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A major part of mastering After Effects is mastering the subject of Alpha channels. In particular, knowing how to solve problems with Alphas when they come up. Now one common problem is if you have an object with anti-aliased or partially transparent edge and part of the background color from behind that object has crept into that edge. Your first recourse is always go to the Interpret Footage dialog and set the Premultiplied pop-up to match the color of that background. But if that doesn't work, there is a plug-in effect, Remove Color Matting, which may also solve your problem.
Here is an object from the stock footage library that has an Alpha channel, but which has a little bit of an issue with the white fringe around parts of its body. I'll zoom in so you can see this in more detail. You see a little bit of white there and little bit white edge around here. I want to remove this white fringe. I will back down to 100% for now. Now I suspect since fringe is white that this was shot on a white background, I'll double-click it to open it up in its Layer panel and I'll set this to RGB Straight. It says take the Alpha channel out of the equation, show me just the color channels, now behold it is shot against the white background.
To get rid of that fringe, I'll select my layer and apply Effect > Channel > Remove Color Matting. Now Remove Color Matting defaults to attempting to remove black from an object and you will see it actually made this edge worse. Before, after. What you need to do is to enter the correct background color into Remove Color Matting. In this case I've already identified there was white background, I'll click OK and now you will see that my edge, my white fringe, has disappeared. Before, after. I'll zoom in again so you can see this in greater detail.
Here is the base. Before and after. Quite a nice improvement. Now another case where this comes up is if someone supplies you stock footage that has the Alpha channel separate from the actual image. Here is footage of the gun being fired and here is the separate matte that's been supplied by the stock footage house. You will see they have done a nice favor here cutting out the gun and attempting to reduce the black background that's in it that flame. To marry these two layers together, I need to bring up the Track Matte panel; F4 toggles between Switches and Modes and set the bottom, the footage to use the luminance of the Matte layer to cut it out and now I have my muzzle flash against an Alpha channel. There it is against the black background.
This looks pretty good so far but let's say I need to composite this against something else, maybe something with a white background. Now I'm seeing some of that black background from the original footage creep into and contaminate the edges of this muzzle flash. Well again, I'll use Remove Color Matting to attempt to fix this. In this case, it's default to black, it wasn't correct, because the footage was shot against and you can see where it's cut the black out, left me with just color; before and after. So this is the case where it works really well.
By the way, this would not have worked if I attempted to apply it back in my original source footage. I'll set my background color to white, OK, select my footage layer and apply Effect > Remove Color Matting, no effect. I'll select my Matte layer and apply Remove Color Matting, no effect. Remove Color Matting needs to work on something with an Alpha channel and this stock footage doe not have an Alpha channel. It fills the whole screen. Therefore, I needed to have the two in their own comp with Luma Matte applied creating their own alpha in this pre- comp and take that bundle and put it into a second composition to Remove Color Matting to form this magic.
I am running After Effect CS4, so I'll tab the Shift key to bring up the mini flowchart and you will see on my little stack on my two channels with the separate Alpha go into this Remove Color Matting comp and that's where I'm applying my effect. Now Remove Color Matting is good for solving problem with your source, but sometimes effects can introduce problems with the Alpha channel. To learn how to solve those, go watch the third movie on Channel Combiner. We'll talk more about Premultiplied and Straight Alphas.
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