Autodesk 62205-051408-9001 User Guide - Page 278

10 Using Masks In the following illustration, Invert is enabled in the Global Apply Mode settings. The masks and the alpha channel of the layer are inverted. Draw Mask View Mode w/Background Alpha View Mode To use one or more masks, select Intersect and enable Invert in the Global Apply Mode settings. Invert Intersect The masks remove the area inside the mask shape and keep the area outside the mask shape, like a "garbage mask." Using Masks with Keys Use a mask operator to mask out unwanted portions of a key (for example, a rig in a shot or "dirt" left from a key). "Garbage mask" applied "Dirt" on matte To clean up the matte, a mask operator can be applied above the Keyer operator in the stack. Note: Since the mask and the keyer both affect the layer's alpha channel, the stacking order (of the mask and keyer) does not matter. You can also apply a mask operator to remove a rig or other problem area prior to pulling the key (applying the Keyer operator). In the following example, Invert is disabled and the mask removes the area outside the mask shape and keeps the area inside the mask shape. This is a "holdout mask." Mask operator Keyer operator Original shot with rig 262