I am looking for an app to help me chop bits off start and end of an mp4 file efficiently and losslessly.
e.g. is there something that lets me find a keyframe and copy the lot between that and another keyframe, without needing to re-encode the lot (slow and lossy), and without losing AV sync?
By mp4, I mean H264 video + AAC audio in an MP4 container
Pre h264 days Avidemux was an excellent tool for this as you can reuse the original encoded streams meaning no loss and take just a few seconds to create the trimmed version, but it is hopeless with h264 - opening one gives you two options: use safe mode (which loses AV sync massively) or use the dangerous mode which immediately crashes it.
AFAIK, Openshot, Kdenlive, Pitivi etc. will all do this by decoding and recoding (slow and lossy).
Seems a simple thing (probably showing my lack of knowledge here!), so was wondering if I'm missing something.