Variable quality image compression system based on SPIHT

摘要

An algorithm for variable quality image compression is given. The idea is to encode different parts of an image with different bit-rates depending on their importance. Variable quality image compression (VQIC) can be applied when a priori knowledge on some regions or details being more important than others is available. Our target application is digital mammography, where high compression rates achieved with lossy compression are necessary due to the vast image sizes, while relatively small regions containing signs of cancer must remain practically unchanged. We show how VQIC can be implemented on top of SPIHT (Said and Pearlman, 1996), an embedded wavelet encoding scheme. We have revised the algorithm to use matrixes, which gives more efficient implementation both in terms of memory usage and execution time. The effect of the VQIC on the quality of compressed images is demonstrated with two test pictures: a drawing and a more relevant mammogram image.