I have some clips that need to be both chromakeyed and time stretched. Right now I'm timestretching (at best quality) *and* using Keylight on the same layer.
Will precomping (or prerendering) the time stretch give me better results? Should I pre-render the key first? Or does it matter?
I'd do my keying first. Time-stretching (using frame blending) can introduce artifacts that would make the keying harder.
I agree with Todd. Key first, render the key, or better yet render the composite, then monkey with the time. When you render the key use a lossless or nearly lossless codec so you are not adding GOP or compression artifacts to the time artifacts you'll get with the Time Stretch.
I think Rick's suggestion of time-stretching the final product will work best. Todd has a point about the artifacts, but then the same could happen on already keyed footage - pixel motion based tools like Timewarp, Twixtor or RSMB just don't particularly like Alpha because it prevents them from analysing neighboring RGB data for smooth blending...