How To Trim Videos
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Keyframes: the anchor of modern video
This matters for trimming because you can only cut cleanly at a keyframe. Cut between keyframes and the resulting file starts on a delta that has no anchor, which means either the decoder fails, or the player recomputes by decoding from the previous keyframe and skipping — which means re-encoding.
Lossless trim (stream copy)
The catch: your cut point snaps to the nearest prior keyframe. If you asked for 10.0 seconds but the nearest keyframe is at 8.5 seconds, your output starts at 8.5s. For most casual cuts this is fine. For precise frame-accurate edits, it’s not.
Precise (re-encoding) trim
To hit an exact frame, you have to re-encode — decode from the prior keyframe to your target, then re-encode the output starting at that frame as a new keyframe.
The -ss position trick
Re-encoding always costs some quality (generation loss) and takes time proportional to clip length. For a 2-minute output, expect 30 seconds to 5 minutes depending on your machine and preset.
Start and duration vs start and end
The combined form is the common compromise — fast seek to roughly the right region, then precise adjust within that region. It’s much faster than pure output-side seek on a long file.
Container formats and their quirks
Even if you don’t need frame-precise cuts, re-encode when:
When to re-encode intentionally
Audio tracks also have their own frame boundaries (AAC frames are ~23ms). Lossless trim doesn’t guarantee frame-accurate audio, but the drift is imperceptible (<25ms). If you’re trimming right at speech boundaries, you may prefer to re-encode audio with a precise cut.