Motion compensation requires a large amount of computation. Consider finding a matching block for an 8×8 block. Each comparison requires taking 64 differences and then computing the sum of the absolute value of the differences. If we assume that the closest block in the previous frame is located within 20 pixels in either the horizontal or vertical direction of the block to be encoded, we need to perform 1681 comparisons. There are several ways we can reduce the total number of computations
One way is to increase the size of the block. Increasing the size of the block means more computations per comparison. However, it also means that we will have fewer blocks per frame, so the number of times we have to perform the motion compensation will decrease. However, different objects in a frame may be moving in different directions. The drawback to increasing the size of the block is that the probability that a block will contain objects moving in different directions increases with size. Consider the two images in Figure 18.11. If we use blocks that are made up of 2×2 squares, we can find a block that exactly matches the 2×2 block that contains the circle. However, if we increase the size of the block to 4×4 squares, the block that contains the circle also contains the upper part of the octagon. We cannot find a similar 4×4 block in the previous frame. Thus, there is a trade-off involved. Larger blocks reduce the amount of computation; however, they can also result in poor prediction, which in turn can lead to poor compression performance.
The Loop Filter
Sometimes sharp edges in the block used for prediction can result in the generation of sharp changes in the prediction error. This in turn can cause high values for the high-frequency coefficients in the transforms, which can increase the transmission rate. To avoid this prior to taking the difference, the prediction block can be smoothed by using a two-dimensional spatial filter. The filter is separable; it can be implemented as a one-dimensional filter that first operates on the rows, then on the columns. The filter coefficients are 1/ 4 1/ 2 1/ 4 , except at block boundaries where one of the filter taps would fall outside the block. To prevent this from happening, the block boundaries remain unchanged by the filtering operation.