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In this chapter we're going to play around with an alternate way of creating transparency, track mattes. I've gone head and closed all of my previous comps. I am going to open up a blank comp, 08a-Alpha Matte*starter. If you don't have the exercise files, just create a blank comp of your own, find a piece of footage, and find something with new interesting alpha like a piece of text. I am going to go down to my Sources folder. I want to pick a background layer such as this VirtualInsanity. I can drag it to my comp or I can use the shortcut, which is holding Command on Mac, Ctrl on Windows and typing forward slash.
Then I am going to select a layer with an interesting alpha such as this piece of text Night Vision. Again Command or Ctrl+/ and there it is. A very common use of track mattes is to use one layer to fill the alpha of another layer. For that to happen the layer with the interesting alpha channel must be on top of the fill layer and you must have the Modes panel open. If you can't see the Modes panel, right- click on any header and select Column > Modes.
If Modes is invisible, other shortcuts include using the Toggle Switches Modes at the bottom of the Timeline panel, or pressing the F4 key, which will bring it open. Now again this Night Vision layer has an interesting alpha. I'll Solo it and then turn on my transparency grid so you can see that only the text itself is solid. Everything else is transparent. Turn Solo off. I want to choose the layer underneath VirtualInsanity in this case, my full frame background, and I want to use its Track Matte popup and say use the Alpha of the layer on top of me to give me a brand new alpha channel.
And the result is this. VirtualInsanity layer is now borrowing the alpha from the layer on top to create its own transparency. And there it is filling that text. When you select Track Matte, you'll see some special icons appear in the timeline panel. This little icon with the transparency grid indicates the layer that is getting the new alpha channel and then the layer with a black and white icon dot is showing the layer contributing the alpha channel. Note that when you set the Track Matte popup it automatically turned off the Video switch for the layer on top.
You don't want to actually see the layer on top. You just obscure the composite. By turning it off you get to see underneath the result of using its alpha channel imposed upon that second layer, the layer underneath. Just for comparison we'll pick Alpha Inverted and now you'll see that the Night Vision text actually cuts out the background layer. Again I'll turn on transparency grid and now you'll see through the alpha of the layer on top, while the rest of the original background is being displayed. But if you want to fill one layer with another you would use something like an Alpha Matte.
Note that if the layer on top moved, I'll type P to reveal its position, the exposed area of the layer underneath will also change. It's basically a different transparency window is being used for that layer underneath. I'll also type T to reveal Transparency and fade out the layer on top. It would fade out the final composite because again its alpha channel is going down to 0, being a fully transparent which would make the final composite transparent. The same effect if you went ahead and faded out the layer underneath as well and you're fading out the whole composite in that case rather than the alpha channel.
And that's literally all there is to it. The main thing is you need to treat these as pairs. The alpha channel on top, the fill layer underneath.
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