"Transparent video" sounds like a property of a file, but inside FFmpeg it is a property of a frame in memory: either the frame has a fourth plane, or it does not. FFmpeg names that difference yuva420p versus yuv420p . Every step on the path keeps the plane, drops it, or makes up a new one.

Source: [Dev.to](https://dev.to/iterandum/alpha-is-just-a-plane-in-the-pixel-format-and-ffmpeg-drops-it-in-three-quiet-places-2a1g)

Sponsored