LosslessCut trims, splits, joins, remuxes and extracts media streams through FFmpeg. Its normal export copies already compressed audio and video streams instead of decoding and encoding every frame again. That keeps the original stream quality and makes many jobs faster. LosslessCut writes a separate output by design and does not overwrite the input file.
A lossless cut follows keyframes
Compressed inter-frame video cannot begin cleanly at every displayed frame. A normal lossless cut usually moves its starting boundary to an earlier usable keyframe. The exported file can therefore show frames before the exact position selected in the preview.
Experimental Smart Cut re-encodes material around the boundary and copies the rest. This can approach a frame-specific start without transcoding the whole file, but the documentation says it does not work with many inputs. A failed Smart Cut is not evidence that ordinary keyframe-aligned cutting cannot export the source.
Segments rearrange one media workflow
LosslessCut can divide a source into segments, rearrange them and export selected sections. A project saves segment information in an .llc JSON5 file. CSV, CUE and other supported formats can move segment lists through separate interchange paths.
The project file records editing decisions rather than embedding the source media. Moving it without the referenced media can leave segment timings with no file to open. The source and output remain independent files even when the project sits beside them.
Container matching is not codec matching
Remuxing changes the container while copying compatible streams. The target container must support the existing audio, video and subtitle streams. LosslessCut can also extract or remove individual tracks without building a conventional multitrack edit. Changing a filename extension alone does not establish that compatibility.
Merging files also requires matching codec parameters. Two inputs can share the same visible container extension and still differ in codec settings that prevent a stream-copy merge. LosslessCut does not place several unrelated sources on one shared editing timeline, and it omits common re-encoding operations such as scaling, overlays and audio mixing.
A playback proxy does not replace the source
The embedded Chromium player cannot decode every format that FFmpeg can process. Fastest FFmpeg-assisted playback creates a low-quality proxy for preview when the original will not play in the interface. Export still uses the original source, so proxy softness does not reduce output quality.
The reverse can also happen: a source may preview but fail during a requested merge or container copy because its stream parameters conflict with the output. Checking the exported boundaries and playing the final file remains necessary. A separate output also needs enough destination space even when processing finishes quickly. The speed of stream copying removes most re-encoding time; it does not remove keyframe, container or decoder constraints.
