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Chroma key controls

From: Compositing with Premiere Pro CS5.5

Video: Chroma key controls

A chroma key is an absolutely standard effect type and it's been in use for years and years, either using live vision mixers or in post production in the way that we're working here. The principle is extremely simple but the art and craft of getting a good key is something that will take a little time for you to develop. In particular, you'll find that really clean media will give you a good key quickly, but if you've got more complicated, what should we call it? Dodgy media that is perhaps badly lit or has some spill from the background into the foreground.

Chroma key controls

A chroma key is an absolutely standard effect type and it's been in use for years and years, either using live vision mixers or in post production in the way that we're working here. The principle is extremely simple but the art and craft of getting a good key is something that will take a little time for you to develop. In particular, you'll find that really clean media will give you a good key quickly, but if you've got more complicated, what should we call it? Dodgy media that is perhaps badly lit or has some spill from the background into the foreground.

In these kinds of situations you'll need to play around a little bit to get the result you need. I'm just going to start off explaining Chroma keys using the regular Chroma key effect rather than the more advanced Ultra effect. Because the Chroma key effect is a little bit simpler to meet these settings. And although this is Windows only effect at least you'll get an idea of what the controls are. I've got a sequence set up here where I've got a color wheel set up and there's a gradient along the bottom. You can see as soon as I put the key on it's giving me some transparency on this gradient.

It's not doing a great job of it but what's happening is because the color, by default, is white, and my gradient is white, it's picking out areas of that gradient and thinking, oh, I suppose that's supposed to be transparent. Behind this color wheel image I've got a, an animated lightning bolt background generated in auto effects. I just put this on here to make it nice and clear when we're making a hole through our image. Essentially all you need to do is pick a color range and in theory the Chroma key will do the rest. So I'm going to either click on the color picker and manually choose a color or, more likely I'm going to use the eye dropper and I'm going to click on something that I think I want to make transparent.

So maybe I'll go for the blue range of this color wheel. Now you'll see right away that even having clicked into the picture, haven't got much of a key. There's a tiny little bit where I clicked, a little tiny tear that you see inside my color wheel. I'm just pressing the grave key as it's called, looks like a backwards apostrophe, to go full screen with these panels. So what I need to do is expand the range of colors that are used. Now more advanced keying applications will give you plus and minus eyedroppers to add to the color range or subtract, or you might get a slider control to choose from a range of hues. Here, in this more simple chronic effect, I can just expand the similarity range. And you can see, as I do so, it's tearing into my color wheel. I'm just going to expand this a bit so you can see the image a bit more clearly. Let me get rid of these other sequences.

Okay. So you can see in the background there, you can also see that the curve is still applied from the color wheel. Of course I haven't chosen black as my transparency color. If I choose my Eye Dropper and click into the black background, right away, I'm going to get a massive, effectively a lunar key, a large area key where the black background of my color wheel, which is being used as my key color.

And you can see because I've got my similarity set quite high. I'm getting a big chunk of the middle of the color wheel as well. If I drag this back out far enough. There you go. That's the inverse of the selection I had in the beginning. Whereas the beginning with white selected I was getting these white areas transparent now I'm getting the black areas. It's not doing a fantastic job of making the selection either. So if I go in now and maybe choose the green, you can see I can just expand this out and get a larger area and ordinarily if you were filming somebody in front of a green screen or a blue screen you'd have a solid area of color and you'd expect to get a pretty reasonable key. Of course one problem with this particular effect is you can see this tearing I'm getting in the image.

This is just not a clean edge for my key. And I might want to do some softening on there to make it a bit more acceptable. If I adjust the blend controls, here, I am going to get that kind of softening between the selected and unselected parts of the image. But you can see, if I go beyond my own similarity setting I'm starting to get even the black background there taken in and that's just no use at all I'm also going to begin to get that would go at 100% whole other areas of color not because I've selected them as a key color but because I've blended so much on the edges.

Which is no use at all, so let's bring that back down, and just have a little bit of smoothing. If I adjust the threshold, I'm going to get more or less, of the background included. Now let me increase my similarity a bit here, so you can see the results of this. As I adjust, it's pretty subtle, but you can see, just in this area where the, the gray area of the key that I'm creating. Effectively, the alpha channel that I'm creating is being affected by me adjusting this threshold.

What this is doing is adjusting the subtle gray tones, or rather the selection of the gray tones from within my key. That is, how much that's quite green or not green. It's not a clean cut edge in this image, of course, because my color wheel blends seamlessly from one color to the next. If I were using a piece of media that was somewhat in front of a green screen this would be quite useful for avoiding the kind of edging and fringing that you can sometimes get. The cutoff adjustment, well you can see for yourself if I increase the threshold a little bit here, this sets an absolute cutoff for what is and is not keyed.

I need to keep this below my similarity threshold again, but you can see. What this is doing is finding areas of the image, remember this is just an image of a color wheel, it's not an active color wheel, and it's setting a point beyond which it's just not going to key it at all. The Smoothing control is for dealing with this aliasing, as it's called, these jagged edges in the image. If I choose Low, you can see some smoothing occurs, if I choose High Quite a lot. And if I really want to see what's going on, I can turn on viewing the mask, and you can see what I've got here is a pretty awful key.

And this is partly because it's not a great effect for the new ultra here, which is Mac and PC, does a better job of it, and partly because this is completely the wrong kind of media to use. For keying but it is very useful media to show you the core concept of a Chroma key which is that everything I'm doing here could be applied to the Magenta the R and G Red regions, the Blue regions. It doesn't really matter what the color is. A Chroma key will be able to pick it up. And those are the core controls of using a Chroma key with Premiere Pro.

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Compositing with Premiere Pro CS5.5

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