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While the AddMix node is a favorite for blue screen and green screen compositing, because of the added edge control, it's not our only choice. Since we have here the pre-multiplied foreground, we now have a four-channel, pre-multiplied image. That means I could use the Merge node, because the Merge node expects a four- channel, pre-multiplied foreground. So I'm going to type M on the keyboard to select a Merge node, hook that up to the pre-multiply, hook the B side up to the background, and switch my viewer over to the Merge node. and there is my composite.
An important common feature of the Merge node and the AddMix node is they both output the alpha channel after the composite. There is the Merge node, and there is the AddMix node. However, the next node, the KeyMix node, does not. I'll put my viewer back to RGB, and let's go get a KeyMix node and see why. From the Merges tab, we'll get a KeyMix node. Now the KeyMix node expects the alpha channel or the key to come in on the mask input, so we'll hook that up here, because this is our master key.
On the A side it wants the un- premultiplied color-corrected foreground. We will hook that up there, and I'll hook the viewer up to KeyMix node. There we go. The B side goes directly to the background, and there is our composite. I'll select the Merge node and connect it to the second input of the viewer, and as I toggle between the two inputs, you can see that the composite is absolutely identical, because these are mathematically equivalent.
However, the KeyMix node does not output the alpha channel like the Merge node and the AddMix node. I'll put my viewer back to RGB. And of course, the KeyMix node has absolutely no adjustments at all. Since no one keyer can really do the job alone, this technique of combining multiple keys into a master key is the proven approach to divide and conquer your blue screen and green screen composites.
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