From the course: Nuke New Features Consolidated

Keylight: A basic key and composite - Nuke Tutorial

From the course: Nuke New Features Consolidated

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Keylight: A basic key and composite

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- Keylight is a color difference keyer developed by the computer film company that is distributed and supported by The Foundry and is now available for Nuke. We'll start by setting up a basic composite. You can pull in these two elements from our Keylight media file. So we'll select the green screen boy, come up to the Keyer tab, and select the Keylight node. Keylight hooks into the source input, which is where the green screen goes, and then we hook up the bg input to the background. The first step in Keylight is to tell it what the backing color is, what they call the screen color. And here is the screen color here, first return on the eyedropper, make sure that's turned on, and then in the viewer, "Command + click" to select the color. Now, Keylight is twitching because it's actually sampling the output of Keylight, which is being changed based on my selection. So we have to use the Nuke keys "Alt + Command" in order to sample the incoming image that is directly off the source image right here, okay? And of course if we want to sample a box, we'll do "Shift + Alt + Command" like this. And of course, if you are using a Windows machine or a Linux machine, you'll be using "Control" instead of "Command". When you select the screen color, you are selecting a single RGB value, even if you use the sample rectangle, it's simply taking the average of all the pixels inside the rectangle. Now to make sure that you don't disturb your selection you can turn the eyedropper off, and that way you won't accidentally change your screen color. So when you first start, the view is set to the final result, and this is Keylight speak for the premultiplied output. If you want an unpremultiplied output, it would turn this on. Let's take a look at the view pop-up menu. There's a lot of different views here, the source is the incoming green screen image. The source alpha means show me the alpha channel of the incoming green screen image. Now here, there's an issue. That is not the alpha channel. If we hook the viewer directly up to the green screen and then switch the viewer to the alpha channel, you can see this is a typical three channel green screen image with a black alpha, but yet somehow, Keylight thinks that it's white, 'kay? So if the source input has no alpha, Keylight will tell you the alpha channel's white. However, if the incoming image is a four channel image and I'll fake that here by putting this in line, I've now added a real alpha channel with some content. Keylight sees that and passes it through correctly, but if it's a three channel image, just remember Keylight will tell you it's white when it's not. The next view we'll take a look at is the screen matte. The screen matte is, in fact, the matte that is created by Keylight and there's lot's of other things to see in the viewer. For example, the inside mask and outside mask, these are holdout and garbage mattes which we'll take a look at later. The status is a very interesting and important view. The status sorts all the pixels of the matte into three categories: 100% solid or white, 100% transparent or black, and everything else, all the semi-transparent pixels, are simply set to a 50% gray. And this is a diagnostic view that will help you later. We'll see how. The final result as we saw before, is the premultiplied output, don't forget about the unpremultiplied option, and then there is of course the composite. In the next video, we'll learn about the screen controls.

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