I saw this feature at another studio, but since it was executed via scripts, I never got to understand how it worked under the hood, and was wondering if anybody here had an idea:
The script degrained the plate for you, using Neat Video's Reduce Noise, and then also created a precomp that extracted the original grain pattern from the plate to use as a regrain layer at the top of the stack. This avoids Match Grain, which often doesn't match well, or Add Grain, which is good, but manual.
and created a grain layer at the top of the layer stack to regrain everything using the same grain rather than using a Match Grain or manually-tweaked Add Grain effect.
I suspect it's something like the degrained plate Difference'd against the original plate, but how do I adjust that comp so that when applied over the degrained comp, it will match the grain from the original plate?