Using alpha channels

Depending on the format and how the image was created, an image might contain an alpha channel. An alpha channel is a grayscale image that is added to the color channels (such as the red, green, and blue channels in an RGB image) that make up the image.

You can use an alpha channel to create and store a matte. As with a matte, an alpha channel enables you to define a shape for the image object. For example, an image of a flower displayed without an alpha channel might be shown in a white rectangle, which would show up against a colored background. By using an alpha channel in the shape of that flower, only the flower is visible.


Original image, image with an alpha channel, and without

To use alpha channels:

1. Select the image object.
2. Choose Window > Properties.
3. Select one of the following:
• No Alpha if you want the image to retain its original shape out to the edges of the selection border. If the image is not rectangular, the remaining area out to the edges of the selection border is filled with white.
• Use Alpha Channel to use the image's alpha channel as a matte, if the image was imported with one.
• Build Alpha From Image to create an alpha channel using the luminosity of the color in the image. With color images, the effect is equivalent to converting the color to grayscale, and using the grayscale as the alpha channel. However, this option is most useful for removing the white background from simple, black-and-white line drawings and for using a grayscale drawing as a shape.