Use video files as track mattes in After Effects by working with both alpha and luma mattes to control layer visibility. This article walks through using transparency and contrast-based video files to create dynamic visual effects and transitions.
Key Insights
- Alpha mattes use a layer's transparency—such as a keyed-out green screen clip of a dancer—to reveal or hide another layer, offering precise control over visual composition.
- Luma mattes rely on brightness values in high-contrast video clips to determine visibility, making them effective for stylized masking effects like time-lapse or threshold animations.
- Noble Desktop's example project includes multiple matte and background layers, allowing users to experiment with techniques such as scaling and animating matte layers to create smooth transitions and dynamic visuals.
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In this lesson, we'll be looking at how to use video files as track mattes. There are two kinds of track mattes that exist. One is an alpha matte, that's this.
It's using transparency of one layer to control the visibility of another. This is specifically a young woman dancing in front of a green screen. That's where that comes from.
The other thing you can use as track mattes is actually video that has high contrast. In that case, you create what's called a luma matte. That's an example of this.
And there's a whole bunch of different like types of videos you can use for it. I've got a couple of examples here in the video as track mattes started project. If I'll open up, I will save this as, again, you can call us whatever you want.
I call it usually my name and the name of the project. So there are two prebuilt columns here, alpha matte and luma matte. In the video folder, there's a bunch of backgrounds and a bunch of other matte layers.
So I'm going to show you this on two of them, but I gave you enough files in this project that you can play with other ones. Okay. So let's start with the alpha matte.
It's probably the easiest one to understand. So this file has three different layers. The dancing girl alpha is basically just a video file with key light, key cleaner, advanceable suppressor.
So if I go back to the toggle switches, I can disable the effects. And this is the video. It's a young lady in front of a green screen.
That's all it is. The three effects in here are part of the key light preset that's just removed to error, just used to remove backgrounds. So it's a basic king effect, no problem at all.
And that leaves her with transparency alpha. There are two other layers. I'll solo color background for a second.
It's just this, all it is really is just a video with background like that. It's just a regular video. It's actually kind of cool.
It looks like I'll un-solo that. I'll solo grayscale background. It's basically the same video, but with grayscale.
I'm actually going to make it a little more distinct. So I'm going to search for an effect called threshold, like so. And I will, I put that on the wrong layer.
Actually, I need the grayscale background highlighted, double click on threshold. And I get that. And it basically gives you this like really kind of high contrast effect.
And the video itself is kind of like a stop motion animation, a time lapse actually is what it is. So it's kind of cool. Okay.
Now, un-solo that, and so I got. So what I basically want to do is use the dancing girl to set the alpha for color background. So it'll only be visible where she is or vice versa, by the way.
I'll need that toggle switches and modes button to go back to the layer modes. And you can see here, I have blending modes and track matte. I'm going to set color background to use dancing girl as this track matte.
I'll drag the little pick whip from track matte to dancing girl. And that's what I'll get. The color background is now only visible where the track matte layer is visible.
So where she was. So I turn her back on because she gets turned off with this effect. You can only see the colored areas where she is, where she exists.
Like, so like that. Okay. Now there's an option for this and to the right of the track matte column, you'll see that this is alpha.
It's using transparency from that layer. And this is invert. It basically reverses the behavior.
So now the color background is visible wherever she isn't. So now the color background layer is hidden wherever the track matte layer is visible. So now it's actually going to cut out more of like that.
And it's simply a matter of how you want to do it. Sometimes you want alpha, sometimes you want alpha inverted. Not a problem at all.
Now the advantage of this system is that you can actually then animate the track matte. So for example, I could animate the track matte. I'm just going to animate its scale, by the way.
So that maybe for a couple seconds, it's normal sized. Then maybe at two seconds, its scale is going to animate up. I'll just do a one second animation for this.
And I'm going to put the anchor point pretty much right in the middle of her body like this. And then I'm just going to scale it up like so until it fills the screen. I'm going to add a little easing on that because, well, why not? I like easing.
I'm also to deal with any like weird edge artifacts I may get. I'm going to go back to my switches and modes and just turn on the motion blur. So it'll basically make the edge a little blurriest animating up.
I'll just set my work area. And now I got that. So I can use this as a transition to get from one to the other.
Okay. Now what I should really do is make sure that it's big enough that it won't see any edge distortion like that. I could also animate the position.