Extract exact segment from avi

Discussion in 'Amateur Video Production' started by wassa, Dec 6, 2006.

  1. wassa

    wassa Guest

    Sometimes when I'm trying to extract segments from an avi video using
    "direct stream copy" function of virtualdub I can't get it to extract
    from the exact starting frame I've specified. Instead it chooses to go
    back a few seconds and start from there. Can anyone explain why this is
    happening and how can I get around it (without recompressing). Thanks.
     
    wassa, Dec 6, 2006
    #1
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  2. wassa

    Jukka Aho Guest

    What is the codec that is being used in these problematic files?

    Observe the keyframe indicator ("[K]") to the left of the timestamp
    display, at the bottom of the window. If your original AVI file is using
    a codec/format in which not all frames are keyframes, VirtualDub will
    (probably) only allow cutting at keyframes in "Direct stream copy" mode.

    For more information, see
    <file:///F:/apps/mmedia/VirtualDub/help/p-mainui.html> (the "Timestamp
    display" section.)
     
    Jukka Aho, Dec 6, 2006
    #2
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  3. wassa

    Bill's News Guest

    Everything Mr. Aho said plus:

    MPEG4ish codecs allow for long GOPs, 300 or more frames
    (compared with MPEG2's 18 or so). In such cases, forced cutting
    at a key frame can leave quite a bit of unwanted video. In
    order to cut at a precise frame other than a key frame the video
    of that portion might have to be reconstructed; VDub never
    included this sort of programming - nor any warning that you are
    not viewing a keyframe when making the cut. For my own xvid
    conversions, I set the max GOP to 60 frames. If I need to do a
    cut later, it's more reasonable to deal with, as I'd usually
    want to cut in a blackout area any way.

    The only way around it, depending upon your purpose in making
    the cut, it to overlap the cuts. Cut at I-frame n, then
    reposition to I-frame n-1 to begin the next segment. This works
    fine when there will be a long pause at playback time between
    the cuts - a media change, for example.
     
    Bill's News, Dec 9, 2006
    #3
  4. wassa

    HealthBob Guest

    HealthBob, Dec 12, 2006
    #4
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