Reduced-complexity shape-adaptive DCT for region-based image coding

We propose a computationally-efficient variant of the shape-adaptive discrete cosine transform (SA-DCT) currently considered for MPEG-4. Although the SA-DCT complexity is acceptable for 8/spl times/8 blocks, it is very high when complete regions are processed at once. To reduce the SA-DCT complexity, we replace its 1-D DCT with a quasi-DCT algorithm and we assure that the quasi-DCT basis functions are very close to those of the DCT. Unlike in our previous approach, we carry out an optimization of the shape of low-index basis functions. We test the new method numerically and subjectively, and conclude that, in terms of energy compaction performance, the new method gains up to 0.5 dB compared to our previous quasi-DCT approach.