Factors influencing image coding performance of the shape-adaptive DCT

In the paper two factors influencing image compression efficiency of SA DCT are examined. Firstly, it is shown that the transform performance is strongly dependent on the smoothness of coded shape border. It is shown that even miniscule corrections of the contour may result in important improvement of restored images, both in terms of measured error, as well as when evaluated subjectively. Secondly, relation between important for coding efficiency orthogonality and DC preserving property of the SA DCT is discussed. The general condition for a transform to be DC preserving is formulated. It implies that there is no easy way to guarantee that a separable shape adaptive transform is both DC preserving and orthogonal. The DC corrected SA DCT is a good compromise solution to this problem.