Subsampling-Adaptive Directional Wavelet Transform for Image Coding

In lifting-based directional wavelet transforms, different subsampling patterns may show significant difference for directional signals in image coding. This paper investigates the influence of subsampling in directional wavelet transform. We show that the best subsampling depends on the direction and the directionality strength of the signal. To improve the coding performance, we further propose a subsampling-adaptive directional wavelet transform, which can use different subsampling patterns adaptively and according to the local characteristics of the image. To handle the boundary transition when subsampling changes, a phase completion process is applied to ensure that wavelet transform with various subsampling can be performed without introducing boundary effects and performance loss. Experimental results show that the proposed transform can achieve significant coding gain in image coding compared to other existing directional wavelet transforms.