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I have a video of powder exploding on a solid black background. I want to use the powder itself as a mask to reveal some copy, so basically as this powder explodes, the copy is revealed. Is this possible in AE?
Actually just figured out a way. I just drew a mask with the pen, feathered the edges and animated the mask expansion.
Alpha matte would work:
You're right to use a Track Matte but it's going to have to be a Luma Matte. Alpha mattes work on shapes, type or anything with transparency. Your powder layer is going to be a video with no transparency, just black and white values. That's what Luma Mattes are for. You may need to adjust the contrast of the powder layer to get a good mask effect.
Oh ok I didnt even know that was called track matte but im very familiar with that. But that isnt what I was looking for. Im trying to reveal some text through an explosion of powder, and the shape of the powder is constantly changing.
So I had to draw a mask and do a semi-frame-by-frame animation of the mask. Not every frame but like every 5 frames or so set a keyframe and then change the mask. That was the only way I could figure out how to do it.
Also, look into the Set Matte effect if you want to matte several layers with just one powder layer. Just choose Lightness instead of Alpha as your "Use for Matte" option.
The issue that I was having is this powder video has a black background, it doesnt have an alpha or anything. Its just a flat video of colorful powder on black, but I wanted it to look like the text im adding is like IN the powder.
If I could do this as particles in 3d studio Id have more control over it but I dont have time to do that. So I ended up putting the powder video on top of the text, and changing the mode to difference, then the animated mask to reveal the text with the powder.
You could have turned your powder footage into black&white, and then applied it as a Luma Matte to reveal the copy. Without seeing what the powder looks like it's hard to be sure... your powder would have to be dense enough to reveal everything, if it's thin and shows the background through you'll have to use another layer on top with the copy and fade or animate it in with a mask to complete the effect.