file size and frame size - correlated?

Discussion in 'Amateur Video Production' started by Doctor Zaius, Aug 27, 2004.

  1. Doctor Zaius

    Doctor Zaius Guest

    I took a 20 sec 704x480 uncompressed RGB AVI (formerly a VOB file that I
    opened, deleted all but 20 sec, and saved w/vdub) and then opened it up and
    compressed it in vdub using the "DivX MPEG-4 (Low-Motion)" codec using a
    "Data Rate" slider setting of 510 Kilobits/s at 3 different frame sizes:
    640x480, 320x240, and 180x120.

    To my surprise all three clips were almost equal in size, about 1600KB.

    I had assumed the clip size would increasr proportionately with frame size.
    What am I missing?

    Doctor Zaius
    Doctor Zaius, Aug 27, 2004
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  2. Doctor Zaius

    DeepOne Guest

    Bitrate and running time are really the only factors which determine
    the size of the resulting file. The resolution only affects how high
    a bitrate you will need in order to achieve acceptable results.
    DeepOne, Aug 27, 2004
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  3. Doctor Zaius

    Doctor Zaius Guest

    Thank you for your reply. Is there a rule of thumb or direct calculation
    for determining how much higher a bitrate one would need to use when going
    from a lower frame size (which played fine, w/good quality) to a higher
    frame size?

    IOW, theoretically speaking, would a bitrate of twice that used to produce
    a good 320x240 clip be required when producing (compressing in vdub) a
    640x480 version of the same clip?
    Doctor Zaius, Aug 28, 2004
  4. Probably four times :)

    You've doubled both dimensions.

    Gene E. Bloch, Aug 28, 2004
  5. Doctor Zaius

    DeepOne Guest

    The following web page has graphs of appropriate bitrates for
    different DVD-compliant resolutions (the resolutions you mention are
    not DVD compliant):
    (Scroll down about half way for the graphs)

    Those graphs are for MPEG(2). You mentioned that you were using DivX,
    and I have no idea what bitrates are appropriate for that except that
    I think they might be lower.
    DeepOne, Aug 28, 2004
  6. Over the weekend, I thought about my answer above. I realize that in
    truth I know nothing about this, and my naive response could be
    quite wrong, depending on how these things are encoded. In
    particular, there's the interlaced/non-interlaced problem...

    Anyone care to comment? Politely, if you would :)

    Gene E. Bloch, Aug 30, 2004
  7. Doctor Zaius

    DeepOne Guest

    If you follow the link I posted and look at the graphs, it seems that
    your assumption was fairly accurate.
    DeepOne, Aug 30, 2004
  8. wrote in
    Cool! I should've followed your link sooner...without needing to be
    led by the hand. Oh well.

    Looking at the shoulders of the graphs, the desired bit rate follows
    the pixel area quite linearly, given the resolution of the graphs.

    As a reward, I present you with Bloch's Lemma: Everything is linear,
    to a first approximation.

    (Insert smiley here.)

    Gene E. Bloch, Aug 31, 2004
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