Compressed image reproduction based on block decomposition

A compressed image reproduction scheme is proposed by properly decomposing and manipulating the coefficients of discrete cosine transform (DCT) directly in the compressed domain. The basic idea of the proposed approach is to decompose each DCT block into several sub-blocks and to adjust the brightness and detail components of a given image for compressing dynamic range and enhancing contrast. Image reproduction based on the subblock decomposition can be done more precisely than any approach based on the normal block-sized approach. First, DCT coefficients of each block are decomposed into several sub-blocks. Next each sub-block´s coefficients are separated into brightness and detail components, and treated differently according to content analysis. Then, the enhanced coefficients are projected on the constraint sets to avoid some artefacts, and are composed back to the original order. The main advantages of the proposed algorithm are that (i) it can enhance the dynamic range and details without affecting the compressibility of the given image since it operates directly in the compressed domain, and (ii) it does not boost blocking artefacts around big edges without any further processing. In order to evaluate the proposed scheme, several base-line approaches are described and compared using enhancement quality measures.