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Understanding luma keys

From: Compositing with Premiere Pro CS5.5

Video: Understanding luma keys

When I use the word key or keying, what I mean in this context is using some kind of reference to define the transparency, or the opacity, or effectively the alpha level for pixels in my image. A classic key is, of course, the Chroma Key. The traditional weather reporter in front of a blue screen and anything bright blue is made transparent and you see another layer of video behind it. There is also another system of keying called Luma key, which works instead on the brightness, the luminence of individual pixels.

Understanding luma keys

When I use the word key or keying, what I mean in this context is using some kind of reference to define the transparency, or the opacity, or effectively the alpha level for pixels in my image. A classic key is, of course, the Chroma Key. The traditional weather reporter in front of a blue screen and anything bright blue is made transparent and you see another layer of video behind it. There is also another system of keying called Luma key, which works instead on the brightness, the luminence of individual pixels.

It's very easy to apply a Luma key inside of Premiere Pro, and the results, sometimes they're great, sometimes they're not so great. It's one of those effects where you get the most magnificent results by planning for it and shooting, knowing that your shadows, dark part of your pictures are going to become transparent. If you plan for it, and you get nice hard lighting and strong shadows, you can get some really powerful effects where content or subjects, performers seem to emerge from another layer. Putting a Luma key on is just a question of dragging and dropping.

And straight away, you can see here in the example, in my background, I've got some color bars and in the foreground, I've got this black and white gradient. Putting the Luma key on immediately puts some transparency into the gradient. You don't get that many controls. You've got a threshold which will define at the point to which the foreground becomes visible and the point at which the background becomes visible, that's pretty self-explanatory. And there's also a cutoff so you can say, beyond a certain point it's just going to be visible or invisible. You can see I can even invert that by crossing my own threshold and that's pretty much all you get.

You can get some interesting results by combining it obviously with more interesting media. if I switch off my background here, for example, in my foreground now, I've got my balloon shot. If I turn off the Luma key, you can see here's the original. If I put on my background, so you can see, I've got my color bars, and turn on my balloons. There you go. You can see, I can layer these together, and I'm getting a partial key here. I'm getting bits of the clouds, the balloons, which are more solid shapes, and I'm using the fact that there's a reasonable variation in the luminence between the clouds and the sky to create the key.

Now, if you color correct, you can enhance the difference. You could always bring down the midtones to get a stronger key, and you'd get a much better result with this. But even with a partial key like this, if I put a regular sky pattern behind it instead of those nasty color bars, you can see right away, I get a much more interesting composition. Now, of course, these are still frames so they are not quite as alive and convincing as they would be if they were moving. But again, you can see there's my original foreground and here is the background.

But the two together create a much more interesting, compelling image. Since we're working with luminance here rather than chrominance, I can probably just get away with throwing on a Luma curve. And then, if I just take away my background for a moment so I can see what's going on here, just by turning off the eyeball for the layer, I can switch my Luma curve output to Luma, which is just going to give me a clear indication of what's going on without being distracted by the chrominence.

And then, maybe I can pull in the shadows on this a little bit while keeping everything else nice and bright. And what I'm looking to do here is deepen that blue sky while keeping everything else looking fairly natural. And if I set this back to composite, that's probably going to give me a, a much better key. If I turn on my background, you see I'm going to get more of the sky behind this way, because I've deepened the darker parts of my foreground image, but the foreground, the brighter parts, are nice and clear. So, combining the Luma key effect with something like a Luma curve gives you a little bit more control, simply because the Luma key effect itself doesn't give you any of that preempt color correction adjustment that you might need. Still, using the Luma key is well worth the investment of your time. Some of the looks that you'll get by using the Luma rather than a Chroma to define your key are a little bit more subtle, a little bit more nuanced than perhaps you'll get with Chroma keys.

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Compositing with Premiere Pro CS5.5

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Maxim Jago
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