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For the last section, we want to look at our composite, and these three toggles down here. None of these toggles have any effect on the Alpha Channel at all. They all affect just the pixel edges, the blending edges around the perimeter of the matte. The first one, Screen Subtraction, defaults on. It's subtracting the backing color from the edges of your composite. If you turn it off, all of a sudden we can see lots of nasty, green pixels. Normally, you are going to leave this on.
This is not Spill Suppression. The IBKGizmo has no Spill Suppression. And if you have a green spill in the interior of your character, you are going to want to add your own spill suppression after the IBKGizmo Node. Now, let's take a look at this toggle, Use background luminance. Turning this on and off causes the IBKGizmo to factor in - remember, the background is connected here - so it factors in the background pixel colors into the output for the foreground.
Turning it on, it will actually brighten or darken the edges based on whatever the background is. Now this may or may not improve your composite. This last toggle uses the chroma, or color, from the background to influence the edges of the composite. You may have one or both turned on, or both turned off, whatever looks best. In this particular case, having them both off seems to look nice. In our next movie, we are going to take a look at how to fill in those transparencies in our core matte.
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