Make a transparent video from PNG pictures with ffmpeg on Windows: WebM with an alpha plane or…

Guide ID: art_01M7BMCT51BQQ5JKK3EHXMT1V5

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Guide title

Make a transparent video from PNG pictures with ffmpeg on Windows: WebM with an alpha plane or ProRes 4444, the plain commands that quietly drop the transparency, and a script that checks the alpha plane of the result

Coverage

For people with a numbered series of PNG pictures that have transparency and want a video that keeps it. A PowerShell script checks the pictures (gaps in the numbers, alpha channel, even size), writes a WebM (VP9 with an alpha plane) or a ProRes 4444 MOV, and compares the alpha plane of the result with the pictures by measurement. It ran on generated disc animations with ffmpeg 9.0.2, including thirteen refusal cases; no real artwork, player, browser or editor was tried.

Example

Ten plain ffmpeg commands on the same 50 pictures: the right VP9 and ProRes commands kept the alpha (56.0 and 68.4 dB against the pictures), while H.264, ProRes 422 and VP9 without an alpha pixel format wrote files without any error and with the transparency gone (0.9 dB). The WebM, 4.6 KB, was read with its alpha by libvpx and as solid by ffmpeg's native VP9 decoder.

Useful starting points

• Check the alpha plane of the result by measurement; ffprobe shows a WebM with alpha as yuv420p.
• Use libvpx-vp9 with -pix_fmt yuva420p for WebM, and prores_ks profile 4 with yuva444p10le for MOV.
• Open the file in the player or editor you will use, over a coloured background, because a decoder may ignore the alpha plane.
• Keep the picture numbers in one run without gaps, and the size even.