Frame-accurate cuts
Every trim in the editor is one of two things: a copy, or a re-encode. The frame-accurate box decides which.
Why a cut is not always exact
Video is stored as a few complete pictures, the keyframes, with every frame in between described only as a change from the previous one. Keyframes sit a second or a few seconds apart, depending on the camera.
A player can only start decoding at a keyframe. So a file that begins between two keyframes has no complete picture to start from, and the first frames come out broken. That is why an ordinary cut cannot start at any frame you like without touching the pixels.
Off: copy, snapped to a keyframe
With frame-accurate off, the start of the trim snaps to the nearest keyframe. The keyframe marks along the timeline show where those are, and the note under the bar tells you how far apart they sit in this clip.
The video is then copied straight into the new file: no decoding, no encoding, no quality loss. A trim takes a second or two whatever the length of the clip. The end of the trim can land anywhere, since a player does not need a keyframe to stop.
On: exact, re-encoded
With frame-accurate on, the trim starts exactly on the frame you chose. To make that possible, the clip is decoded and encoded again from that frame onward.
- It takes about as long as the clip is, longer on a phone or an older laptop.
- The result is encoded at a high quality setting (CRF 18), so the loss is small, but it is not zero.
- The file is often a different size from a plain copy, because it is a new encode.
When the editor re-encodes anyway
A crop, a tilt, a mirror, a size or format change, a colour grade or a patch all change the pixels, so the clip is re-encoded with or without frame-accurate ticked. In that case ticking it costs nothing extra, and the cut is exact for free. The export dialog says which of the two you are about to get and why.
A plain rotation is the exception: it is written into the file as an orientation flag, so a rotated clip can still be a lossless copy.
Questions
How far off can a snapped cut be?
Up to one keyframe interval, so normally under a couple of seconds. iPhone footage has keyframes about one second apart; some screen recorders go ten seconds or more, which the note under the timeline will tell you.
Does the end of the trim snap too?
No. Only the start needs a keyframe. The end is exact either way.
Can I have exact cuts and no quality loss?
Only when a keyframe happens to sit where you want the cut. Otherwise it is one or the other.