What's the purpose of half resolution?

Discussion in 'Amateur Video Production' started by Doc, Sep 22, 2004.

  1. Doc

    Doc Guest

    What is the half resolution capture setting for? I find that when I
    capture in that mode, playback looks elongated.
     
    Doc, Sep 22, 2004
    #1
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  2. Doc

    Martin Guest

    Think in terms of picture quality.

    Half resolution = more bits per pixel = better quality.

    martin.
     
    Martin, Sep 22, 2004
    #2
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  3. Doc

    Doc Guest

    Okay, but what do you do with it? Obviously the elongated image isn't
    usable. Is it meant for output to tv/video tape?
     
    Doc, Sep 22, 2004
    #3
  4. Doc

    Jukka Aho Guest

    The image is not elongated. Rather, the area that each individual
    pixel is considered to represent is just of some other shape than
    square (rectangular, either way.)

    Or more accurately, the sampling grid does not have the same density
    in the X direction as it has in the Y direction.

    Yet another way to look into it: the pixel aspect ratio (x/y or
    y/x, whichever way you want to represent it) is something else
    than 1/1.

    If you see elongated images on your screen, that is because your
    media player software lacks proper scaling (interpolating) and
    aspect ratio support, and simply dumps the image data pixels 1:1
    to the screen pixels.

    * * *

    Note that in the widely-used ITU-R BT.601 based full screen capture
    resolutions (such as 720×576 or 704×576 for "PAL" and 720×480 or
    704×480 for "NTSC") the pixels are not "square", either, but still
    slightly elongated - so even images captured in these formats need
    corrective scaling/interpolating in order to view them properly on
    a computer using square pixels.

    You might want to take a look at

    <http://www.iki.fi/znark/video/conversion/> and
    <http://www.lurkertech.com/lg/pixelaspect.html>

    * * *

    As for the purpose of half resolution, DVD players, for example,
    can display 352×576 ("PAL") and 352×480 ("NTSC") images interpolated
    over the full screen.

    There are also other variations, such as the 2/3 BT.601 sampling
    employed by the SVCD format (480 pixels across the screen, making
    the image 480×576 for "PAL" and 480×480 for "NTSC" - yet the image
    correctly fills up the screen when viewed on an SVCD [software or
    hardware] player.)
     
    Jukka Aho, Sep 22, 2004
    #4
  5. Doc

    Martin Guest

    And let's not forget VCD resolution - it's valid for DVD too.
    352x288 PAL or 352x240 NTSC.

    Martin.
     
    Martin, Sep 22, 2004
    #5
  6. Doc

    Doc Guest

    Hmm. Not sure if I get it. I'll elaborate where the settings are.

    The settings occur in a box labeled "Frame Adjust"

    Under "Horizontal" are selections for Full Res. and Half Res.

    Under "Vertical" are selections for One or Both Fields.

    Then there's the cropping selections.- None or TV.

    When "half resolution" is picked,the AV_IO interface becomes narrow, and on
    playback on the computer, the image does the same whether played back within
    AV_IO, Windows Media Player or WinDVD. However, I do notice that if I
    playback the image to a TV, it looks normal.
     
    Doc, Sep 23, 2004
    #6
  7. Doc

    Jukka Aho Guest

    This is quite normal. AV_IO is primarily a capture utility; it
    probably does not care about aspect ratios at all. (Instead of
    trying to make the image look "right" on a computer screen, it
    simply displays the raw, captured pixel data, mapped 1:1 to the
    screen pixels.)

    As for why you get the same behavior from the Windows Media
    Player or WinDVD, I can only guess, but maybe the capture
    utility (or rather, the codec that you use while capturing)
    does not record the meta information about the correct frame
    aspect ratio into the AVI files.

    You might want to try viewing them in a bit more versatile
    program, such as Media Player Classic [1], and experimenting
    with the different scaling options:

    [1] <http://www.afterdawn.com/software/video_software/
    video_players/media_player_classic.cfm>

    * * *

    Anyhow, even though there was some sort of problem in getting the
    standard Windows Media player to display the files correctly, you
    can still use them as a source material for e.g. creating a
    half-res DVD. It all really boils down to your application. What
    is your source material? What is your target format? Where do you
    want to use the video files?

    These days, people usually want to make DVD's out of their captured
    video. Is this what you are trying to do? Or is it something else?
    Using half-res captures may help you in saving space if you need to
    squeeze lots of material on a single DVD disc. If encoded to MPEG-2
    properly, the images will stil display correctly on a tv as far as
    proportions go; they are just going to have a bit more mushy look
    since there is less of horizontal resolution.

    Knowing what you actually intend to _do_ with the captured video
    would help in making a better judgement about whether using the
    halved capture resolution would be of any real use (or benefit)
    to you.
     
    Jukka Aho, Sep 23, 2004
    #7
  8. Doc

    Doc Guest

    Your explanation helped make things clearer, thanks.
    Primarily DVD perhaps some VCD too.
     
    Doc, Sep 23, 2004
    #8
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