How to Cut Video Without Re-encoding in Avidemux (Lossless Cut)

Q: Why does my cut video show green or garbled frames at the start?

The cut started on a non-keyframe. In Copy mode Avidemux copies compressed data as-is, and frames between keyframes are stored as differences from the keyframe they reference. Re-cut with markers snapped to keyframes using the up/down arrow keys.

Q: Which formats support Copy mode?

Any format Avidemux can open, including MP4, MKV, AVI, TS, and WebM. The output container must support the codec inside; when in doubt, MKV Muxer accepts nearly everything.

Q: How much quality do I lose with lossless cutting vs re-encoding?

Nothing. Copy mode does not touch the encoded data, so output is pixel-identical to the source. Re-encoding always loses some quality but allows frame-exact cuts and smaller files.

Q: Can I join videos without re-encoding too?

Yes. Open the first file, then use File > Append for the rest (same codec, resolution, and container), keep Video and Audio on Copy, and save. The keyframe rule applies at join points.

To cut video without re-encoding in Avidemux, set both Video Output and Audio Output to Copy and choose a matching output container (MP4 Muxer for MP4 sources, MKV Muxer for MKV). Copy mode streams the original encoded data straight into the new file, so the cut is fast and completely lossless — but cuts can only start cleanly on keyframes.

Avidemux.net is an independent community resource site — not the official Avidemux project (avidemux.org, GitHub mean00/avidemux2).

Why “Copy” means lossless

When Video Output is set to Copy, Avidemux does not decode and re-encode your video at all — it copies the original compressed frames byte-for-byte into the new file. That means:

Zero quality loss — the output pixels are identical to the source.
Very fast — saving takes seconds instead of minutes, since no encoding happens.
Small learning curve — three dropdowns and you’re done.

The trade-off: you can only cut on keyframes (see the caveat below). If you need frame-exact cuts anywhere in the timeline, you must re-encode instead.

Step-by-step: lossless cut in Avidemux

Open your video — File → Open, or drag the file into the Avidemux window.
Mark your selection — navigate to the cut start and press A (or click the “A” marker button); navigate to the cut end and press B. Use the ↑ / ↓ arrow keys to jump between keyframes for clean cut points.
Video Output → Copy (left panel, top dropdown).
Audio Output → Copy (left panel, second dropdown).
Output Format → the matching muxer — MP4 Muxer for an MP4 source, MKV Muxer for MKV, and so on (see table below).
File → Save, give the file a name with the right extension (e.g. clip.mp4), and you’re done.

Pick the right container (muxer)

Rule of thumb: keep the same container family as the source unless you have a reason to change it.

Source file	Output Format to choose
.mp4 / .m4v	MP4 Muxer
.mkv	MKV Muxer
.avi	AVI Muxer
.ts / .m2ts	Mpeg TS Muxer
Anything else / unsure	MKV Muxer (most tolerant fallback)

The keyframe caveat (read this before you cut)

Copy-mode cuts can only start cleanly on keyframes (I-frames). Video between keyframes is stored as differences from the previous keyframe, so a cut that begins mid-group-of-pictures has no complete picture to start from — the result is green/garbled frames or audio-video sync drift at the cut point. This is a fundamental limit of lossless cutting, not an Avidemux bug.

Avidemux warns you (“cuts will not be on keyframes”) when your markers sit on non-keyframes — take the warning seriously.
Use ↑ / ↓ to snap your A/B markers to the nearest keyframes.
HEVC/H.265 sources deserve extra care: Avidemux specifically detects and warns when HEVC cut points may cause playback issues even on keyframes.
If you absolutely must cut at an exact non-keyframe frame, switch Video Output to a real encoder (e.g. x264/x265) and re-encode — you lose a little quality and a lot of speed, but gain frame-exact cuts.

“MUXER Cannot open” and other common errors

This error almost always means an encoder/container mismatch, not a broken file. Run through this checklist:

Video Output and Audio Output are both set to Copy.
The Output Format muxer matches the source container (MP4 source → MP4 Muxer).
If it still fails, try MKV Muxer as the fallback — it accepts the widest range of codecs.
Audio quirk: some codecs legal in MKV are not legal in MP4. If MP4 Muxer refuses, either use MKV Muxer or re-encode just the audio (e.g. AAC) while keeping video on Copy.
Known edge case: files encoded with third-party hardware encoders (e.g. HandBrake’s H.265 Apple VideoToolbox preset) can hit demuxer quirks in Avidemux — re-encoding with Avidemux’s own encoder works around it.

Frequently asked questions

Why does my cut video show green or garbled frames at the start?

Your cut started on a non-keyframe. In Copy mode Avidemux copies compressed data as-is, and frames between keyframes are stored as differences — without the keyframe they reference, the decoder shows garbage. Re-cut with your A/B markers snapped to keyframes (use the ↑ / ↓ keys), and heed Avidemux’s “not on keyframes” warning.

Which formats support Copy mode?

Any format Avidemux can open (demux) — MP4, MKV, AVI, TS, WebM, and more. The constraint is on the output side: the container you save to must support the codec inside. When in doubt, MKV Muxer accepts nearly everything.

How much quality do I lose with lossless cutting vs re-encoding?

Nothing — Copy mode does not touch the encoded data, so the output is pixel-identical to the source. Re-encoding always loses some quality (how much depends on the bitrate), but it is the only way to get frame-exact cuts at arbitrary points or to shrink the file.

Can I join videos without re-encoding too?

Yes — open the first file, then File → Append the rest (they must share the same codec, resolution, and container), keep Video/Audio on Copy, and save. The same keyframe rule applies at the join points.