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Re: [videoblogging] Re: H264 and quicktime 7

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  • Michael Verdi
    ... Hey Steve, Actually, doubling the framerate doesn t double the file size. In fact it only produces a slight increase. This is something that seems very
    Message 1 of 13 , May 1, 2005
      On May 1, 2005, at 11:18 AM, Steve Watkins wrote:

      > Thanks for the samples.
      >
      > OK well I see your tests as being extremely promising for h.264.
      >
      > If I am thinking correctly Your h.264 example is handling 8 times
      > more information than
      > the 3ivx sample:
      >
      > Your 3ivx is at 320x240 wheras the h.264 at 640x480 has 4 times as
      > many pixels. And
      > the framerate on your H.264 clip is 30fps as opposed to 15fps in
      > the 3ivx example. So
      > thats 8 times as much raw video being handled in the h.264 clip,
      > but the filesize is only
      > twice as big as the 3ivx.
      >
      > have I got that right?
      >

      Hey Steve,
      Actually, doubling the framerate doesn't double the file size. In
      fact it only produces a slight increase. This is something that
      seems very counter intuitive but I've run across it before. I have
      no idea why this is. Does anybody out there know?
      Thanks,

      -Verdi
      http://michaelverdi.com
      http://freevlog.blogspot.com
    • Steve Watkins
      Yes its because of the way many forms of compression works. They record complete frames of video only occasionally (keyframes) and other frames are stored
      Message 2 of 13 , May 1, 2005
        Yes its because of the way many forms of compression works. They record complete
        frames of video only occasionally (keyframes) and other frames are stored like ' what has
        changed since the last frame' rather than the entire frame being stored.

        If you tell the encoder to make video of a certain datarate, it cant go above that no matter
        what framerate and resolution you use, so the file wont be any larger, just more
        compressed and more ugly.

        When I was talking about 4 and 8 times more information, I was talking about
        uncompressed video, how much 'raw information' the video encoder has to squeeze into
        whatever datarate you have specified.

        So returnign to the tiger store opening video as an example:

        The datarate for the 3ivx version is 258kbits/sec

        The datarate for the h264 version has beeen set to around 658kbits/sec

        These numbers probably include the audio too, but I wont worry about that for now.

        So the h.264 codec has been given just over twice as many bits per second in which to
        store the video. But it is being told to store the video with 4 times as many pixels and
        twice the number of frames = 8 times more raw video information in just over twice as
        much space.

        Anyway the words Ive used to explain this has probably made my explanation not 100%
        technically accurate, but hopefully its of some use.

        A starting point for testing how good h.264 really is would be to encode with exactly the
        same framerate, datarate, keyframe settings, resolution and audio settings as you already
        use in 3ivx. Does it look better than 3ivx then?

        More from me later once Ive actually managed to do testing on my mac.

        Cheers

        Steve of Elbows

        --- In videoblogging@yahoogroups.com, Michael Verdi <michael@m...> wrote:
        > Hey Steve,
        > Actually, doubling the framerate doesn't double the file size. In
        > fact it only produces a slight increase. This is something that
        > seems very counter intuitive but I've run across it before. I have
        > no idea why this is. Does anybody out there know?
        > Thanks,
        >
        > -Verdi
        > http://michaelverdi.com
        > http://freevlog.blogspot.com
      • Adrian Miles
        around the 1/5/05 Michael Verdi mentioned about Re: [videoblogging] ... 1. frame rate can affect file size but it is not symmetrical (double fps doesn t =
        Message 3 of 13 , May 4, 2005
          around the 1/5/05 Michael Verdi mentioned about Re: [videoblogging]
          Re: H264 and quicktime 7 that:
          >Actually, doubling the framerate doesn't double the file size. In
          >fact it only produces a slight increase. This is something that
          >seems very counter intuitive but I've run across it before. I have
          >no idea why this is. Does anybody out there know?

          1. frame rate can affect file size but it is not symmetrical (double
          fps doesn't = double size). This is due to delta frames, key frames
          and frame differencing.

          2. this is *basic* compression.

          3. A good modern codec (eg H.264) should be allowed to auto insert
          keyframes. If you override this you're going to produce stoopid sizes.

          4. a key frame is the codec saying, hey, I need to remember *all* the
          info for this frame, because it is so different from the last one.
          Then the codec only remembers what is different from that key frame
          for the next n frames. Until *it* decides it needs a new keyframe.
          (this is why you let it decide.) Very good codecs do this forwards
          and backwards (so how different from last keyframe, how different
          from next keyframe).

          5. this is why very good codecs do 2 passes, they have to analyse all
          this first.

          6. if you film yourself not moving, against a still background, you
          can compress down to about 1 kbit a second and it will look great
          (I'm serious). If you jump around all over the place then more
          keyframes are needed, there is much more differencing, file size is
          bigger.

          7. However, frame size makes a big difference. double frame size (say
          160 x 120 to 320 x 240) and you quadruple the amount of data required.

          8. However, dropping frame rate can also help since it doesn't have
          to draw so much, and you get less key frames (potentially).

          9. Never increase the frame rate in compression.

          10. Compression is lossy, it throws data away. If you try to add data
          then you are asking the codec to invent data it doesn't have. It
          would be like compressing to jpeg in photoshop, then converting it to
          32bit RGB, and complaining about the crappy quality of your eps
          artwork. :-)

          11. Never increase the frame size in compression.

          12. Never increase anything in compression.

          13. If you manually set keyframes (say every 5 seconds), and you've
          got 10 seconds of a vase, then you've added a pile of data that is
          100% irrelevant and unneeded.

          14. If you are serving off RTSP (real time) then you do manually
          insert keyframes. Videbloggers don't use rtsp, we use http (for very
          good reasons).
          --
          cheers
          Adrian Miles
          ____________
          hypertext.RMIT
          http://hypertext.rmit.edu.au/vlog
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