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Chapter 13: Compositing

Compositing, alpha channels, and adjusting clip opacity

Adobe After Effects, another program in Adobe Creative Suite Production Premium, gives you a greatly expanded range of compositing tools. You can easily import composites made in After Effects into Premiere Pro.

To create a composite from multiple images, you can make parts of one or more of the images transparent so that other images can show through. You can make portions of an image transparent using any of several features in Premiere Pro, inlcuding mattes and effects.

To make an entire clip uniformly transparent or semi-transparent, use the Opacity effect. You can set a selected clip’s opacity in the Effect Controls panel or Timeline panel, and you can fade a clip down or up over time by animating opacity.

If you simply want to create a fade to black, consider applying a transition such as Dip To Black to the clip instead of animating opacity keyframes manually.

When part of a clip is transparent, transparency information is stored in its alpha channel.

Images can also be composited together without modifying the transparency of the clips themselves. For example, you can use “Blending modes” on page 432 or some of the “Channel effects” on page 349 to blend image data from multiple clips into a composite.

Clips on upper tracks cover clips on lower tracks except where alpha channels indicate transparency. Premiere Pro composites clips from the lowest track up, to create a composite of clips on all visible tracks. Areas where all tracks are empty or transparent appear black.

Rendering order affects how opacity interacts with visual effects. The Video Effects list is rendered first, then geometric effects such as Motion are rendered, and then alpha channel adjustments are applied. Within each effects group, effects are rendered from the top down in the list. Because Opacity is in the Fixed Effects list, it renders after the Video Effects list. If you want opacity to render earlier or later than certain effects, or if you want to control additional opacity options, apply the Alpha Adjust video effect.

You can choose how to interpret the alpha channel in a file in the Interpret Footage dialog box. Choose Invert Alpha Channel to swap areas of opacity with areas of transparency, or choose Ignore Alpha Channel to not use the alpha channel information at all.

If you have difficulty identifying which parts of a clip are transparent, choose Alpha from the Program view menu in the Program Monitor. Another way to see areas of transparency is to add a bright solid color matte on a track below the image you are keying.

Alpha channels and mattes

Color information is contained in three channels: red, green, and blue. In addition, an image can include an invisible fourth channel, called an alpha channel, that contains transparency information.

An alpha channel provides a way to store images and their transparency information in a single file without disturbing the color channels.

When you view an alpha channel in the After Effects Composition panel or a Premiere Pro Monitor panel, white indicates complete opacity, black indicates complete transparency, and shades of gray indicate partial transparency.

Last updated 1/16/2012