Using Masks to Replace or Modify Alpha Channels

The mask filters allow you to replace, contract, or expand a clip’s alpha channel. If a clip has no alpha channel, a new one can be assigned to it using a still image or an alpha channel copied from another clip.

Mask Filters Available in Final Cut Pro

There are several mask filters you can use to modify alpha channels in a clip:

  • Mask Feather: Lets you selectively feather the edge of just the alpha channel of a clip, making it softer, without blurring the image channels of the clip. The Mask Feather filter works with other filters that generate alpha channel information, allowing you to feather the edge of a mask created with the Mask Shape filter, for example.
  • Image Mask: Uses the luma or alpha channel in one clip to create an alpha channel in another. The clip used to create the alpha channel can be a still image or a QuickTime movie. Using the Image Mask filter, you can create complex alpha channels; for example, you can put the kind of rough border used in art photography around a video clip. The alpha channel created with the Image Mask filter can be inverted, if necessary.
  • Mask Shape: Allows you to create an alpha channel in a clip that consists of a simple shape—a diamond, oval, rectangle, or round rectangle. The solid part of the clip that remains after applying the Mask Shape filter appears inside the shape that’s been created. The vertical and horizontal scale of the shape can be altered, as can the mask’s center. The Mask Shape filter can also be inverted to reveal areas of the clip outside the shape.
  • Soft Edges: Allows you to feather the edges of a clip. You can extend the softening of the left, right, top, and bottom edges of the clip into the clip by different amounts. The feathering appears as a gradient that always extends to the edge of the clip. The Soft Edges filter overrides all other alpha channel information created by any other filters appearing before it.

Example: Using the Image Mask and Mask Feather Filters

In the following example, a still image is used to create an alpha channel in another clip.

To use the Image Mask and Mask Feather Filters
  1. Open the clip to be masked in the Viewer.

  2. Choose Effects > Video Filters > Matte > Image Mask.

  3. Click the Filters tab in the Viewer.

    Figure. Viewer window showing an example of a grayscale TIFF file that has a white center surrounded by a rough black border.
  4. Drag the Border.tif clip to the Mask clip well of the Image Mask filter.

    Figure. Filters tab showing a clip being dragged into the clip well of the Mask option.
  5. Choose Luminance from the Channel pop-up menu.

    Note: When using a clip’s luma to define transparency, white is used for 100 percent opacity, and black is used for 100 percent transparency.

    Figure. Viewer window showing that the resulting clip now has a border that matches the clip placed in the clip well.

    You can further modify this new alpha channel, blurring it with the Mask Feather filter, for example.

  6. Choose Effects > Video Filters > Matte > Mask Feather to apply the Mask Feather filter to the clip you want to blur.

  7. Open this clip in the Viewer, then click the Filters tab.

  8. In the Filters tab, drag the Mask Feather filter so it appears below any other filters that may be creating or modifying the clip’s alpha channel (such as the Image Mask filter that’s already been applied).

  9. Adjust the Soft slider to soften the edge of the clip’s alpha channel.

    Figure. Viewer window showing the resulting masked composite image after adjusting the Soft slider.