Extending RD-OPT with global thresholding for JPEG optimization

In this paper, the RD-OPT algorithm for optimizing discrete cosine transform quantization tables [Ratnakar and Livny, 1995] is extended to incorporate global thresholding. Performance gains are possible by zeroing off some DCT coefficients in DCT-based image compression. We describe a global thresholding scheme, in which the zeroing thresholds for coefficients can be arbitrarily different from those determined by the quantization table. Unlike local thresholding [Crouse and Ramchandran, 1995], the zeroing decisions are not made separately for each image block. This simplifies the use of thresholding, is easier to optimize and is almost as effective as local thresholding.