Ah, foxyshadis is entering plugin land ... /me rubs hands
Nice extension to AddGrain. Less juggling-around with separating & merging color planes.
BTW: It seems there's a long standing issue with AddGrain. Somehow, if during processing a AddGrain'ed clip is duplicated, used otherwise, and later on combined again with the original AddGrain'ed clip, then the noise pattern is no more the same. Seems like the filter starts with a different seed for processing the child frame, instead of using the seed from the mother frame.
At times, this is highly unpropitious. The usual workaround is to not apply AddGrain directly to a clip, but to apply it to a neutral grey clip instead, and subtracting this one from the clip you want to process. This way mother and child keep having the same pattern.
Sorry for I've no example at hand right now (and can't make one, since I'm already sleeping
)
From the sourcecode I can tell nothing (no clue about how these things are handled). Perhaps someone in the knowings can see where it's going wrong.