Why do we transmit the IBP frames out of order?

Question

We transmit the IBP frames like IPBBPBB and then we display them in IBBPBBP. This is the question, why do we do that. I can't just visualize it in my head. I mean, why not just transmit them in the order they are to be displayed?.

Answer 1

With bi-directional frames in temporal compression, decoding order (order in which the data needs to be transmitted for sequential decoding) is different from presentation order. This explains the effect you are referring to.

On the picture below, you need data for frame P2 to decode frame B1, so when it comes to transmission, P2 goes ahead.

See more on this: B-Frames in DirectShow

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Since MPEG-2 had appeared a new frame type was introduced - the bi-directionally predicted frame - B frame. As the name suggests the frame is derived from at least two other frames - one from the past and one from the future (Figure 2).

Since the B1 frame is derived from I0 and P2 frames both of them must be available to the decoder prior to the start of the decoding of B1. This means the transmission/decoding order is not the same as the presentation order. That’s why there are two types of time stamps for video frames - PTS (presentation time stamp) and DTS (decoding time stamp).

Answer 2

Briefly: it's done for decoder's speedup. You've mentioned usual MPEG2 GOP (Group of Pictures), so I'll try to explain answer for MPEG2. Though, H264 uses absolutely same logic.

For coder, picture difference is less, when calculated using not only previous frames, but successive frames too. That's why coder (usually) processes frames in display order. So, in IBBPBBP GOP every B frame can use previous I frame & next P frame to make prediction.

For decoder, it's better when every successive frame uses only previous frames for prediction. That's why pictures in bitstream are reordered. In reordered group IPBBPBB, every B frame uses I frame & P frame, which both are previous, and that's faster.

Also, every frame has it's own PTS (presentation timestamp), that implicitly determines display order - so it's no big deal that reordering is made.

This wikipedia article can give you answer to your question.