How do I convert a ProRes MOV with alpha to WebM in FFmpeg?

Q: How do I convert a ProRes MOV with alpha to WebM in FFmpeg?

If the MOV already has alpha, encode VP9 with libvpx-vp9 and yuva420p. Confirm pix_fmt with ffprobe before and after. FFmpeg will not create transparency from an opaque file.

Q: What if my ProRes MOV has no alpha channel?

Stop. Re-encoding cannot invent a matte. Export ProRes 4444 with RGB+Alpha from the editor, or use ExportAlpha for AI background removal first.

Q: How do I verify the WebM still has alpha?

ffprobe should report yuva420p or another yuva pixel format. Play the file in Chrome over a colored page. Desktop players that composite onto black are not a reliable test.

This is a convert step, not a background-removal step. FFmpeg can take a ProRes 4444 MOV that already stores alpha and encode VP9 WebM with yuva420p. If the source is opaque, FFmpeg cannot invent a matte.

Source has no alpha? Remove the background first

ExportAlpha runs AI removal, then encodes true-alpha WebM or ProRes. Do not run FFmpeg on a filled-in MP4 and expect holes.

Try 5 free credits — no card See pricing

5 free credits on first sign-in for most new accounts. Trial stills capped at 5. No card required. Fair-use limits apply.

Hard rule: alpha in, alpha out

A convert copies or recompresses channels that already exist. It does not segment a person from a room. Before you encode:

Proceed if the MOV is ProRes 4444 / 4444 XQ (or another codec that actually stored alpha) and ffprobe shows an alpha-capable pixel format or an alpha mode.
Stop if the file is H.264, ProRes 422, or any MOV whose transparent areas are already black or white. Re-encoding will keep the fill.

If you still need a matte, export RGB+Alpha from After Effects or Premiere, or upload the filmed clip to ExportAlpha for AI removal. Then come back to FFmpeg if you want a local WebM encode.

Check the source first

ffprobe -v error -select_streams v:0 -show_entries stream=codec_name,pix_fmt -of default=noprint_wrappers=1 input.mov

You want a ProRes family codec and a pixel format that includes alpha (names vary; look for yuva, rgba, or an explicit alpha mode). If pix_fmt is plain yuv422p10le or yuv420p with no A, there is nothing to convert into transparency.

Encode VP9 WebM with yuva420p

Same intent as the snippet on How do I make a WebM video with a transparent background?—this page is the dedicated ProRes → WebM convert:

ffmpeg -i input.mov -c:v libvpx-vp9 -pix_fmt yuva420p -c:a libopus output.webm

This is a reference command. FFmpeg builds differ. You need libvpx-vp9 compiled in. Drop audio (-an) if the stinger or overlay is silent. Bitrate and CRF flags are optional; check current FFmpeg VP9 docs for your version. If FFmpeg errors on yuva420p, the input likely has no alpha or your libvpx build cannot write it.

Verify the WebM

ffprobe -v error -select_streams v:0 -show_entries stream=pix_fmt -of default=noprint_wrappers=1:nokey=1 output.webm

Expect yuva420p. Then open the file in Chrome over a colored page. A desktop player showing black does not prove failure. Safari will not composite WebM alpha; that is a browser limit, not a bad convert. ExportAlpha does not ship HEVC-with-alpha—use another encoder if Safari must show the holes. See transparent WebM in Safari.

When to skip FFmpeg

If the clip still has a real background, local FFmpeg is the wrong tool. ExportAlpha removes the background and writes VP9 WebM (yuva420p) or ProRes 4444 MOV. Uploads: 60 seconds, 1080p, 200MB. New accounts get 5 free processing seconds. That is AI removal plus encode—not a live keyer.

Frequently Asked Questions