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24th September 2012, 16:51 | #1 | Link |
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Join Date: Aug 2012
Posts: 15
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Is reencoding lossless?
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? |
24th September 2012, 19:39 | #3 | Link |
Software Developer
Join Date: Jun 2005
Location: Last House on Slunk Street
Posts: 13,248
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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.
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