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
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
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
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