Interpreting Alpha Channels

In some cases, certain file formats may be interpreted incorrectly. For example, if you notice fringing or other unusual edge artifacts in your image or clip, change the footage channel settings.

You can modify an interpretation for each layer by selecting or changing the type of alpha channel present in the footage item, inverting the alpha channel, or turning it off altogether.

Alpha channels are displayed over a background color or transparency, depending on the view mode. For more information, see Using View Modes.

To change a layer's alpha channel settings:

1. Select the layer in the active viewport.
2. In the Workspace panel, expand the appropriate branch, composite, and layer, and then select the Footage or expand the Footage operator and select the footage from the Footage Library.



The Footage Controls panel appears.



3. Click Source.
4. Use the options under the Channels & Alpha heading to determine how the alpha channel is interpreted. You can use the following interpretation modes.

Color

Ignores the alpha channel information present in the footage and displays only the RGB color information.

Alpha

Copies the RGB color information into the alpha channel of the footage.

Color+Alpha

Interprets both the color channels and the alpha channel.

If the image contains no alpha channel, it appears solid white in Alpha View Mode (because there is no transparency information). When Alpha is selected in the Channels & Alpha group, the color information of the image is copied into the "empty" alpha channel.

This is similar to creating a luma matte from an image with no alpha channel. The transparency information generated from the Alpha option can be used as a matte or stencil for another layer.

Guess

combustion determines the type of alpha channel present in the footage. This option is used when the footage file is first opened.

Inverted Alpha

Inverts the alpha channel. When checked, pixels that were opaque become transparent, and vice versa.

Premultiplied with

Interprets the color channel as premultiplied.

Premultiplied color channels combine the color information with the alpha channel information by using a specific color as a base, almost always white or black. Deselect this option to interpret the alpha channel as a straight alpha channel (that is, a 256-level grayscale image embedded in the file).

Enabling this option helps eliminate a problem that occurs when an image has already been composited over a background with an alpha channel, but then saved with that same alpha. So if you see a fringe around a composited image, try enabling the Premultiplied option in the Footage operator for that layer.

Note: Make sure to set the Premultiplied color to the color of the fringe. The Guess option also attempts to guess premultiplication.

Color box

Specifies with which color the RGB channel is premultiplied. Click the color box and select a color using the picker.