1. This site uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn More.

MPEG compression or lack of

Discussion in 'MPEG-1 and MPEG-2 encoding (AVI to DVD)' started by dchetwyn, Mar 10, 2005.

  1. dchetwyn

    dchetwyn Member

    Joined:
    Mar 9, 2005
    Messages:
    3
    Likes Received:
    0
    Trophy Points:
    11
    I am writing some software that analyses some underwater footage of the Great Barrier Reef, we are towing a camera at roughly 2 mph and capturing on DVC-Pro.

    I am converting the footage to MPEG. When we are analysing the footage and something of interst goes past the user will pause the image and take a snapshot of that frame, be it a fish, crab, prawn, reef, shark etc. The problem is that we store (in a database) a reference to that frame in either milliseconds or a frame number but when we attempt to return to that exact frame it is close but never exact. Now I understand a little about the compression and that it will return to the I frame frame which generally occurs about every 12ish frames. My question is; is there any way using mpegs I can return to that exact frame, is there an I frame only MPEG, is this a special CODEC.

    Any help would be appreciated.

    I do not want to use AVI files that I know we can return to the exact frame.
     
  2. rebootjim

    rebootjim Active member

    Joined:
    May 13, 2004
    Messages:
    2,631
    Likes Received:
    0
    Trophy Points:
    66
    You could make a non-standard mpeg-2, with a GOP of 1. That may work with your database.
    BTW, I frames (in NTSC) occur every 18 frames, as there is usually 1 I frame per GOP.
     
  3. dchetwyn

    dchetwyn Member

    Joined:
    Mar 9, 2005
    Messages:
    3
    Likes Received:
    0
    Trophy Points:
    11
    Cheers Jim,
    Can you feed me a little more info. How would I go about doing this. We are, I believe using Pinnacle software to convert from the DVC-Pro to mpeg. I am not 100% sure what GOP is either.
     
  4. rebootjim

    rebootjim Active member

    Joined:
    May 13, 2004
    Messages:
    2,631
    Likes Received:
    0
    Trophy Points:
    66
    GOP is a Group Of Pictures. Standard NTSC is 18 frames per GOP, each containing 1 I frame, 5 P frames, and 2 B frames. I know that doesn't add up to 18, but somewhere I'm sure there's an explanation for that...anyhow...
    When an mpeg is encoded, the I frames are where the "edit points" are (you know this already).
    What I'm suggesting is to put a closed GOP of 1, instead of 18, thus forcing an I frame every GOP, or a GOP every I frame...
    This is totally non-standard, and I'm not sure it would play on anything but a computer, but it should give you your editing/database accuracy.
    You would have to encode a test video this way, and see if it works.
    Alternatively, you could force an I frame for every frame, in a standard 18frame GOP.
    I have NO idea if Pinnacle can do this. Hunt through it's encoding section options/preferences, and see.
    I know Tmmpgenc 2.5 Plus can do it, but again, I have NO idea if it would actually play on a standalone player.
    These are just suggestions for you to try, and may be so far out of DVD spec, that it will never work.
     
  5. Minion

    Minion Senior member

    Joined:
    Sep 28, 2003
    Messages:
    5,768
    Likes Received:
    0
    Trophy Points:
    116
    I"m petty Sure that you can Make DVD Compliant I Frame Only Files..I believe the NTSC Standard Just Calls for a Gop Length of 18 frames or Less so with a GOP length of 1 frame it being an I frame it should be DVD compliant...

    I actually just Checked in Tmpgenc and used one of the Standard NTSC DVD Templates and I was able to set the GOP to 1 Frame it being an I Frame and with the DVD Templates it will not let you change anything to a setting that is Not DVD Compliant....

    The Problem with I Frame only Video Files is that the File size will be Much Higher because I Frames are Less compressed than B or P Frames but the Quality should also be Very good....

    So I suggest you maybe try the 1 I Frame GOP and see how it works for you...Cheers
     
  6. rebootjim

    rebootjim Active member

    Joined:
    May 13, 2004
    Messages:
    2,631
    Likes Received:
    0
    Trophy Points:
    66
    Cool! Thanks for running that test.
     
  7. dchetwyn

    dchetwyn Member

    Joined:
    Mar 9, 2005
    Messages:
    3
    Likes Received:
    0
    Trophy Points:
    11
    Thanks Guys am onto it now and will keep you posted.
     

Share This Page