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In the previous chapter we've stabilized this footage which was shot on a green screen stage. The next thing we need to do is key out the screen so that we can see our new background behind it. The process of keying is picking a color such as this screen background, making it transparent, but making sure that the action we want to keep these actors remain opaque, and preferably with a nice anti-aliased or feathered edge at the boundary between the two. Now if you did not follow along in the previous chapter, but you do have access to the Exercise Files go into the Comps folder and open up 05-Keying*starter3.
This is the same shot already stabilized for you and you can go ahead and key with that. In my case, I'm going to continue with a shot that I've already been tracking. I am going to go back early in the shot where I have a nice isolation on this actor and screen and apply Keylight. I can find it in the Effects menu underneath Keying. You can also search for Keylight in the Effects and Presets panel. Working with Keylight requires going through several steps. I'm going to show you the basic workflow that will get you a pretty good key most of the time.
However, there is other documentation out there on Keylight including a manual from The Foundry, the company who did create Keylight. The first thing you need to do is to find the color you want to get rid of. That's called the Screen Color in Keylight. I'm going to click on an Eyedropper icon and pick the screen fairly close to the actor. The reason I'm doing that is I want to pick up the lighting and shading closest to what I'm trying to cut out. If I pick somewhere in a far corner it maybe different in illumination than the area I really want to key near the actor. So I'll start there, and you might be tempted to say, fantastic, I'm done.
Well, not entirely true. You can see some of the wood here through his head. I'm going to turn off the background here. I had the Transparency Grid turned on, you can see part of Transparency Grid through his neck. Obviously, I've a little bit of work to do to really make this a perfect Alpha Channel where it's completely opaque where his body is solid. The best way to do that in Keylight is to change its view pop-up from Final Result to Status. Status is a very exaggerated version of the Alpha Channel. White is opaque, black is transparent, and anything that lies in-between the two will be represented as gray.
So you see I've a lot of partial transparency in the shot that I need to clean up. To do that I need to go into the Screen Matte section of Keylight. I need to alter the Black point and the White point of the matte or the Alpha channel to get rid of any ambiguity and make sure that background is completely transparent and the body is completely opaque. The general philosophy of these parameters is make the minimum change you have to, to get the desired result. If you overadjust you'll get a harsher edge between your foreground and background.
The first parameter I'm going to tackle is Clip Black to clean up the background. I'm going to use Clip Black to get rid of my worst problems, then use other parameters to clean it up from there. I'll click and start dragging, you'll see a little goes a long way. With the philosophy of making a minimum change I'm going to hold Command on Mac, Ctrl on Windows so I can scrub in finer increments. I'm going to focus here on getting as much black around the actor himself as possible, but not worry about trying to get the whole thing black, just make sure I've got a good key around the actor.
I am going to back off a little bit more here. Once I've gotten rid of the majority of the black I will then go up and use the Screen Gain parameter to make any further little tweaks and adjustments. So this is my baseline. Now I'm going to improve it by slightly increasing Screen Gain and making the rest of the background completely black around say there. Now since I do have a problem with this corner wanting to peek through, an ideal approach would be not to try to key out this area where no actor ever goes, but instead create a garbage mask around the actors, so I'm only keying the area close to them and masking out everything else.
And I'll discuss the concept of the garbage mask more in a separate sidebar movie. But right now let's press ahead. I've got black where my background is supposed to be. That's good. Now we need to take his head that's supposed to be opaque and turn it white, and to do that I need to reduce the Clip White parameter. Again, I'll hold Command on Mac or Ctrl on Windows to scroll by very small increments and you'll see I'll either get white or green-colored white. Now the green shading is basically Keylight telling me this is an area where it's detected green spill from the background and it's going to perform some color correction to clean that up.
So it's still good. It still has a nice solid Alpha. It's just telling me ahead of time it's going to perform some color correction. So let's key point on Clip White until I filled out his neck right in through here, down to about there. I don't mind these gray areas around his hair, because those are supposed to be semitransparent. I want a nice feathered Alpha Channel where his hair is soft. Let's go ahead and clean the rest of his neck and that looks good.
Never look at just one frame, go ahead and scroll through more of a shot to make sure you're getting the desired result. I'm seeing that it might be getting little bit of transparency here. To clean that up I might need to back off my Screen Gain a little bit or further increase Clip White. I've a little bit of sparkling down here in the corner; I can clean that up later with a garbage mask. Again, I'm trying to make the minimum change from the defaults and still get a good key. Once I'm satisfied that I've got a nice black area for the background that's transparent, a nice white or light green area for the foreground so it's be opaque.
I'll switch my Status back to Final Result. And now I see that I've a much nicer Alpha channel and I'm not seeing the checkerboard transparency grid through the actor. I do have some edge issues, but I'll clean those up in the next movie. Scroll through, this little line here is again from the very edge of the video. A garbage map; we will fix that up. And there is my keyed actors; a good start. Again, I want to emphasize that I'm really worried about the edges around the actors and their hair.
I don't want to make that too harsh, because otherwise it would be very obvious that they have been cut off from a different background. If I decrease Clip White too much you'll see that I start to get a really harsh looking edge around the hair. I'll undo. Same if I increase Clip Black, I'll start chewing into the hair. Of course, if I invert the two I've real problems. If I go too soft, then I start seeing tracking dots. So I want to use the minimum value change I can to still have a good key and nice anti-aliased edges particularly where there is fast motion like those guy moving quickly.
You will see where he is moving quickly I do indeed have partial transparency around his motion blurred shoulder and that's exactly what I want. Okay, there is our basic key. Let's improve it and do a little bit of color correction in the next movie.
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