Think of garbage mattes as a way to cut up the image – like making a jigsaw puzzle, only hopefully, with not too many pieces. I’ve used them to exclude areas from a key. So, if you had a device with a green reflection on your table, you could use a garbage matte around it so nothing would be keyed in the green area. Of course, you’d still see the green. Another way to use them is when you have an unevenly lit green screen. You break up the frame into sections and key each separately. By feathering the edges, the various keys join together seamlessly. This is also helpful when you have tenuous edges like hair. I suggest you shoot a test and try to key. Use the test to help determine light and camera placement. For instance, a higher camera will see less of the green screen and more of the table. And if you shoot from your POV, you won’t see any green – just table and, perhaps, some floor in front of it.
John
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