Green screen video export settings
There is no settings dialog here. There are three export formats, and each makes a different set of promises about resolution, frame rate and alpha. This page is what those promises actually are — read off the code that does the writing, not off a spec sheet.
The three things you can export
- Transparent PNG — a still, keyed at the source image's native width and height. Alpha kept, losslessly.
- PNG sequence (.zip) — one PNG per frame, at the source video's native size. Alpha kept on every frame.
- Transparent WebM — a real-time canvas recording with an alpha channel, encoded as VP9 where the browser supports it.
There is no MP4 export: the MP4 profiles that play everywhere carry no alpha channel, so an MP4 of a keyed clip would have to be composited onto something first. That belongs in an editor.
Resolution: your source size, or nothing
The tool never resamples. There is no "export at 1080p" control, because there is nothing to control: the keyed frame is written at whatever the source actually is.
For the PNG still and the PNG sequence that is exact. A 1080×1920 phone clip exports 1080×1920 frames; a 4K clip exports 4K frames, slower to key and much heavier to store. If you want a smaller file, downscale the source first — the export will not do it for you.
The WebM path has one small exception: both dimensions are rounded down to even numbers before the recording canvas is created, so a 1081-pixel-wide source exports at 1080 and you lose one column and one row. The encoder works in 4:2:0 chroma subsampling, which wants even values. You will only notice this on cropped phone footage.
Frame rate: two different rules
The PNG sequence and the WebM do not share a frame rate setting, and only one of them is under your control.
PNG sequence. This one uses the frame rate slider. It defaults to 12 frames per second, and the number of frames written is the clip's duration multiplied by that rate, rounded down. There is a hard ceiling of 1200 frames, because every frame is held in memory before the archive is written. A 60-second clip at 12 fps is 720 frames and fits; the same clip at 30 fps would be 1800 and is refused, with the rate that would fit reported instead.
Transparent WebM. You cannot set this one. The canvas is captured at up to 30 frames per second and the keying loop runs on animation frames while the clip plays in real time. Three consequences:
- Output is capped at 30 fps whatever your source is — a 60 fps clip comes out at roughly 30.
- It is a real-time capture, not a frame-accurate transcode. If the keying loop cannot keep up, frames are dropped and the result plays slightly faster than reality.
- Because it records while playing, a 30-second clip takes about 30 seconds to export. There is no offline render mode.
Alpha: what survives each path
- PNG still and PNG sequence: 8-bit per-pixel alpha, lossless. A pixel the key removed is fully transparent in the file, and there is no second encoding pass to soften the edge.
- WebM: the recording canvas has an alpha channel and the recorder keeps it, so the transparency is in the file. Whether you see it depends on what you open it with — that is a player problem, not an export problem. The transparent WebM page covers which players and editors keep it.
Expect one difference on the WebM path: it is a re-encode. The keyed frames go through a lossy VP9 pass at 8 Mbps, so a fine alpha edge — hair, a soft rim — can come out slightly softer than it looked in the preview. The PNG sequence has no such pass. If the edge is the point, export frames.
Audio
The WebM export has no audio track, and this surprises people. The video element is muted for programmatic playback and the recorded stream is taken from the canvas alone, so nothing carries the sound across. If you need the audio, keep the original and mux the track back in an editor, or record it separately and sync afterwards.
File size and how long it takes
The PNG sequence is the heavy option. Each frame is a full-resolution PNG and the archive is written in stored mode — the frames are not compressed further on top of the PNG encoding they already went through — so the download is close to the sum of the individual frames. The 1200-frame ceiling exists for this reason: the frames are assembled in memory before the archive is handed to you.
The WebM is the light option. Video is encoded at 8 Mbps, so half a minute lands somewhere around 30 MB, as a single file rather than a folder of stills. The trade is the real-time export and the lossy pass above.
Which one to pick
- Compositing over a new background: PNG sequence if your editor imports image sequences cleanly, WebM if it prefers a single clip. Test one short clip before committing to a long one.
- A single still: PNG.
- One file that plays with transparency: WebM — then check it in the player you actually intend to use.
- Frame-accurate timing, or you care about the edge: PNG sequence. No re-encode, no dropped frames, no rounding. The PNG sequence vs WebM page goes through the trade in more detail.
When this does not work
- You want MP4 with alpha. Not offered, and not possible in the common MP4 profiles. Composite in an editor.
- You want to choose an output resolution. There is no picker; the export matches the source.
- You want to set the WebM frame rate. Fixed at up to 30 fps, driven by real-time capture.
- You need the audio in the WebM. It is not there. Mux it separately.
- Your machine is too slow to keep up. The WebM drops frames and runs fast; there is no slower-but-complete option. Use the PNG sequence, which is not time-bound.
- Your browser cannot record a canvas. The WebM path refuses and says so, pointing you at the PNG sequence rather than producing a broken file.
- The clip is long and you wanted frames. Over 1200 frames the sequence is refused — lower the rate or cut the clip up.
- The source has an odd pixel dimension. The WebM loses one row and one column; the PNG exports do not.
Then export it locally
None of this involves an upload, an account or a watermark — the key runs in the tab and the file is written straight to your disk. The no-upload page covers what keeping it local avoids, and the video page covers the keying side rather than the export side. The measurement fixtures behind the numbers on this site are in the reference repository, with the write-up archived at doi.org/10.5281/zenodo.22916767 — synthetic fixtures, not real footage, so read them as the shape of the problem.