Can someome explain I, B & P frames......pleeeeeeease!

[Emanef]

I've been through all the help files I can find here, and those with my Hercules Radeon All in Wonder 7500, but can't find anything actually expleining what the differance between I, B & P frames, other than the I frames are easier to process than B & P.

Can someone, please, please, please just explain the difference between the three frame types. Does all digital video consist of the three frame types, or they only relevant to captured video? Are they different ways of processing the same video parts? Please....I'm not stoopid, I just want a proper explanation (but not tooooo technical!) so I can understand reasons for using the different settings with regards to frame types.

Cheers guys.....

[mute]

From http://members.aol.com/symbandgrl/

P Frames

Starting with an intra, or I frame, the encoder can forward predict a future frame. This is commonly referred to as a P frame, and it may also be predicted from other P frames, although only in a forward time manner. As an example, consider a group of pictures that lasts for 6 frames. In this case, the frame ordering is given as I,P,P,P,P,P,I,P,P,P,P,…

Each P frame in this sequence is predicted from the frame immediately preceding it, whether it is an I frame or a P frame. As a reminder, I frames are coded spatially with no reference to any other frame in the sequence.



B Frames

The encoder also has the option of using forward/backward interpolated prediction. These frames are commonly referred to as bi-directional interpolated prediction frames, or B frames for short. As an example of the usage of I, P, and B frames, consider a group of pictures that lasts for 6 frames, and is given as I,B,P,B,P,B,I,B,P,B,P,B,… As in the previous I & P only example, I frames are coded spatially only and the P frames are forward predicted based on previous I and P frames. The B frames however, are coded based on a forward prediction from a previous I or P frame, as well as a backward prediction from a succeeding I or P frame. As such, the example sequence is processed by the encoder such that the first B frame is predicted from the first I frame and first P frame, the second B frame is predicted from the second and third P frames, and the third B frame is predicted from the third P frame and the first I frame of the next group of pictures. From this example, it can be seen that backward prediction requires that the future frames that are to be used for backward prediction be encoded and transmitted first, out of order. This process is summarized in Figure 19. There is no defined limit to the number of consecutive B frames that may be used in a group of pictures, and of course the optimal number is application dependent. Most broadcast quality applications however, have tended to use 2 consecutive B frames (I,B,B,P,B,B,P,…) as the ideal trade-off between compression efficiency and video quality.

The main advantage of the usage of B frames is coding efficiency. In most cases, B frames will result in less bits being coded overall. Quality can also be improved in the case of moving objects that reveal hidden areas within a video sequence. Backward prediction in this case allows the encoder to make more intelligent decisions on how to encode the video within these areas. Also, since B frames are not used to predict future frames, errors generated will not be propagated further within the sequence.

One disadvantage is that the frame reconstruction memory buffers within the encoder and decoder must be doubled in size to accommodate the 2 anchor frames. This is almost never an issue for the relatively expensive encoder, and in these days of inexpensive DRAM it has become much less of an issue for the decoder as well. Another disadvantage is that there will necessarily be a delay throughout the system as the frames are delivered out of order as was shown in Figure 19. Most one-way systems can tolerate these delays, as they are more objectionable in applications such as video conferencing systems.

Found a smaller description at http://icsl.ee.washington.edu/~woobin/papers/General/node4.html#SECTION00021100000000000000

Temporal Redundancy Reduction
To support random access to the stored video while exploring the maximum redundancy reduction using temporal predictions, three types of pictures are defined in MPEG: intra ( I) pictures, predicted ( P) pictures, and bidirectionally interpolated ( B) pictures. I pictures provide access points for random access, but only with a moderate compression. P pictured are coded with reference to a previous picture, which can be either an I or P picture. B pictures are intended to be compressed with a low bit rate, using both the previous and future references. The B pictures are never used as the references. The relationship between the three picture types is illustrated in Fig. 1. The MPEG standard does not impose any limit to the number of B pictures between the two references, or the number of pictures between two I pictures.

HTH

[Emanef]

Excellent, thank you....I'll have a proper read later.