You can use garbage masks to key undesired elements in an image and to do rotoscope work. A garbage 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 you can use a mask to add to an alpha channel.

Garbage matte A matte that isolates unwanted elements from the primary element in an image. Garbage mattes are also referred to as garbage masks.

Auto Tangent When selected, the mask is created with tangents.

Intensity Defines the alpha value defined by the mask.

Opacity Defines the transparency of the mask.

Invert The area outside of the mask is covered by the garbage mask settings.

Edge Distance The inner or outer offset from the edge of the mask.

Edge Position The position of the edge handle from the first control point of the mask.

Edge In/Out The inner/outer offsets from the edge of the mask.