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A Couple of movies ago we took advantage of the Alpha channel of the layer to go ahead and create transparency for a second layer underneath. That's what track mattes do. But in addition to borrowing the Alpha channel from the layer on top you can also use another property of it. Its Luminance. So let's show that example. I'll go back to my Project panel. Open up another comp. in this case 09-Luma Matte*starter. It's just black and transparent to begin with. Now I'll drag a couple more sources in. I've got some full frame fire that I'll go ahead and add it to my comp by selecting and typing Command or Ctrl+/ and there it fills the frame.
And then I am going pick something that has an interesting black and white contrast to it. And I'll select Cloud matte.mov and also type Command or Ctrl+/ and add it as well. And it's a bit of smoke emanating out from the side of the screen. It does not have an alpha channel. I am going to solo it, toggle the transparency grid and I've got nothing. I'll go ahead and look at RGB channels, say just show me the alpha. Solid white. Nothing going on there. Go back to RGB for now.
However, I can still use is as a track matte. I put the matte, the thing with the interesting outline, on top, add the fill layer underneath, make sure that Modes column is visible. If not, I can use F4 to toggle it on and off. And set the Track Matte popup for the layer underneath to Luma Matte. Use of the luminance or grayscale values of the Cloud matte layer on top to go ahead and give me, the Firestorm layer, new transparency. And there we are.
Now as the smoke billows off in the side and it is filled with this fire. Notice again that After Effect automatically turned off the visibility for the Cloud matte, as we don't want to see it. We just want to borrow its luminance to create a new alpha channel for our fire. Now you can use virtually any layer as a luminance matte. Something that's high contrast, black and white is your best choice but you can pick other layers and convert them to grayscale or just play around with them little bit and see how they work out. For example, I'll set this back to No Track Matte for now and bring in that VirtualInsanity layer we've playing around with earlier.
I am going to put it on top. It flashes between dark, black, and some bright areas. Let's try it as a luminance matte for the Firestorm. And now you'll see that the fire is playing inside the flashes of that VirtualInsanity layer. If it's too dark, I'll select the VirtualInsanity layer and use an effect such as Color Correction > Levels. Drag that a little bit wider and I can see that the luminance range of VirtualInsanity does not go all the way out to full range.
So I'll pull my white point slider down to make it a little bit brighter and maybe even play with my Gamma to capture more of those brightness values. Make sure we bury our blacks as well while we are at it. Now we've got quite a bit of contrast flashing on the screen and that's kind of fun composite as well. Go back to No Track Matte, move the Cloud matte back on top of Firestorm, and there is it as luminance matte. And again Luminance Matte has an Inverted option. If I use that, now all the areas outside of that puff of smoke is filled with fire and the inside of the puff is transparent.
And I can drag VirtualInsanity on top of Firestorm and it will get used instead. There you see the opposite effect.
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