Compositing overview

Compositing involves layering multiple video clips or images to create a single, combined visual.

This technique allows you to craft complex scenes by stacking elements, where parts of one layer may be transparent, revealing content beneath.

Every image or video can contain an alpha channel, which holds information about the transparency of different areas.

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

Alpha channels store transparency information in files in one of two ways: straight or premultiplied.

With straight (or unmatted) channels, transparency information is stored only in the alpha channel, not in any of the visible color channels. With straight channels, the effects of transparency aren’t visible until the image is displayed in an application that supports straight channels.

With premultiplied (or matted) channels, transparency information is stored in the alpha channel and also in the visible RGB channels, which are multiplied with a background color. The colors of semitransparent areas, such as feathered edges, are shifted toward the background color in proportion to their degree of transparency.

Straight channels retain more accurate color information than premultiplied channels. Premultiplied channels are compatible with a wider range of programs. Often, the choice of whether to use images with straight or premultiplied channels has been made before you receive the assets to edit and composite.

Premiere Pro and After Effects recognize both straight and premultiplied channels, but they recognize only the first alpha channel in a file containing multiple alpha channels.

These file formats contain alpha channels:

• Adobe Photoshop (.psd)
• ElectricImage,
• TGA
• TIFF
• EPS
• PDF
• Adobe Illustrator (.ai)

AVI and QuickTime (saved at a bit depth of Millions Of Colors+) can also contain alpha channels, depending upon the codecs used to generate these file types.

You can make composites in Premiere Pro in multiple ways:

• You can make portions of an image transparent using effects such as Track matte, Garbage matte, or some of the Channel effects.
• You can also use blending modes
• You can define transparency by a particular color value (with a color key or chroma key) or brightness value (with a luminance key). This is known as keying.

How compositing works in Premiere Pro

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.