Is reencoding lossless?

[senze]

If I do some cutting with e.g. virtual dub at P- or B-Frames what happens then?

Will there be an I Frame generated at the position? And more importantly, can someone tell me if this process will diminuish the perceived quality of the video?

In general, are there some intra or inter-prediction codecs which I can use to decode and then reencode into the same quality without having any quality loss?

[SassBot]

You can't cut in virtualdub at P or B frames. Any applications that allow cutting on P or B frames will reencode the GOP so it would not technically be lossless.

[LoRd_MuldeR]

Re-encoding is never lossless, except when encoding to an un-compressed (or lossless compressed) format. There are no lossy formats that can re-encode in a lossless way.

When dealing with lossy compressed formats (MPEG2, MPEG-4 ASP, H.264/AVC, etc) then re-encoding will unavoidably cause some "generation loss".

Nevertheless, if you only are cutting and don't apply any filters, re-encoding may be avoided altogether. But this requires that you are cutting only at key-frames (I-Frames).

As soon as you are not cutting at an I-Frame (i.e. in the middle of a GOP), re-encoding will be required!

Some tools use "smart" re-encoding though, which means that only the first/last GOP of a segment (where you did the cut) will be re-encoded, while the rest is copied over 1:1.

Intra-only compression (e.g. MJPEG, AVC-Intra, etc) allows you to cut at any point without re-encoding, simply because all frames are I-Frames.