Education, Pro Tips

Video Compositing: How to Work With Alpha Mattes and Alpha Channels

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If you’ve spent any time working with green screens or green-screen footage, you’re aware of how much you can do when you have an isolated subject to work with. This goes doubly for clips with alpha mattes or alpha channels, because you don’t need to worry about keying out or removing anything (as it’s removed already), finishing part of your work before you even put it into your project.

But how is it removed? The key is in the channels. Three main channels contain all the color information: red, green, and blue (RGB). There is also a fourth channel that’s invisible and contains transparency information, called the alpha channel. This allows you to have transparent information that doesn’t affect any of the colors in the video. Here is an example of a clip with an alpha channel. Note the checkered background, indicating that it’s transparent:

Dragon Flying and Breathing Fire, Looped Clip with Alpha Channel by Zuxela

(Alpha) mattes, on the other hand, are layers that tell the software which layer or area within a layer is where the transparent parts are. Many mattes are displayed as black and white, with black being the transparent area and white being the opaque area. A lot of times, the alpha channel is the matte, but not always — that’s why I put “alpha” in parenthesis. You can make a matte in place of the alpha channel in situations where a matte works better, or if the file doesn’t actually have an alpha channel built in. Here’s an example of a clip with an alpha matte. Note the black and white section in the back half, which tells your editing software which area is transparent:

4K Money: Slow Motion Scattering on White, Alpha Matte by Arsgera

There are many image-file types that contain alpha channels (EPS, PNG, TIFF, TGA, PSD, AI, etc.), and a few video codecs that contain an alpha channel, like PNG, ProRes 4444, Animation, AVI, and certain Quicktime options. If you see the bit depth has a 32-bit option, then you can usually check a box or select the option to include an alpha channel.
 

Working with Alpha Channels and Mattes

Now that I’ve described roughly what’s going on, it’s time to move on to how to work with these types of items. The good news is, if you’re working with an item with an alpha channel, most of your work is done for you. You can really just import the file into your editing software (or VFX software), move it to the desired spot, and voila! Here’s another example of a clip with an alpha channel:

Cartoon Cheetah Running, Animation Cyclical by AlphaNature

When you start working with mattes, things get a little more tricky, but they’re still relatively simple to use. The first thing you need to do is find an item with an alpha matte, which you can specifically choose in the advanced search on Pond5.

Pond5 Advanced Search Options

For my own composite, I’m going to use this glass breaking clip:

Cracked And Shattered Black Glass with Slow Motion by Arsgera

I’m putting it into my project(s), first with Premiere and then with After Effects, so you can see how it works with both.

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Working in Premiere

Place the file in your canvas or drag it onto your timeline/sequence, and splice/edit the clip where the matte starts. Most clips are cut perfectly in half, so if the two halves don’t line up, check your edit point and try again. Place the matte (black-and-white section) on V1 (or V2, it’s not super important, and you’ll see why in a second), then place the “regular” footage on V3 or any higher layer than your alpha matte.

alpha-matte-layer-layout-adobe-premiere

Now place your footage (or whatever you want to be revealed) under the “regular” clip, on V1 or V2. It can be above or below the matte, but it can’t be under the regular clip. Go to your effects panel and find the effect “Set Matte.” Place it on your regular clip on V3, then set the “Take Matte From Layer” to whichever layer has the matte on it (in this case, it’s V1).

Set the “Use for Matte” to “Luminance,” since it’s black and white, (or “grayscale”), then turn off the matte layer with the eyeball tool.

set-matte-effects-panel-adobe-premiere

The reason you turn off the layer is because the effect just needs to use the clip’s information for the matte, and V1 doesn’t need to actually be visible.

take-matte-from-layer-1-adobe-premiere

set-luminance-for-matte-adobe-premiere

Scrub through your timeline to make sure it works the way you want, and presto! Final result:

 

Working in After Effects

Drag your clip into your sequence, cut it at the correct moment, and place the alpha clip on layer 1, with the regular clip on layer 2. Then place your footage layer on layer 3.

layer-layout-alpha-matte-adobe-after-effects

Make sure your “Modes” column is visible, then on layer 2, click on the “TrkMat” dropdown. Select layer 1 as a “Luma Matte” (again, since it’s black and white), and you’re already done! Scrub through to make sure it looks the way you want, and you’re all set.

luma-matte-dropdown-adobe-after-effects

Getting a basic understanding of mattes and alpha channels is a great entry point into the world of compositing, which can open up many new ways to create. You can utilize these techniques to blend real footage and VFX together, helping to keep your projects fresh and interesting, while improving your skills.

For more inspiration, explore thousands of alpha-channel clips on Pond5 to see what else you can start creating:

Alpha Channel Collection