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 based on the sequential JPEG algorithm. This minimum information-overhead method implies a transform coding technique with two different block sizes: N/spl times/N and 2N/spl times/2N pixels. The input image is divided into blocks of 2N/spl times/2N pixels and each of these blocks is classified according to its image activity. Depending on this classification, either four N-point or a single 2N-point 2-D DCT is applied on the block. The purpose of the algorithm is to take advantage of large uniform regions 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.