ClipToProp

std.ClipToProp(clip clip, clip mclip[, string prop='_Alpha'])

Stores each frame of mclip as a frame property named prop in clip. This is primarily intended to attach mask/alpha clips to another clip so that editing operations will apply to both.

If the attached mclip does not represent the alpha channel, you should set prop to something else.

It is the inverse of PropToClip().