Adaptive Coding Based on Quad Tree Segmentation

In the last chapter we saw one adaptive coding method which classifies each block and then, depending on which class the block belongs to, uses an appropriate set of quantizers. In this and the following chapter we will discuss a second adaptation technique which varies the block size, rather than the quantizers, according to image activity. In this way it will be possible to ensure that the prediction is only made over regions which are relatively smooth and so avoid the large prediction errors which can occur when a block contains a sharp edge. This will then provide a residual which will be a much better candidate for a stationary source. In section 6.2 we will discuss adaptive segmentation in general, and then in section 6.3 we will describe the specific quad tree approach which leads to unequal block sizes. We will complete the segmentation topic in section 6.4 with ways to minimize the overhead incurred by the adaptivity, and in section 6.5 we will discuss modifications to the reconstruction made possible because of the unequal block sizes. We will then turn, in section 6.6, to the second half of the algorithm—coding the residual, and conclude with some results in section 6.7 and conclusions in section 6.8.