After Effects is a timeline based compositing app, and its primary directive is to composite multiple layers of video together. Controlling and manipulating the transparency of each layer is an essential feature, and there are many ways to do this.
One of the most obvious and original tools that After Effects has is the track matte – where one layer has its overall transparency set by another layer. However, in many cases the same thing can be done by using the Set Matte effect.
So what’s the difference? Is one better than the other? Why do we need both? Let’s have a look…
If you’re in the mood for some trivia, then you might be wondering about the difference between the terms “matte” and “alpha channel”. They both refer to the same thing, which is the transparency of an image. However we use the term “alpha channel” to refer to the transparency that’s part of the image data, in the same way that red, green and blue channels are. The term “matte” is used when that transparency comes from a separate layer.
So if we import some footage into After Effects, apply a separate track matte and render it out as a Prores4444 – the track matte becomes an alpha channel. We can also take some footage that has an alpha channel, and render just the alpha channel out as a separate file. This is converting the alpha channel to a matte. It doesn’t really matter which term you use, most people won’t care about the technical difference.
The video refers to an older article of mine that demonstrates the After Effects composition rendering order. This is a fantastic resource for anyone starting out with After Effects, as it starts with very basic examples and moves through to some more complex and advanced scenarios. It can take years of experience to understand the composition rendering order in After Effects, and so watching this will give you a head start.
I also briefly mention the “remove color matting” effect, which can be essential when dealing with footage that has semi-transparent areas. This is another topic altogether, and again it’s something I’ve look at before. This is closely linked to the difference between a straight and pre-multiplied alpha channel, which you can learn about here…
Otherwise – if you’ve found this helpful please check out the other articles I’ve written on After Effects.