Lifting-Based Directional DCT-Like Transform for Image Coding
Traditional 2-D discrete cosine transform (DCT) implemented by separable 1-D transform in horizontal and vertical directions does not take image orientation features in a local window into account. To improve it, we propose to introduce directional primary operations to the lifting-based DCT and thereby derive a new directional DCT-like transform, whose transform matrix is dependent on directional angle and interpolation used there. Furthermore, the proposed transform is compared with the straightforward one of first rotated and then transformed. A JPEG-wise image coding scheme is also proposed to evaluate the performance of the proposed directional DCT-like transform. The first 1-D transform is performed according to image orientation features, and the second 1-D transform still in the horizontal or vertical direction. At the same time, an approach is proposed to optimally select transform direction of each block because selected directions of neighboring blocks will influence each other. The experimental results show that the performance of the proposed directional DCT-like transform can dramatically outperform the conventional DCT up to 2 dB even without modifying entropy coding.