042 Screen leveling for even better luma keys

- [Steve] Hi, this is Steve Wright, welcoming you to this week's Nuke Nugget, How to Use Screen Leveling to Pre-Process Your Clips for a Better Lumakey. When making a lumakey with a plate that is not uniform, and they're never uniform, you have to crank harder on the key to clear the backing region around your target, this chokes in on the key, resulting in hard edges. Here we'll see how to first, level the plate, to make it much more uniform, so when we pull the key, the resulting edges are much nicer.

So let's push in and see what we've got here, and we're going to make a little more room for our picture, okay, we'll hook up to the De-Noise Node, because, of course, the first thing we're going to do is de-noise our picture. Now to show you the non-uniform screen that we have here, it's dark and saturated at the top and light and desaturated at the bottom. So we can use the Sampler Node to do that, the problem with the Sampler Node is it only will strike a horizontal scan line across the picture, and I want to measure it top to bottom, so, I am going to have to first, flip the picture on its side.

Then I'll run a little bit of a blur, because I don't want to see all the noise and the chop in the picture, I just want to see a nice gradient of what my backing region looks like. Then we'll open up the Sampler Node, and there you see it, so this edge over here is the dark, saturated on the left side, which is the top of the picture, and over here is the brighter, desaturated bottom of the picture. Alright, so, back to our picture, and I'm going to close the Sampler Node to get him off the screen.

So here's the procedure, we need to make a matching gradient that is the same as this background plate, and that's pretty darn easy to do. First, we'll make a ramp, now this ramp goes from one to zero, there you go. Okay, then I'm going to use a color look up node to recolor the white area up here and the black area down there, to give me a matching blue gradient to the sky.

Now, we don't need this ramp anymore, here's how we'll do that. We'll hook the viewer up to both the original image and the ramp, there we go, one, two, one, two. We'll then go to the White Controls, and we have the de-noise on one side and the ramp on the other. The De-Noise Node, here of course, is our original image. So I'm going to scoot this edge over here, and my mission is to make my ramp up here match the sky right there, and then down at the bottom, make the ramp match the sky down here.

And I'm going to do that with this color look up node right here, I'm going to hook my viewer up to it, and nope, broke my guy here. Alright, so let's push into the top section here, and let's light up the red curve, I'm going to switch the viewer to the red channel. So this is what the red looks like in the original clip, and by the way, we can gamma up to get a deeper look into our blacks.

So these are practically zero, now this is the white end of my ramp, so that means I want the white end of my curve, and I'm going to pull the red curve way down to nearly or almost all the way to black. So tiny, there, let's call that good. Alright, next, switch the viewer to the green channel, grab the green curve, pull the green, white point down until we get a match to our background plate.

Let's say, like so. Again, I'm just trying to match this area right here, switching the viewer to the blue channel, grabbing the blue, white point, pull that down until we get a match, like so. Alright, there we go. Now, viewer back to normal, so I got a pretty good match between my ramp and the original picture, right here. Zoom out, go down here to the bottom, we're going to repeat the process for the blacks, this is the white part of the sky, remember, I know, this makes no sense whatsoever, but it works, alright.

Okay, so, viewer, gamma back up, in order to exaggerate our differences, select the red channel, let's go get the red curve, and now we want to do the black point. This is the black point for the red channel, and I want to match the original clip, let's call that good. Switch to the green channel, grab the green curve, black point, raise it up until I get a match, go to the original plate, call that good.

Blue channel, blue curve, black point, and a match, okay, let's call that good. Knock our viewer, gamma back to normal, switch to RGB, re-home the viewer, and look at this. I have a lovely gradient, so now if I bounce between my color look up node and my original plate, you can see, I've made a very good match to that uneven backing region.

By the way, in any photographic image of the sky, there will also be a gradient left to right, okay, in this picture, it's not very bad at all, and we'll ignore that for purposes of this demonstration. But if you had a serious one, you might want to apply a gradient also going the other way, so that you could really match it in all four corners. Okay, so, what do we do, now that we've got a nice clean gradient matching the sky? This is what we do, first, we use the Mirror Node to flip it upside down, so now this is the bright bottom of the screen and that's the dark top, which is the opposite of my picture.

And now, if I plus or sum those two together, I have now made this uniform from top to bottom. The ramp, exactly offsetting the gradient in my picture, I can prove that to you, by hooking up my Sampler Node to this Merge Node, where I add the pictures together. So if we open up Mr. Sampler, and we tell it to resample, look at that, boom, nice, level, reasonably, level colors, all the way across all three channels.

This means that my backing is now much more uniform, so let's see how much good this does us. Let's close this, we'll switch to this. This is the lumakey attached to the original clip, see it right there, the de-noise, so let's pull a lumakey on the original clip, we'll pull our best lumakey and see what we get. Again, viewer, gamma up, switch to the alpha channel, and grab our lumakey, and start cranking down on the alpha channel.

And again, you can see, the top is cleared, but the bottom is not, so I've got to keep cranking and choking, and cranking and choking, until I've cleared the backing region out, like so. Alright, so that was the original clip without any correction. Let's try this same trick with our corrected screen, with our leveled screen, so we're going to grab this guy, we're going to pull down on him, now this time, you can see, within the degree that my screen has leveled it, I no longer am just clearing the top and having to choke the bottom, it fairly well, uniformly clears everywhere at the same time, okay.

So let me just get rid of all the little bits of noise that I can, now, let's compare the two, this is my leveled screen, there's the original. Leveled, original. You can see that the leveled screen has much more edge detail, many more fine little white baby clouds. I've got a much more complete, nicer edged key, by leveling the screen first. So there you have it, how to level a non-uniform screen to sweeten the plate for a better key.

Be sure to check in next week, so you don't miss out on my next Nuke Nugget.