Inter-frame (P-frame) Coding

The previously discussed intra frame coding techniques were limited to processing the video signal on a spatial basis, relative only to information within the current video frame. Considerably more compression efficiency can be obtained however, if the inherent temporal, or time-based redundancies, are exploited as well. Anyone who has ever taken a reel of the old-style super-8 movie film and held it up to a light can certainly remember seeing that most consecutive frames within a sequence are very similar to the frames both before and after the frame of interest. Temporal processing to exploit this redundancy uses a technique known as block-based motion compensated prediction, using motion estimation. A block diagram of the basic encoder with extensions for non-intra frame coding techniques is given in Figure 7.14. Of course, this encoder can also support intra frame coding as a subset.

 

P-Frame Coding

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,Š

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.

P-coding can be summarised as follows:

• An Coding Example (P-frame)

• Previous image is called reference image.

• Image to code is called target image.

• Actually, the difference is encoded.

• Subtle points:

  1.

  Need to use decoded image as reference image, not original. Why?

  2.

  We're using "Mean Absolute Difference" (MAD) to decide best block. Can also use "Mean Squared Error" (MSE) = sum(E*E)