Masking is a very powerful tool used for roto scoping and creating shapes. You have control over how your mask will behave, you can choose to add or subtract from your mask shape. We will focus on the subtract feature:
- Drag another shape on top of the current solid shape.
- Find the new mask parameter on that solid in the lower left side of the timeline.
- To the right of the mask is a tab called add with a arrow pointing down. Click on that.
- Choose Subtract from that list:
Fig. Subtract mask
See how it cuts that shape out? This is how you can create interesting shapes in After Effects. Have a lot of fun with this, there's so much you can do with it. The skies the limit! Masking becomes very important when you need to cut layers, video, images, and so on. Think of layers as scissors to cut out what you don't want to see. I'd like for you guys to play with these shapes. Start editing these mask points. You can create complex interesting shapes with these techniques.