Switching between the SA-DCT and the EI-DCT in arbitrarily shaped image coding

In object-oriented image coding applications, the use of the SA-DCT (Shape-Adaptive Discrete Cosine Transform) and of the block-based DCT schemes associated to the EI (Extension-Interpolation) padding technique have shown promising results at high and low bit rates, respectively. Both the SA-DCT and the EI padding algorithm consist of two 1D-DCT processing. Recent works have shown that their efficiencies can be further improved by the most appropriate selection of the first direction to be processed. This paper introduces novel methods to determine the preferential direction of processing boundary blocks of the SA-DCT and of the EI padding technique. These methods are based on two known strategies related to the variances of the lengths of object segments in both directions, and a third one, which measures the energy compaction efficiency of the transforms. The proposed methods use the morphological feature TNOP (distribution of Texture according to the Number of Object Pixels) to adaptively select the most adequate strategy for each group of boundary blocks exhibiting similar number of object pixels. At last, it is introduced an adaptive switching scheme that selects between the proposed scheme for the block-based DCT or for the SA-DCT, when the available bit rate is known. This novel scheme allows different switching rules for distinct groups of boundary blocks, according to the feature TNOP. It outperforms the isolated use of both the SA-DCT and the block-based DCT schemes, at any specific bit rate.