frame_description.txt

I frame - "Intra-coded" frames : average 7:1 reduction.

- like JPEG, every video frame is broken into blocks of 8x8
pixels of Y, R-Y, and B-Y (although, I am not sure how
this "1/4 pixels" divx5 has plays into all this)

- blocks are grouped into "macroblocks" of 16x16

- macroblocks are grouped horizontally into slices which
have similar average block levels.

- multiple slices form a frame, and these frames are the
resulting "I" frames.

P frame - P frames are predicted based on prior I or P frames plus
the addition of data for changed macroblocks.

- average about 20:1 reduction, or about half the size of I
frames

- I don't think divx5 uses these, MPEG2 does though.

B frame - Bidirectionally predicted frames based on appearance and
positions of past and future frames macroblocks.

- B frames require less data than P frames, averaging about
50:1 reduction.

- B frames require more decoder buffer memory because 2
frames are compared during the reconstruction process.

- B frames also require manipulation of the coding order:
frames moving from the coder to the decoder are NOT in
presentation sequence. 

basically, the the B frame will say something like "this frame is the same as the GOP's "I" frame except this one part, I will only contain the data needed to encode this one part, and combine it with the info from the I frame", in laymen's terms of course. This give DivX5 it's optimal reduction capability.


MPEG2 at least, a GOP order is always IPBBPBBPBBIPBBPBB etc etc. (pending on your GOP size), but it is always 1 I, 1 P, and 2 B's, then you can stack more groups of "PBB"'s in that one GOP if needed (usually up to 15 total frames.