About combining multiple masks
Often you may find that it’s impossible to create the mask shape you need using only a single mask. For example, when masking an image that has a hole in it, you need at least two masks—one with its blend mode set to Add (to mask the overall object), and a second one with its blend mode set to Subtract (to cut the necessary hole in the middle of the image). In the following example, the only way to mask the space between the diver’s arms and legs is to create three overlapping masks, two with blend modes set to Subtract.
Multiple masks are also useful when you need to mask an image with complex contours or when you’re rotoscoping an object with multiple moving parts. For more information, see Rotoscope a mask’s shape.
Whatever the reason, you can easily combine multiple masks for any object by adjusting each mask’s Mask Blend Mode parameter in the Mask Inspector.
Each mask’s blend mode determines whether the mask adds to, subtracts from, or replaces a layer’s previously existing alpha channel. Additionally, blend modes affect how masks interact with one another. Ultimately, every mask applied to a layer combines according to the specified blend modes to create that layer’s final alpha channel.
Blend mode operations are also determined by the order of each mask in the Layers list, particularly with the Replace and Intersect blend modes. For example, a mask set to Intersect affects all masks nested underneath it in the Layers list, masking only the areas of the layer where the masks overlap one another. Similarly, a mask set to Replace mode effectively turns off any masks beneath it in the Layers list (including the image layer’s built-in alpha channel, if one exists).
For more information on mask blend modes, see Mask controls in the Inspector.