Using Alpha Transitions

Alpha transitions are wipes with animated media assets. With alpha transitions, you can easily create complex transitions that you can reuse and retime.

Having a logo or other visual element wipe across the screen to transition from one scene to another is a common technique. It is possible to create this effect in a Final Cut Pro sequence using alpha mattes, luma mattes, and a few video tracks. However, moving the transition, duplicating it, or changing its speed can be cumbersome. Alpha transitions make this common editing technique simple and easily reproducible.

Adding Alpha Transitions to the Timeline

You apply an alpha transition just as you would any other transition. After you apply the alpha transition in the Timeline, you can create the specific visual effect you want by building and modifying the transition in the Transition Editor window.

To apply an alpha transition from the Effects menu
  1. Do one of the following:

    • Click an edit point between two clips in your sequence to select it.

    • Position the Canvas or Timeline playhead at the desired edit point.

    • Position the Canvas or Timeline playhead on a transition that’s already been edited into your sequence.

  2. Choose Effects > Video Transitions > Wipe > Alpha Transition.

For more information about adding transitions, see Adding Transitions to Clips in Your Sequence.

To open the Alpha transition in the Transition Editor
Do one of the following:
  • Double-click the transition.

  • Select the transition’s icon, then choose View > Transition in Editor.

  • Control-click the transition, then choose Open from the shortcut menu.

About the Alpha Transition Controls

Use the following controls to create and modify the alpha transition in the Transition Editor window. For more information about the Transition Editor, see Refining Transitions Using the Transition Editor.

Figure. Viewer showing an alpha transition.
  • “Clip” clip well: This is where you place the main element clip that wipes across the screen revealing the next scene underneath it. This clip may or may not contain a straight alpha channel.
  • Clip Alpha Matte clip well: If the element clip that you dropped into the “Clip” clip well does not contain an alpha channel, you can drop a clip into this clip well to create an alpha channel from its luma values.
  • Wipe Matte clip well: The actual wipe animation between the two clips in the Timeline is determined by the luma values of the clip dropped into this clip well. Any white areas in this clip will display the first clip. Any black areas in this clip will display the second clip. Any levels of gray will display the appropriate mix of the two clips. The wipe generated by this clip well is generally invisible to the viewer because it happens underneath the main element clip. However, if you place a clip in this well only, you can create a luma-based wipe.
  • Scale Transition Asset Durations checkbox: Select this checkbox to scale the duration of the media in the clip wells to fit the duration of the transition in the Timeline.

    Note: Lengthening or shortening the duration of the media asset may result in a possible loss of temporal quality. You can deselect this checkbox to make sure the media assets retain their native duration regardless of the length of the transition in the Timeline. This may cause the media assets to be abridged if the transition in the Timeline is not long enough.

  • Reverse Transition Assets checkbox: Select this checkbox to reverse the transition.
  • Opacity slider and field: Use this slider to adjust the opacity of the element clip that is in the “Clip” clip well.
  • Mid-point Offset slider and field: If there is no clip in the Wipe Matte clip well, the effect will simply cut from the first Timeline clip to the second Timeline clip. By default, this cut occurs at the midpoint of the transition. The Mid-point Offset parameter allows you to roll the cut point to the left or right. This can be useful if the element clip in the “Clip” clip well completely obscures the screen at some point other than the transition’s center, making a background wipe unnecessary.

Creating and Modifying Alpha Transitions

You can use several different methods to create alpha transitions.

Asset Wipe Transition Using Three Clips

In this method, you drag three different elements into the three different clip wells to create an effect in which an animated media element wipes across the screen, revealing the next scene underneath it. This three-clip method generally ensures the best results with alpha transitions.

To create an asset wipe transition using three clips
  1. Drag the main element clip to the “Clip” clip well.

    Figure. Media element moving across the screen.

    In this example image, the media element moves across the screen, essentially “wiping” the frame from right to left.

  2. Drag the luma track matte clip to the Clip Alpha Matte clip well.

    Figure. Luma matte moving across the screen.

    The white area in this clip should be an exact outline of the media element you added in Step 1. White represents 100% opaque and black represents 100% transparent. Levels of opacity can also be assigned using shades of gray.

  3. Drag the wipe matte clip to the Wipe Matte clip well.

    Figure. Wipe matte moving across the screen.

    This clip determines the wipe between the two Timeline clips. Any white areas in this clip will display the first clip. Any black areas in this clip will display the second clip.

In the resulting effect, the media element wipes across the screen to reveal the next shot from right to left.

Figure. Series of still images showing the final wipe effect.

Asset Wipe Transition Using Two Clips

In this technique, you drag an element clip that already contains an alpha channel into the “Clip” clip well. This makes the track matte element in the Clip Alpha Matte clip well unnecessary.

Note: This method requires a video codec that supports alpha channels.

To create an asset wipe transition using two clips
  1. Drag the main element clip to the “Clip” clip well.

    Figure. Media element moving across the screen.

    This clip contains the main media element as well as an embedded alpha channel.

  2. Drag the wipe matte clip to the Wipe Matte clip well.

    Figure. Wipe matte moving across the screen.

    This clip determines the wipe between the two Timeline clips. Any white areas in this clip will display the first clip. Any black areas in this clip will display the second clip.

In the resulting effect, the media element wipes across the screen to reveal the next shot from right to left.

Custom Transition Using One Clip

You can use the animated luma values of just a single clip to create complex animated transition effects.

To create a custom transition using one clip
  • Drag a wipe matte clip to the Wipe Matte clip well.

    Figure. Wipe matte moving across the screen.

    This clip determines the wipe between the two Timeline clips. Any white areas in this clip will display the first clip. Any black areas in this clip will display the second clip.

Alpha Transition Using the Mid-point Offset Slider

Instead of using the wipe matte to determine the change from the first Timeline clip to the second Timeline clip, your main media element clip could momentarily fill the frame at some point during the transition. Using the Mid-point Offset slider, you can time the cut from the first clip to the second clip to occur exactly at the moment the frame is filled with the media element.

By default, this cut between the two Timeline clips occurs at the midpoint of the transition. The Mid-point Offset allows you to roll the cut point to the left or right.

To create an alpha transition with the Mid-point Offset slider
  • Drag the slider so that the cut between the two Timeline clips occurs while the main media element fills the frame.

Tip: You can use the Opacity slider to temporarily reduce the opacity of the media element (to make it easier for you to find the cut between the two Timeline clips).

Creating Your Own Alpha Transition Elements

Use the following guidelines for best results when creating your own alpha transition media elements.

Frame Rates

If you are creating transitions to be applied to interlaced content, create the media elements with progressive frame rates at double the frame rate of the content. This allows Final Cut Pro to regenerate the interlacing properly if the transition time is different from the clip time.

If you are creating transitions to be applied to progressive content, create the media elements at the frame rate of the content.

If you want to stretch the length (duration) of your transitions, you may want to create them at the maximum frame rate multiple possible. For example, a transition used in a 24-frames-per-second sequence could be rendered at 96 frames per second, which would allow you to stretch the transition to four times its native length without a loss in temporal quality.

Media Elements with Alpha Channels

For the best results, create a second clip that works as a luma track matte for the three-clip method. (See Asset Wipe Transition Using Three Clips.) If you do decide to use an embedded alpha channel, you will get best results with transition assets that do not have areas of semitransparency. Alpha channels should be rendered as Straight. Rendering the alpha channel as Premultiplied will yield undesirable results. If you are rendering assets from Motion, be sure to deselect the Premultiply Alpha checkbox in the Output settings.

Media Elements Without Alpha Channels

Even when you are using media elements without alpha channels, it is still important that you force the clip to render with Straight color at Millions color depth. Playing these clips in QuickTime Player will display unexpected results, but they will composite correctly in Final Cut Pro or any editing or compositing application.