Zerotree coding of DCT coefficients

We show that an efficient quantizer originally developed for wavelet compression can be applied to DCT coding with excellent results. We have implemented a system for block-based, embedded, progressive, DCT image compression which makes use of the embedded zerotree wavelet (EZW) algorithm of Shapiro (1993). The image is partitioned into a number of distinct square blocks and the DCT applied to each of these individually. The coefficients are then rearranged into a sub-band structure which outwardly resembles a wavelet transformed image, with the number of sub-bands being related to the size of the DCT used. The zerotree coding algorithm is then employed to quantize the coefficients. The system outperforms the embedded zerotree wavelet scheme in a rate-distortion sense, with good performance from lossless (to the fixed point DCT precision) to high compression. The system gives the best trade between accuracy and high frequency ringing with a DCT block size of 16×16, corresponding to four sub-band scales