Make a straight-line mask on a solid: beginning of line, end of line and a mask vertex at the middle point. Make a mask path keyframe. Copy it and paste it later in the comp. Go back about midway in the comp. Alter the mask path vertices to make a U-shaped mask path.
There you go.
Thanks for the reply!
How do you create a straight-line mask on a solid? It appears that you can only make closed path masks in AE.
When I use the pen tool and make a straight line on a solid layer, it creates a mask but the solid color is still visible and I can't change the color of the mask stroke.
If you're doing it with a mask on a solid, add the Stroke effect. That will allow you to paint On Transparent and with whatever color you desire.
1. I tried the stroke effect and it appeared to work for what I want to do, but I can't figure out how to animate the number of points on the stroke. I need to keyframe more/less points over time.
2. The stroke effect creates strokes with a rounded cap. How do I create a stroke effect with a butt cap?
Here's an example of what I'm trying to do:
The blue dots represent points I need to add/remove to get the desired shapes.