How to create an alpha movie mask?

[NightNote]

I learned that movie-images can be created with:

Code: Select all

image whatever = Movie(play="blah.ogv", mask="helloworld.ogv")

Mask movie supposed to be like a some sort of transparent sheet, but it does actually nothing! My movie-sprite with or without alpha-mask is the same!

How can I make actually working alpha-mask? Plase help, stuck on this for a long time. Thanks.

[Remix]

As far as I know, the mask uses just the red channel to dictate which pixels in the other clip are shown. Most people just opt to use pure red and black in the mask.

You should also note that recent releases support putting both the movie and mask into one movie side by side (to make sure frames do not go out of synch)

[NightNote]

Please, can you describe both methods more detailed?