Predicted and difference frames
The motion compensation vector alone is not sufficient to define the video contents of a picture frame. It may define a moving block but it fails to define any new elements such as the background that may have been revealed by the movement of the block. Further information is therefore necessary. This is obtained by first predicting what a frame known as the P-frame would look like if it were reconstructed using only the motion compensation vector and then comparing this with the actual frame. The difference between the two contains the necessary additional information which, together with the motion compensation vector, fully defines the contents of the picture frame. The P-frame is constructed by adding the motion vector to the same frame that was used to obtain the very same motion vector. The P-frame is then subtracted from the current frame to
generate a difference frame, which is also known as the residual or pre- diction error. The difference frame now consists of a series of pixel values, a format suitable for subsequent spatial data compression.
Referring to Figure 4.9, current frame F0 is fed into Buffer 1 and held there for a while. It is also fed into the movement vector generator which uses the contents of the previous frame F-1 stored in video memory to obtain the motion vector MV0. The motion vector is then added to F-1 to produce predicted frame P0 which is compared with the contents of the current frame F0 in Buffer 2 to produce residual error or difference frame D0. Residual error D0 is fed into the spatial DCT encoder and sent out for transmission. Encoded D0 is decoded back to reproduce D0 as it would be reproduced at the receiving end. D0 is then added to P0 which has been waiting in Buffer-2 to reconstruct the current frame F0 for storage in the video memory for the next frame and so on.