Adaptive block-size transform coding for image compression

In this paper we report the results of an adaptive blocksize transform coding scheme that is based on the sequential JPEG algorithm. This minimum informationoverhead method implies a transform coding technique with two di erent block sizes: N N and 2N 2N pixels. The input image is divided into blocks of 2N 2N pixels and each of these blocks is classi ed according to its image activity. Depending on this classi cation, 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 xed block-size scheme. For the same reconstruction quality, the results of the adaptive algorithm show a signi cant improvement of the compression ratio with respect to the non-adaptive scheme.