In the conventional iterated function system(IFS) image compression

the threshold of distortion between range block and domain block is prefixed which does not take the statistics characteristics of current range block into account. Thus the encoding scheme can not adapt to different input images and sometimes the encoding performance relies on the skill of the operator. In order to avoid that problem

a new scheme to compress still grayscale image is presented. The difference between this new scheme and the conventional one is that the threshold in the new scheme is calculated from the variance of the range block. Therefore the encoding system can adapt to the given image. The threshold is analyzed and a conclusion is drawn that the threshold is a positive portion of the variance of current range block. Furthermore

this new encoding scheme is proved to be in accordance with features of human visual system. The adaptive encoding scheme improves the effectiveness of encoding. This new scheme also makes the IFS encoding algorithm more practical.