Cutting a video without re-encoding copies the compressed picture and sound into a new file, exactly as they are. Nothing is decoded or compressed again, so there is no quality loss at all, and even a long recording is cut in seconds: the work is little more than copying the part you keep.
How to cut without re-encoding
- Upload the video (MP4, MOV, MKV, WebM, AVI, WMV, TS/MTS and more; up to 2 GB and 3 hours).
- Enter the start and the end (or a length), for example
12:30and15:00. - Keep Cut: Fast (no re-encoding) and start. The clip keeps the original format.
Why the start moves to a keyframe
Compressed video is stored in groups of frames. Only the first frame of each group, the keyframe, is a complete picture; the others describe changes from it. A copy therefore has to start at a keyframe, or the first frames couldn't be shown. The clip starts at the keyframe at or before your start time. Keyframes are usually one to ten seconds apart, so the clip may start a few seconds early; the result tells you exactly where it starts. The end is cut where you asked.
If the start has to be exact, choose Accurate: the clip is re-encoded from the exact frame, at high quality.
What is kept
- Every sound track, unchanged, in step with the picture.
- Subtitles in MKV, MP4, MOV and WebM files.
- The upright orientation of phone videos (the rotation flag is copied).
- The format: an MKV stays an MKV, an AVI an AVI, a TS or MTS keeps its packet format.
MXF files are copied into MKV. RealMedia and Y4M files can't be copied this way; they are cut accurately instead, and the result says so.
More about what this tool reads and writes: Video Trimmer.