About Masks

Use a mask to isolate a region of an image, and make the rest of the image completely transparent. The mask affects the image's alpha channel, which is where the transparency information is stored. You can use a mask to create the alpha channel, or add to an existing alpha channel.

The inside of the mask has an opacity between 0 and 100%, while the outside has an opacity equal to 0%.

When you invert a mask, you make the rest of the image completely opaque. You still control the opacity of the inside of the mask.

You can use the isolated region from a mask in a composite. For instance, use masks to make the sky in a foreground clip transparent and then place a more dramatic sky in the background.

If you simply want to isolate a region of an image without affecting the transparency, use a selection instead of a mask. When you apply an operator (such as a Color Corrector) above the selection, only the isolated area is affected and the rest of the image is still visible. For more information on selections, see Paint: Drawing Selections.

When you key a blue-screen image, you can use masks to isolate the regions you want to either key out or leave in the key. As you clean up the matte, you can add masks before or after the Keyer operator since you are affecting the alpha channel in either case.

You can also use masks to create a transparency for elements of a clip that cannot be keyed out (for instance, if the subject was not shot in front of a blue or green screen).

To better define the regions of transparency for a clip, you can animate the shape and opacity of masks.