Compositing, alpha channels, and adjusting clip opacity

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.

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 or some of the Channel effects 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.

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.

A matte is a layer (or any of its channels) that defines the transparent areas of that layer or another layer. White defines opaque areas, and black defines transparent areas. An alpha channel is often used as a matte, but you can use a matte other than the alpha channel if you have a channel or layer that defines the desired area of transparency better than the alpha channel does, or in cases where the source image doesn’t include an alpha channel.

Many file formats can include an alpha channel, including Adobe Photoshop, ElectricImage, TGA, TIFF, EPS, PDF, and Adobe Illustrator. AVI and QuickTime (saved at a bit depth of Millions Of Colors+), also can contain alpha channels, depending upon the codecs used to generate these file types.

About straight and premultiplied channels

Alpha channels store transparency information in files in one of two ways: straight or premultiplied. Although the alpha channels are the same, the color channels differ.

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.

Some software lets you specify the background color with which the channels are premultiplied; otherwise, the background color is usually black or white.

Straight channels retain more accurate color information than premultiplied channels. Premultiplied channels are compatible with a wider range of programs, such as Apple QuickTime Player. 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 only the first alpha channel they encounter in a file containing multiple alpha channels. Adobe Flash recognizes only premultiplied alpha channels.

About keying

Keying is defining transparency by a particular color value (with a color key or chroma key) or brightness value (with a luminance key) in an image. When you key out a value, all pixels that have similar colors or luminance values become transparent.

Keying makes it easy to replace a background of a consistent color or brightness with another image, an especially useful technique when you work with objects too complex to mask easily. The technique of keying out a background of a consistent color is often called bluescreening or greenscreening, although you don’t have to use blue or green; you can use any solid color for a background.

Difference keying defines transparency with respect to a particular baseline background image. Instead of keying out a single-color screen, you can key out an arbitrary background.

Keying in Premiere Pro is performed with keying effects. For more information, see Keying effects.

Blend clips with a Garbage Matte effect

You can use a Garbage Matte effect to blend images. For more information about using the Garbage Matte effect, see Eight-Point, Four-Point, and Sixteen-Point Garbage Matte effects.

Blend clips with a matte using the Track Matte Key effect

You can blend clips with a matte (typically, a black and white image, or an image from the Titler.) using the Track Matte Key effect. For more information, see the article, Track Matte Key effect.

Specify clip opacity in a Timeline panel

Compositing tips

Keep the following guidelines in mind when compositing clips and tracks:

• If you want to apply the same amount of transparency to an entire clip, simply adjust the clip’s opacity in the Effect Controls panel.

• It’s often most efficient to import a source file already containing an alpha channel defining the areas that you want to be transparent. Because the transparency information is stored with the file, Premiere Pro preserves and displays the clip with its transparency in all sequences where you use the file as a clip.

• If a clip’s source file doesn’t contain an alpha channel, you must manually apply transparency to individual clip instances where you want transparency. You can apply transparency to a video clip in a sequence by adjusting clip opacity or by applying effects.

• Applications such as Adobe After Effects, Adobe Photoshop, and Adobe Illustrator can save clips with their original alpha channels, or add alpha channels, when the file is saved to a format that supports an alpha channel.