In this walkthrough, we've shared how to mask in After Effects using built-in features and keyboard shortcuts, so you can isolate a section of an image for a variety of creative purposes.
How to Mask in After Effects
1. First, load your current After Effects project and zoom in on the area that you'd like to mask. Before creating a new mask, go to "Edit" > "Preferences" > "Appearance" (or "After Effects" > "Preferences" > "Appearance" on Mac) and make sure that the "Cycle Mask Colors" box is selected. This option assigns a different color for every mask you create. Otherwise, all of your masks will have the same color (yellow, by default).
2. Next, select the Pen tool (or press the G key) and then click and drag on your image to create a new anchor point, with bezier handles. When you hover your cursor over one of these handles, it will change to a Convert Vertex tool, allowing you to click and drag the handle in any direction. This will tell After Effects the direction you want the mask to go in. Pulling on a bezier handle will affect the path's curve, so you can get extremely precise with your shapes.
3. Continue clicking and dragging with the Pen tool to create new points along the path, and then drag on the handles to adjust their direction. At any time in the process, you can hold the Ctrl key (Command on Mac) while hovering your cursor over an anchor point, and it will change to a normal pointer, allowing you to tweak the point's location. Generally, the best way to achieve a smooth mask is by using as few anchor points as possible.
4. To complete your mask, click and drag the Pen path until it connects with your original anchor point. You'll know the section is isolated because everything outside of the mask will look like a grey and white checkerboard. If you type MM on your keyboard, After Effects will show your full list of masks, and you adjust settings like Feather and Opacity for each individual mask. Here, you can also click on any mask and rename it.
5. Finally, if you want to subtract certain sections from your current mask, just start drawing a new mask with the Pen tool over the area that you wish to remove. Then, go back to your list of masks, find the new addition, and change the "Add" setting to "Subtract".