An Alpha channel defines the transparency in a video clip or image. Basically, transparency is assigned to the pixel. In After Effects, if you create a composition that you intend for something to be seen underneath, or seen through the “empty” area when you are using it in post-production, you need to export your clip with alpha.
When you add your composition to the render queue, click on the output module settings. In the video output section, you’ll see a dropdown by the word “Channels.” This is where you select RGB, Alpha, or RGB+Alpha. RGB+Alpha should be selected, and this will export your video with the alpha channel.
However, not all video formats/codecs support alpha. In After Effects, the Animation codec does, as well as Apple ProRes 4444, Apple ProRes 4444 XQ, and a few others. You must select one of these codecs to have the RGB+Alpha option become available.
If you export your composition using Adobe Media Encoder, once you select a proper format that supports alpha (such as QuickTime), you will see a list under the presets dropdown. Media Encoder lets you know which options include alpha.
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