Consider the three video GOP structures in video coding standards MPEG-2 and H.2
ID: 3710203 • Letter: C
Question
Consider the three video GOP structures in video coding standards MPEG-2 and H.264/AVC: a) IBBPBBBPBB b) IPPPP c) IIIIII .... (I: Intra coded, P: Predictive coded(undirectional) B: Bidirectional predictive codes)
Rank the GUP suucures3D video. Explain. (5 pts) For each GOP structure, what is the decode order of the frames and what is the maximum number of frames that need to be stored in memory at the decoder side (5 pts) a. b. hich GnP structure is most robust under transmission errors? Why? (5 pts)Explanation / Answer
GOP: Group of Pictures
1) Generally, this terminology is associated with MPEG video encoding. Here the group of pictures is assembled which helps to enhance the visual result of the video. It means that each and every coded video stream has some group of GOPs.
2) A MPEG vedio stream is consists of a succession of I, P and B frames. - I ONLY: The video stream is consists of only I frames. (In Question given as (c))
- IP ONLY: The video stream is consists of I and P frames. In this case, the I-frames are called as key-frames. (In Question given as (b))
- IPB: The video stream is consists of all three frame types. In this case, GOP-size must be a multiple of 3. Here also, I-frames are called as key-frames. (In Question given as (a))
3) GOP begins with an I frame, which is usually followed by a number of P and B frames.
4) For GOPs, frames are coded and decoded in a different order than they are displayed because of the bidirectional prediction for B frames.
(a) IBBPBBBPBB
The source order/ Encoder input order:
I(1) B(2) B(3) P(4) B(5) B(6) B(7) P(8) B(9) B(10)
Transmission order:
I(1) P(4) B(2) B(3) P(8) B(5) B(6) B(7) B(9) B(10)
Decoder output:
I(1) B(2) B(3) P(4) B(5) B(6) B(7) P(8)B(9) B(10)
Explanation - Due to the bidirectional nature of B frames prediction the decoder must first process the previous and next reference frames(This referencing structure adapted in MPEG and H.264 reduces the buffer load). In above example, to decode B(2) the decoder must first have I(1) and P(4).
(b)IPPPP
(c)IIIIII ...
Here, the GOP structure is repeated throughout the bit stream.
If a cut appears in the middle of a GOP, usually the software will decode all the frames in the GOP and recode with an I frame in a suitable position.
?