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An example luma key

From: Compositing with Premiere Pro CS5.5

Video: An example luma key

Sometimes you'll find that you've got media assets that you perhaps would anticipate having an alpha channel. And then, at the last minute, well it turns out you don't. Here, for example, I've got some media that has no alpha channel. This is just an animated cloudy, foggy area generated in After Effects. But the file's got no alpha channel at all. And I can see that by selecting the item in my Project panel. And looking up at the top. Now, never mind that the file is called Clouds No Alpha. If this had alpha, like for example, the other example of the clouds has, it would say so in the information at the top of the panel. So, let's presume we don't have an alpha version of this and we don't have access to After Effects to go in and create some alpha, how can we get this to work in front of our Background layer? Here, on the Timeline I've got a, a dodgy bit of green screen that I created a few days ago. And perhaps to make the picture look even worse, it'll be interesting to put some smoke in the foreground.

An example luma key

Sometimes you'll find that you've got media assets that you perhaps would anticipate having an alpha channel. And then, at the last minute, well it turns out you don't. Here, for example, I've got some media that has no alpha channel. This is just an animated cloudy, foggy area generated in After Effects. But the file's got no alpha channel at all. And I can see that by selecting the item in my Project panel. And looking up at the top. Now, never mind that the file is called Clouds No Alpha. If this had alpha, like for example, the other example of the clouds has, it would say so in the information at the top of the panel. So, let's presume we don't have an alpha version of this and we don't have access to After Effects to go in and create some alpha, how can we get this to work in front of our Background layer? Here, on the Timeline I've got a, a dodgy bit of green screen that I created a few days ago. And perhaps to make the picture look even worse, it'll be interesting to put some smoke in the foreground.

Well, this is exactly what the Luma key is for. If I bring my Luma key effect, drop it on the Foreground layer, right away, you can see I've got my effect. If I play this through now. > > This is meant to by white, right? > > There you go, that's my incredible stage performance. So, you can see that it's done a really lovely job of putting the Luma key in, instead of the original alpha. And I can still make some adjustments here to change how much of the foreground and the background you get. But the more that you reduce the thresh hold, you'll see, if I bring this in a bit, we're beginning to get more and more fringing.

We're getting more of the black part of the image which looks less and less convincing. If I bring this in even more, we're definitely getting into the Harry Potter territory of CG clouds. Which is perhaps, perhaps not what you're really looking for. It could be exactly what you're looking for. But bare way, you're going to start to get this fringing if you depend on the Luma key by adjusting the threshold. And of course, equally, we've got the cut off. So, if I really wanted to give this hard edges, I could increase the cutoff point.

So that it was very, very close to my threshold. The threshold is just defining how dark a pixel needs to be before it's interpreted as transparent or rather before that luminance level is translated into the alpha channel level. And the cutoff just give me no grey scale, if you like, no softening of that adjustment. If I set the cutoff low, then I get lots of subtle shades. If I set it closer and closer to my threshold, if I, if I set this to a full 37, just one point beyond my threshold or below it, you can see I'm getting clouds that look almost like weather report clouds.

Again, that could be exactly what you're looking for. This isn't a question of necessarily achieving the most realistic results. It's about fulfilling your creative aspirations. You'll notice, by the by, that the Luma key effect inside of Premiere Pro, is not a coder enabled one. So, even if you have the right graphics card, you're going to get a red line. Saying that though, in Premiere Pro, nowadays the red line doesn't necessarily mean that you'll have to render to play it back. It just means that the quality isn't going to be that high. Again, if I set my threshold up and I set my cutoff down. I can get pretty convincing results by using original media that doesn't actually have an alpha channel.

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Compositing with Premiere Pro CS5.5

23 video lessons · 1205 viewers

Maxim Jago
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