Adaptive block-size transform coding for image compression

In this paper we report the results of an adaptive block-size transform coding scheme that is bascd on the sequential JPEG algorithm. This minimum information-overhead method implies a transform coding technique with two différent block sizes: <i>N</i> x <i>N</i> and 2<i>N</i> x 2<i>N</i> pixels. The input image is divided into blocks of 2<i>N</i> x 2<i>N</i> pixels and each of these bocks is classified according to its image activity. Depending on this classification, either four N-point or a sngle 2<i>N</i>-point 2-D DCT is applied on the block. The purpose of the algorithm is te take advantage of large uniform régions that can be coded as a single large unit instead of four small units — as it is made by a fixed block-size scheme. For the same reconstruction quality, the results of the adaptive algorithm show a significant improvement of the compression ratio with respect to the non-adaptive scheme.