How compressed video is stored
Storing every frame as a full picture would make video enormous. Instead, codecs such as H.264 store a complete picture only now and then. These are keyframes (also called I-frames). The frames in between store only what changed since earlier frames.
A keyframe and the frames that depend on it form a group of pictures (GOP). To show any frame in the middle of a GOP, a player has to start from the keyframe before it and rebuild everything in between.
| Source | Typical keyframe spacing |
|---|---|
| Phone camera | About every 1 to 2 seconds |
| Streaming and social downloads | About every 2 seconds |
| Screen recordings and some encoders | Can be 5 to 10 seconds or more |
These are typical values, not rules. Video Info shows a file's codec and frame rate, which helps you guess how it was made.
Fast trim: stream copy
A fast trim copies the compressed frames between your start and end without decoding them. It is very quick, even for a long file, and the quality is identical to the original because nothing is re-encoded.
The catch is the start point. A copied clip has to begin on a keyframe, otherwise the first frames would have nothing to build on. So the cut lands on the nearest keyframe, which can be a little before the time you chose. With keyframes every two seconds, the start may move by up to about two seconds.
If a fast trim starts slightly early, that is usually fine for sharing. If you need the exact frame, switch to Accurate.
Accurate trim: re-encoding
An accurate trim decodes the video, keeps exactly the frames you selected and encodes them again. The start and end are frame exact. It takes as long as any other re-encode, which in a browser can be several times the clip length for high-resolution video, and it re-compresses the picture, which costs a little detail.
| Fast | Accurate | |
|---|---|---|
| Speed | Seconds | Depends on length and resolution |
| Quality | Identical to the original | Very close, re-encoded once |
| Start point | Nearest keyframe | Exact frame |
| Output format | Same as the input | MP4 (H.264 and AAC) |
Which one to use
- Cutting dead time from the start and end of a recording: Fast.
- Splitting a long video into parts for Stories or Status: Fast, the small shifts do not matter.
- A clip that must start on a specific word or beat: Accurate.
- Removing a section from the middle: always re-encoded, so the join is seamless.
Questions
Why does my trimmed video start a second early?
Fast trim copies the video without re-encoding, so it has to start on a keyframe. The nearest keyframe can be slightly before your chosen time. Use Accurate for an exact start.
Does fast trim reduce quality?
No. The compressed frames are copied as they are, so the quality is identical to the original.
Why is accurate trim slower?
It decodes and encodes every frame of the clip, which takes real computing time, especially in a browser running on one CPU thread.
Can I make keyframes more frequent?
Only by re-encoding. Any re-encode, such as compressing or an accurate trim, places keyframes at regular intervals, which makes later fast trims more precise.