha?

There is [alpha]. it works in most instances I've thought to use it. Not sure if it will work work with [pix_mask]. Give it a whirl!

Well, I just can't seem to get it to work. At least this is the push I feel I need to get learning vvvv.