Masking in Adobe After Effects allows users to cover up, add and subtract sections of an image to create animations or other special effects like the illusion of a ghost. You can focus on certain parts of video footage and use key frames to animate the mask as you create the project.
Choose the type of mask you want to apply to your creation. The pen tool creates a free-form mask as you draw in the monitor window. This is ideal for masking video clips of people or other objects. The various shape tools create masks as well. Choose from shapes including a polygon, star and rectangle.
Applying a Mask
For the pen-tool mask, click on the screen at the desired start point for the mask. Trace around the edges you want the mask to cover. Double-click or match the end point to complete the mask layer. If you want to use a specified shape, select the shape from the top toolbar. Click and drag on the screen to make the shape. Use the selection's corner points to adjust the shape sizes. On the mask properties, select "Add" or "Subtract." The "Add" tool keeps video footage inside the mask and the "Subtract" tool removes footage inside the mask.
Once the mask is applied, you can make custom adjustments so it will fit in your video better. The "Mask Feather" tool sets fuzziness around the mask's edges so they will blend with the composition's other objects. The "Mask Path" tool adjusts the mask's shape. The "Mask Opacity" tool shows how much of the mask is visible.
To change and adjust the mask's length for the video timeline, click on the keyframe icon next to the "Mask Path" selection. Set the timeline navigator to the point that you want to change the mask shape. Adjust the mask points around the object, move the timeline navigator and repeat this process until you've gone through the whole video. Scroll through the timeline to see how the mask adjusts positions and appears on the video.