Using Video and Graphics Clips with Alpha Channels

Ordinary video clips have three channels of information for red, green, and blue. An alpha channel is an additional channel that defines areas of transparency in the clip or image. If you import a QuickTime movie or a still image from another application, or a file that’s been created by a 3D animation program, it may have an alpha channel in addition to its color channels. An alpha channel is a grayscale channel where levels of white and black determine varying degrees of transparency applied to the color channels during compositing.

When you import a QuickTime movie or an image file, its alpha channel is recognized immediately. When you edit the movie or image file into a sequence, Final Cut Pro uses the alpha channel to composite the movie or image file against any clips appearing in video tracks below it.

Note: Alpha channels work the same in video and still-image media files.

Figure. Images of a foreground graphics clip, alpha channel mask, and background graphics clip combined into a final composite image.

Important: When you import a clip with an alpha channel that contains no data, the clip appears white. To display the clip properly, change the alpha channel for the clip to None (see Changing a Clip’s Alpha Channel Type).

Working with Clips That Have Alpha Channels

You can edit clips that have alpha channels into your sequence as you would any other clip, but you must make sure that a clip with an alpha channel is on a higher-numbered video track than any clips that are supposed to be beneath it.

For example: You have a title graphic that was created in Photoshop, so it has an alpha channel already set up. When you import it into Final Cut Pro, the alpha channel is recognized automatically. To create your title sequence, you edit the title graphic and a background image into your sequence, so that the background image is on track V1 and the title graphic is on track V2.

Figure. Canvas showing that the area around the text in the graphics file is transparent because the file contains an alpha channel, and the Timeline showing the title graphic with the alpha channel on a higher-numbered track than the background clip.

Changing a Clip’s Alpha Channel Type

When you import a still image file, Final Cut Pro identifies the type of alpha channel and sets the imported clip’s Alpha property appropriately. You should usually not change this setting, except in rare circumstances where Final Cut Pro identifies an image alpha channel as premultiplied black or premultiplied white and you are certain that the alpha channel type is actually straight.

When you import a video file with an embedded alpha channel from third-party editing and compositing applications, Final Cut Pro always sets the Alpha property to Straight. In these cases, you can manually change or reverse the alpha channel setting if necessary.

To change a clip’s alpha channel type using the Item Properties window
  1. Do one of the following:

    • Select a clip in the Browser, then choose Edit > Item Properties > Format (or press Command-9).

    • Control-click a clip in the Browser, then choose Item Properties > Format from the shortcut menu.

      The Item Properties window appears.

  2. Control-click the Alpha row in the Clip column, then choose the alpha channel type from the shortcut menu.

    • None/Ignore: If a clip has no alpha channel, or if you want to turn off the alpha channel of a clip so that it’s no longer used, you can use the None/Ignore setting.
    • Straight: Typically generated by a 3D animation or compositing program, straight alpha channels contain only the pixels rendered for an image with no background (although the background appears black). Because of this, movies with translucent effects, such as volumetric lighting or lens flares, appear distorted unless they’re composited with something in the background. Movies or images with a straight alpha channel are easy to composite and always have very clean edges when composited in a multilayer sequence.
    • Black: Also known as premultiplied with black. Most other alpha channels, especially channels that are hand drawn in Photoshop, are premultiplied. This means that, unlike a movie with a straight alpha channel that contains only the pixels of the foreground image, movies with alpha channels premultiplied against black always look right, even if they have translucent lighting effects. This is because the entire image is precomposited against black.
    • White: Also known as premultiplied with white. Works the same as the black alpha channel, except that the clip is precomposited against white.

      Tip: If necessary, you can also Control-click the Reverse Alpha row in the Clip column and choose No or Yes from the shortcut menu.

      Figure. Item Properties window showing a shortcut menu listing alpha channel options.
To change the alpha channel type of a clip using the Alpha Type menu item
  1. Select a clip in the Timeline or open a clip in the Viewer.

  2. Choose Modify > Alpha Type, then choose a new alpha channel type from the submenu.

To change the alpha channel type of a clip in the Browser
  • Control-click in the Alpha column in the Browser, then choose a new alpha channel type from the shortcut menu.

To reverse a clip’s alpha channel
  • Do one of the following:

    • Select a clip in the Timeline or open a clip in the Viewer, then choose Modify > Reverse Alpha.

    • In the Browser, Control-click in the Reverse Alpha column next to the clip you want to modify, then choose Yes from the shortcut menu.

      If there is a checkmark by the Reverse Alpha menu item, the clip’s alpha channel is being reversed. If there is no checkmark, the alpha channel is left alone.

Exchanging Media with Alpha Channels

Alpha channels are simply grayscale images or frames, where levels of white and black determine varying degrees of transparency. If you look at a clip with an alpha channel in the Viewer with the Alpha option selected in the View pop-up menu, you’ll see that the solid areas of the image are represented by 100 percent white and that transparent areas of the image are represented by 100 percent black. Lighter to darker shades of gray indicate lesser to greater areas of transparency.

Other editing applications may use white and black differently than Final Cut Pro. If you’re exchanging media with other editors and broadcast graphic designers, let them know how you need clips with alpha channels to be set up for use in Final Cut Pro.

Figure. Viewer window showing that in Final Cut Pro, the white areas of media with an alpha channel are solid and the dark areas are transparent.

Viewing RGB and Alpha Channels in the Viewer

When you’re working in the Viewer, it can sometimes be helpful to have a quick look at the alpha channel of your clip. The alpha channel defines areas of transparency and can change if various filters are applied. For example, if a blue-screen clip is added to the Viewer, and a blue-screen filter is applied to the clip, you can check to see what the alpha channel looks like by itself by choosing View > Channels > Alpha.

There are several channel viewing options in the View menu and the View pop-up menu in the Viewer.

  • RGB: This is the default color option; it shows the three color channels that make up the actual video image.
    Figure. Viewer window showing an image with the default RGB color option.
  • Alpha: This option represents the alpha channel as black against white, where absolute black represents 100 percent transparency, levels of gray represent varying degrees of transparency, and absolute white represents nontransparent areas of the picture.
    Figure.  Viewer window showing an image displayed with the Alpha option.
  • Alpha+RGB: This option lets you see a representation of the alpha channel overlaid on top of the RGB channels that constitute the actual image of the video clip. The alpha channel is overlaid in red, with solid red representing nontransparent areas.
    Figure. Viewer window showing an image displayed with the Alpha+RGB option.

The RGB and Alpha options in the View menu are most effective when used with different backgrounds, described in Choosing a Background.

To choose an alpha channel display option
Do one of the following:
  • Choose View > Channels, then choose an option from the submenu.

  • Choose an alpha channel display option from the View pop-up menu in the Viewer.

    Figure. Shortcut menu showing the alpha channel display options and the background display options.

Choosing a Background

When working with clips that have an alpha channel, you can choose different backgrounds that make it easier to see which areas of your picture are transparent. Translucent clips or generated text clips may be easier to see if you choose a background that emphasizes them, such as Checkerboard 1 or 2. If you’re compositing colored images, a contrasting color would work better.

Figure. Viewer window showing an image against a checkerboard background.

When a clip is rendered for export to tape, the background is always set to black. If it is rendered for export as a QuickTime movie, the background will still appear to be black, even if the alpha channel is exported along with the clip.

There are several backgrounds to choose from.

  • Black

  • White

  • Checkerboard 1

  • Checkerboard 2

  • Red

  • Green

  • Blue

Note: Red, Green, and Blue backgrounds are available from the submenu only when choosing View > Background.

To choose a background for viewing a clip
Do one of the following:
  • Choose View > Background, then choose a background from the submenu.

  • Choose the background from the View pop-up menu in the Viewer.

Superimposing Video When Preparing for EDL Export

In an Edit Decision List (EDL), a Super or Key track is the equivalent of a single superimposed video track in Final Cut Pro. EDLs can recognize only one Key track, so V2 is the only video track other than V1 that appears in an exported EDL. Transitions in track V2 are not allowed in EDLs and are ignored. If you want to use a transition in track V2, use keyframes in the opacity overlay to dissolve V2 up and down. (See Importing and Exporting EDLs.)

The opacity overlay in the Timeline appears as the “Key Level” in your exported EDL. Opacity keyframes, if they exist, are translated into values for the Key Level and are calculated in the EDL in relation to the timecode of the master tape. These numbers are for use by a video switcher. Some online systems can translate these for automatic use by some switchers, but more commonly they are used only as notes for the editor to use in an online session. These values and their locations appear as notes in the EDL.