I got two layers/video clips. First is original video- obiect on background. Second is a black & white image (entire obiect is white, and entire background is black). Now i want to blur only background of original image using second clip as a ?mask?. I'm a begginer in AE and i still didnt find the solution. Help please..
youll have to explain yourself a bit better, perhaps if you added screenshots of the two layers youre talking about, and be a bit more specific in regards to what you're actually trying to achieve. Then I could try and help you.
It sounds like you need to learn how to use a layer as a Track Matte.
Duplicate your background layer, apply blur to it. Then place your black and white matte layer above the new layer. Set the Track Matte value of the blurred layer to Luma Matte.