How to remove a video background without a green screen
AI matting works out where the foreground is in every single frame, which means you can lift a person or a product out of a shot that was never filmed against green. The convenience is real. What you filmed still decides how good the edge looks, so a few minutes of setup saves you a lot of disappointment.
Shoot so the edge is easy
Light your subject evenly and give them something to stand out against. A dark jacket in front of a dark wall gives the model almost nothing to work with; the same jacket against a light, uncluttered room gives it plenty.
Motion blur, flying hair, sheer fabric and reflections are the hard cases, because the subject and the background genuinely share those pixels. A steady camera and enough light solve half of it before any software is involved.
Why a video takes so much longer than a photo
A photo needs one pass. Ten seconds of 30 fps video needs about three hundred of them, one per frame, plus decoding on the way in and alpha encoding on the way out. That is why length and frame rate drive the wait far more than file size does.
Test a short, representative clip first. Ten seconds tells you whether the edges hold up on this particular footage, and it costs you a minute instead of an hour.
Pick an export that keeps the transparency
A normal H.264 MP4 will not carry your alpha channel anywhere useful. VP9 WebM with alpha will, and browsers and compatible editors keep it intact for compositing.
If whatever you are handing it to refuses WebM, do the compositing yourself: put the subject over the final background, then export a plain MP4 that everything accepts.
Watch it move before you trust it
Look at hair, hands and edges during the fastest movement in the clip. A mask that looks perfect on a paused frame can flicker horribly in motion. Be honest about scale, too — a slightly ragged edge nobody will notice at thumbnail size in a feed will be glaring on a full-screen subject.