Invert a key when it is easier to exclude rather than include colours in a secondary.

To invert a key:

1. In the Secondaries menu, toggle the Matte/Key-in option box to display the Key Invert panel, and then enable Invert.

   
   

   The area of the key that was opaque becomes transparent and the area that was transparent becomes opaque.

   Before	After
   Images courtesy of Hungarian Academy of Film & Theatre, 3rd year