There are 4 mattes in Keylight.
| 1. | Screen Matte |
| 2. | Inside Mask |
| 3. | Outside Mask |
| 4. | Alpha (Composite Alpha) |
The Screen Matte is generated by the Keylight algorithm after the screen color has been picked. It can be processed (clipped, eroded, etc) by the screen matte processing tools.
The Inside Mask is the hold out matte. It is used to confirm areas that are definitely foreground. If your subject has blue eyes and is being shot in front of a blue screen, this mask can be used to put back the eyes. This mask is taken from the InM input to Keylight. The embedded alpha channel of the foreground input can be added to this mask using the Source Alpha parameter in the Inside Mask folder.
The Outside Mask is the garbage mask and is used to remove unwanted objects (lighting rigs, etc) from the foreground. The mask is taken from the OutM input to Keylight. The luminance or the alpha of this input is set using the OutM Component parameter.
The matte used to blend the foreground and background in the final composite is the alpha displayed in the alpha channel of the composite. This matte is the combination of the screen matte, inside matte, and outside matte.