How do I compress a video with an alpha channel?

Compressing a video with an alpha channel means keeping the transparency layer while choosing a smaller delivery codec. It is not a way to turn an opaque H.264 MP4 into a transparent file. The source must already have alpha, or you must remove the background first.

Compression keeps alpha. It does not create it.

A convert or recompress copies channels that already exist. If the file is everyday H.264 MP4 with black or white already filled in, the “transparent” areas are gone. Shrinking that file just makes a smaller opaque clip.

• Proceed if you already have ProRes 4444, VP9 WebM with yuva420p, HEVC-with-alpha, or another file whose pixel format actually includes alpha.
• Stop and remove the background if the clip is a filmed room, a social MP4, or any export that was already composited. Then encode.

Everyday MP4 does not carry a usable alpha channel. Details: Does MP4 support a transparent background?

Pick a delivery format, not a fake size promise

Two honest options when you still need the holes in the frame:

• VP9 WebM, pixel format yuva420p: The usual smaller file for Chrome, Firefox, and OBS overlays. Built for delivery, not for a long editorial chain.
• ProRes 4444 MOV: The large editorial intermediate. Premiere, Resolve, and Final Cut expect this. It stays big because it is meant to survive grading and reconform, not because a slider was left on “high.”

We are not quoting megabyte savings or bitrate targets here. Size depends on resolution, motion, and encoder flags. If you only need a web or OBS overlay, WebM is the format that is designed to be smaller. If an editor still has to cut the shot, keep ProRes 4444 until that work is done. Format map: What video formats support a transparent background?

Already have ProRes alpha? Encode WebM with FFmpeg

If the MOV already stores alpha, a local convert is enough. This is the same intent as convert ProRes MOV alpha to WebM in FFmpeg and how to make a transparent WebM:

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

This is a reference command. CRF, bitrate, and deadline flags are optional—example flags vary by FFmpeg build. You need libvpx-vp9 compiled in. Confirm pix_fmt with ffprobe before and after; you want a yuva format, not a filled yuv420p. Current VP9 notes: FFmpeg Wiki.

Safari is a format problem, not a compression slider

Making the WebM smaller will not make Safari composite alpha. Safari ignores WebM transparency. That path needs HEVC-with-alpha from another encoder. ExportAlpha does not ship HEVC. See why transparent WebM is black in Safari.

What ExportAlpha does in this workflow

If the source has no alpha, FFmpeg is the wrong first tool. ExportAlpha removes the background, then writes a real alpha channel as VP9 WebM or ProRes 4444. Product limits:

• Uploads up to 60 seconds, 1080p, and 200MB
• New accounts get 5 free processing seconds
• File-based, not live keying or a webcam plugin
• WebM and ProRes only—no HEVC-with-alpha download

After you have ProRes with alpha, you can still run the FFmpeg convert above if you want a local WebM. After you have WebM from this tool, do not recompress it to H.264 MP4 and expect the holes to survive.

Frequently Asked Questions

How do I compress a video with an alpha channel?

Confirm the source has alpha, then encode VP9 WebM with yuva420p for delivery, or leave ProRes 4444 for editing. If the source is opaque, remove the background first. Compression alone will not invent transparency.

Does compressing a video add an alpha channel?

No. You cannot “compress in” a transparent background. Re-export from a timeline that already has alpha, or use background removal, then pick WebM or ProRes.

Should I use WebM or ProRes 4444 if I need a smaller transparent file?

Use WebM when the next stop is a browser or OBS. Keep ProRes 4444 when the next stop is an editor. We are not publishing a size chart; WebM is the delivery codec, ProRes is the large intermediate.

Can I use FFmpeg to compress ProRes with alpha to WebM?

Yes, when the MOV already has alpha. Use libvpx-vp9 and yuva420p. Treat CRF numbers you see on blogs as examples that vary by FFmpeg build, not as a guaranteed setting.

Sources

• FFmpeg VP9 encoding: FFmpeg Wiki
• WebM project docs: webmproject.org
• Apple ProRes 4444 alpha: Apple support