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Chapter 9. Mask Essentials

Chapter 9. Mask Essentials

An alpha channel, as you will recall, defines the opaque and transparent areas of an image. In Chapter 2, you learned how to import footage items that contain alpha channels. Even if a footage item lacks an alpha channel, you can still define areas of transparency. One way to do this is by using masks.

Masks cut out a path, or shape, in a layer's alpha channel. The image within the masked area remains visible; the area outside the mask reveals the layers below (Figure 9.1). You can also apply a mask to a colored solid to create graphical elements.

Figure 9.1. Masks “cut” a shape in a layer's alpha channel to create opaque and transparent areas in the image.

There are three ways to create a mask in a Layer window: You can draw it manually with a tool; you can define it numerically in the Mask Shape dialog box; or you can paste a path from Adobe Illustrator or Photoshop.

Once you've created a mask, you can soften, or feather, its edges. After Effects 4.0 introduced the ability to create as many as 127 masks in a single layer; it also introduced mask modes to control how those masks interact. These features make it much easier to create more complex shapes. Version 4.0 also borrowed the Transform feature found in other Adobe programs, making it simpler to scale and rotate masks. Version 4.1 added the ability to control mask opacity, lock masks, hide them in the Layer window, and convert an open path to a closed path, and vice versa. New in this version (5.0) are color-coded masks, a Mask Expansion property, and to the ability to manipulate masks directly in the Comp window. If you're new to paths or the Pen tool, take a little extra time with these sections; you'll be able to apply these skills to manipulating motion paths as well. If you're already familiar with path editing in other programs, don't let the minor inconsistencies between programs distract you.

Later chapters explain how to animate mask properties, create motion paths with masks (and masks from motion paths), and apply effects to masks.



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