these terms - alpha channel and matte?

can anybody explain how are they work in photoshop or after effects. i'm learning after effects now and i'm having a difficulty with understanding these terms..

i'm sure this question is silly for the pros but what can you do :)


Thanks,

The alpha channel is a black and white channel of an image (the "A" in RGBA) that says how transparent each pixel is. White is Opaque, Black is transparent. In between is in between.

A matte is that which defines the alpha channel (kinda abstract). Often, this is simply a chunk of the image that is masked off by the editor/ compositor.

a good example would be if you wanted to put on a title and have the video play behind
the letters in the title.. the title would be Alpha matted, sos the title would have zero
opacity and the matte would define the letters in the title.