I was using a mask to reveal parts of a a vector layer, but it's quite complex and it'd be a lot easier if i could just paint the layer on. I can use the eraser tool to erase the alpha, and the paint tool to paint the layer different colors. But what i really need is for the layer to start completely transparent, and to use the brush/paint tool to paint the alpha on. How can i do this? (I see a paint mode for silhouette alpha, but what i need is a STENCIL alpha or similar).
Precomp your vector layer and on the precomp add a paint effect. With "paint on transparent" = off (so you can see what your painting on) set the paint mode to Normal, Channels need only be RGB and the Duration to Write on. Set paint foreground colour to black.
Then Paint where and with the speed that you want the reveal to occur. Once you finish the paint operation the line you draw will vanish. Press play and you should see the line you drew appear at the rate you drew it.
Copy this effect from your precomp layer to a white solid of the same size so your black line appears on white. Then put this solid layer above your vector layer or the precomped vector layer if you wish and use it as a luma inverted Track matte.
Let me know how it goes!
3D animator and VFX specialist