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Chapter 9. Mask Essentials

Chapter 9. Mask Essentials

An alpha channel, as you recall, defines the opaque and transparent areas of an image. In Chapter 2, you learned how to import a footage item containing an alpha channel. 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 might 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.

You can create a mask in a Layer window in three ways: drawing it manually with a tool, defining it numerically in the Mask Shape dialog box, or by pasting a path from Adobe Illustrator or Photoshop.

You can reshape masks and soften, or feather, their edges. After Effects 4.0 introduced the ability to create up to 127 masks in a single layer, as well as mask modes to control how they 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 adds the ability to control the opacity of masks, lock masks, and hide them in the Layer window, and convert an open path to a closed path, and vice versa.

If you're new to paths or the Pen tool, take a little extra time with these sections; you can 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 a motion path with masks (and masks from motion paths), and apply effects to masks.



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