Compositing for Adobe Premiere
You can composite images in several ways: by applying keys, by using alpha channels (in clips that have them), by using the opacity effect, and by simply scaling an image to reveal underlying images. All of these methods, except scaling, require that part of a clip be transparent.
When part of a clip is transparent, transparency information is stored in its alpha channel. You can apply any combination of opacity, masks, mattes, and keying to modify the alpha channel. This partially or totally hides areas of a clip.
Each video track in the Timeline panel contains an alpha channel that stores transparency information. All video track frames are completely transparent except where you’ve added opaque content such as video, still images, or titles. You can make areas of opaque content partially or completely transparent by adjusting a clip’s alpha channel or applying a matte or key to a clip. Clips on upper tracks cover clips on lower tracks except where alpha channels indicate transparency. Adobe Premiere Pro composites clips from the lowest track up, and the final video frame is a composite of clips on all visible tracks. Areas where all tracks are empty or transparent appear black. If necessary, you can use the File > Interpret Footage command to change how Adobe Premiere Pro interprets a clip’s alpha channel throughout a project.
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, Adobe 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. In these applications, you can display a checkerboard pattern that indicates transparency so that you can distinguish transparent areas from opaque white areas.
Adjust the Opacity of Clips
By default, clips on tracks appear at full (100%) opacity except for areas marked by a clip’s mask, matte, or alpha channel. Make an entire clip more transparent by setting an opacity value below 100%. When a clip’s opacity value is set to less than 100%, clips on lower tracks may be visible. At 0% opacity, the clip is completely transparent. If no clips are stacked below a partially transparent clip, the sequence’s black background becomes visible. 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.
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.
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 an Adobe 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, Adobe Flash Video (FLV), 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. For Adobe Illustrator EPS and PDF files, After Effects automatically converts empty areas to an alpha channel.
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.
Move or Change the Transparent Area with Track Matte Key
The Track Matte Key reveals one clip (background clip) through another (superimposed clip), using a third file as a matte that creates transparent areas in the superimposed clip. This effect requires two clips and a matte, each placed on its own track. White areas in the matte are opaque in the superimposed clip, preventing underlying clips from showing through. Black areas in the matte are transparent, and gray areas are partially transparent.
A matte containing motion is called a traveling matte or moving matte. This matte consists of either motion footage, such as a green‑screen silhouette, or a still image matte that has been animated. You can animate a still by applying the Motion effect to the matte. If you animate a still image, consider making the matte frame size larger than the project frame size so that the edges of the matte don’t come into view when you animate the matte.
You can create mattes in various ways:
- Use the Title panel to create text or shapes (grayscale only), save the title, and then import the file as your matte.
- Apply the Chroma, RGB Difference, Difference Matte, Blue Screen, Green Screen, or Non Red Key to any clip and then select the Mask Only option.
- Use Adobe Illustrator or Adobe Photoshop to create a grayscale image and import it into Editing Software.
- Add the background clip to a track in the Timeline panel.
- Add the clip you want to superimpose to any track higher than the track containing the background clip. This is the clip revealed by the track matte.
- Add the track matte clip to a third track above the tracks with the background and superimposed clips.
- If you need to add a new track to the sequence, drag the track matte clip to the empty area above the highest video track in the Timeline panel. A new track is automatically created.
- In the Effects panel, click the triangle to expand the Video Effects bin and then click the triangle to expand the Keying bin.
- Drag the Track Matte Key to the superimposed clip.
- In the Effect Controls panel, click the triangle next to the Track Matte Key name to expand its settings.
- Click the Matte setting pop‑up menu with the down‑pointing triangle and choose the video track containing the track matte clip.
- (Optional) If you’re animating the Track Matte Key over time, make sure that the current‑time indicator is in the position you want. Click the Toggle Animation icons of the settings you want to adjust.
- Click the Composite Using pop‑up menu and choose one of the following:
- Matte Alpha Composites using the track matte clip’s alpha channel values.
- Matte Luma Composites using the track matte clip’s luminance values.
14. (Optional) Select the Reverse option to invert the values of the track matte clip.
15. (Optional) If you’re animating the Track Matte, move the current‑time indicator either in the Effect Controls panel or Timeline panel and change the Track Matte Key settings.
A new keyframe appears in the Effect Controls timeline when you change the settings. You can also adjust the interpolation between keyframes by editing the keyframe graph. Repeat this step as needed.