ClipToProp

std.ClipToProp(vnode clip, vnode 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. Unlike most other filters the output length is derived from the second argument named mclip.

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