Variable complexity DCT approximations driven by an HVQ-based analyzer

Transform approximations are explored for speeding up the software compression of images and video. Approximations are used to replace the regular discrete cosine transform (DCT) whenever only a few DCT coefficients are actually encoded. We employ a novel hierarchical vector quantization (HVQ)-based image analyzer to drive a bank of coders, thus switching to the fastest coder depending on the image contents: smoother areas would produce less bits than detailed ones but faster. The HVQ analyzer is fast and simple enough to not offset the complexity gains brought by the DCT approximations. The approximations in this paper are applicable to high compression environments, where lower complexity is also required.