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Old 18 Jul 2002, 09:18 AM   #1
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Angry-2 I-P-B frames Explained

A quick explaination of an MPEG "GOP", or, "Group Of Pictures",

GOP - Begins with an "I" frame, followed usually by a number of "P" and "B" frames (divx5 only uses B frames I believe)
- each GOP is independant: all frames needed for predictions are
contained within each GOP
- GOP's can be as small as a single I frame, or as large as desired, but usually no more than 15 frames in length.

- the longer the GOP, the more efficient, but less rubust the
coding

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. This also means of course, that your P3500 media box in you living room might struggle with decoding a high rate D5 encode (not sure about that, but D5 is a more intense encoding/decoding process, but DVD's use I, P, and B frames, sooooooo...

Oh, BTW, in 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.

Note: this has no role in 'fps'


One thing though:
DivX5 doesn't use only B-frames, it uses P-frames as well like its predecessor. Contrary to MPEG2, DivX5 uses a "PB" sequence chain instead of "PBB". The latter would result in better compression, but the way I understand it the avi format won't work correctly with more that one sequential B-frame.
Still, using "PB" instead of only P-frames results in a serious size decrease already, so it was definately worth it.
To be entirely accurate it uses "BP" grouping (i.e. frame n is B and frame n+1 is P) but it appears as "PB" because you cant decode a frame until the frames it predicted off are decoded.
And so p frames have to be decoded before the B frames between them. 1/4 Pixel accuracy is used in motion estimation to get the best fit for each macro-block. say you have a panning camera and each frame the picture hasnt moved an entire number of pixels accross the screen then 1/4 accuracy helps get the predicted macro-blocks in a much better possition. This isnt used in I-Frames because motion estimation isnt used in I-frames because it is basically a JPEG encoded frame using a different Quant value (typically 16).
I dont think MPEG-2 uses more than 1 B frame but i could be wrong.
The use of B-Frames does improve the compression/quality of the codec by a considerable amount but requires a lot more motion estimation (a large portion of hte encoding time) to be done hence hte much longer encoding times. B frames are typically 1/2 the size of P-Frames but require more encoding and decoding.


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L A M E R ! ! !

Last edited by UncasMS; 24 Jul 2002 at 09:52 AM
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Old 18 Jul 2002, 12:03 PM   #2
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Old 21 Jul 2002, 06:17 PM   #3
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excellent work.
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Old 24 Jul 2002, 09:17 AM   #4
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Quote:
excellent work
.... or plagiarism!




what you present as YOUR posting is a mixture and cheap plagiarism of doom9 forum members like -i-, acaila and GAteKeeper!!


you were simply copying somebody elses words under your name in this forum WITHOUT the slighest hint, that what you 'present' as your work has been posted by other people at some other place before.

be asured that any such kind of plagiarism will not be accepted and thus i want your opinion on this.

here is the original posting:
http://forum.doom9.org/showthread.php?threadid=19436

next time try to even make up your own mind, do NOT post something under your name, that other people have said before - no matter who and where!

there's hardly anything i despise more than this kind of cheap plagiarism!

Last edited by UncasMS; 24 Jul 2002 at 09:46 AM
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Old 21 Nov 2008, 03:34 AM   #5
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This site is very impressive.....
Thanks.....
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Old 27 Nov 2008, 08:46 PM   #6
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Lightbulb-2 I-P-B frames Explained

hi,

A quick explaination of an MPEG "GOP", or, "Group Of Pictures",

GOP - Begins with an "I" frame, followed usually by a number of "P" and "B" frames (divx5 only uses B frames I believe)
- each GOP is independant: all frames needed for predictions are
contained within each GOP, is it possible to include linkstar.co.uk adverts within an iframe?
- GOP's can be as small as a single I frame, or as large as desired, but usually no more than 15 frames in length.

Last edited by ManuelBanks; 3 Feb 2009 at 02:05 PM
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