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Now let's take a look at Bias. We will start by looking at the Alpha Bias. We'll switch to this picture here, close that Keylight and open this one. The Bias controls are right here and they are designed to cope with seriously discolored elements. I didn't have a green screen discolored badly enough to demonstrate this, so I have just borrowed the one from the Keylight tutorial. The first one we will look at is the Alpha Bias. If we pull a key, we'll sample our image, pull our key, switch to the Composite, because of the serious red Bias in the green screen we got a terrible matte and a semi-transparent composite.
The way we tell Keylight about this red color bias is we turn on the Alpha Bias eyedropper and we do a sample in the foreground. There, much better. Now, with Keylight, every one of these samples results in a single value. So if you sample different areas, you will get different results. So you might look around for the best overall sample to use. Now we will take a look at the Despill Bias. We will put this one in the Viewer, close that Keylight, and open up this one, and home the Viewer.
The Despill Bias here is designed to cope with serious discolorations caused by the Despill algorithm in Keylight. So we'll pull a key, and set it for Composite, and we have this rather discolored hair here and over here. So we will zoom-in a little bit. You will notice the Despill Bias is ghosted out at the moment. That's because the Use Alpha Bias for Despill is enabled. This connects the Alpha Bias to the Despill Bias.
If I turn this off, now the Despill Bias is un-ghosted and can be set independently. This is the way you use it if you only have a problem in the Despill areas. So I am going to enable the Eyedropper. I am going to use Alt+Command because I want to sample through to the Source Image, not the Keylight Output. So I am just going to do a little click and fix, and again, this is very sensitive to where you sample. So if you sample in different areas, you're going to get wildly different results.
So I will put that back where it belongs. You may also open up the Color Picker and dial-in the color you want this way. Next, we will see how to use Keylight's Screen Replace function.
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