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I'm producing a product that involves a composition of several videos, over alpha layers, and there are surprisingly few options aside from image lists. Quicktime's RLE (Run-Length Encoding) codec allows for an alpha channel, but was designed for animation—the encoding is over single rows and relies on swaths of consistent color, which does not apply to my current video and results in an exorbitantly large file size, making it difficult or impossible to play back in real time on my machine.

Surely there are recommended and hopefully open formats which allow for alpha channels, but have a more reasonable compression scheme. Unfortunately I'm not sure what they are.

Is there a codec recommended for high-color-variation videos with alpha, which provides a workable size?

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ProRes 4444 is the go-to for compressed encoding if you want alpha, and you’re doing the encoding on a Mac. It’s not exactly lightweight in terms of file size, but it’s optimized for playback on Mac hardware.

OpenEXR is a heavyweight codec, used mostly in the VFX industry, is widely supported on different operating systems, and supports alpha and any number of auxiliary data channels, such as ones output by 3D rendering software. It does support compression, but it’s not as playback-friendly as ProRes.

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  • I've used OpenEXR before, I thought it was only capable of single frames per file though? Did I "whoosh" some important detail? I guess I could always just archive them and compress after the fact.
    – Michael Macha
    Jun 6, 2022 at 23:02
  • It’s often best practice to use image sequences instead of videos for compositing. It A) eliminates temporal artifacts B) simplifies the replacement of specific frame ranges, and C) allows resuming renders at the last complete frame in the event of a crash or interruption.
    – Jason Conrad
    Jun 6, 2022 at 23:46

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