Low-Complexity Transform Adjustments for Video Coding

Recent video codecs with multiple separable transforms can achieve significant coding gains using asymmetric trigonometric transforms (DCTs and DSTs), because they can exploit diverse statistics of residual block signals. However, they add excessive computational and memory complexity on large transforms (32-point and larger), since their practical software and hardware implementations are not as effi-cient as of the DCT-2. This article introduces a novel technique to design low-complexity approximations of trigonometric transforms. The proposed method uses DCT-2 computations, and applies orthogonal "adjustments" to approximate the most important basis vectors of the desired transform. Experimental results on the Versatile Video Coding (VVC) reference software show that the proposed approach significantly reduces the computational complexity, while providing practically identical coding efficiency.