Image compression coder having improved bit rate control and block

Abstract

A method of bit rate control and block allocation for discrete cosine transform (DCT) image signal compression includes the steps of (a) partitioning the image signals in blocks representing portions of the total image, (b) calculating DCT coefficients for image data in all blocks, (c) obtaining a measure of block activity (BACT) for each block based on DCT coefficients and for the total image activity (ACT) as a sum of the measures of all block activity, and (d) determining a code allocation factor (AF) for each block based on the ratio of block activity (BACT) to target code volume (TCV data) for the coded image data. Thereafter, step (e) includes allocating bits for each block using the allocation factor (AF) for each block and the target code volume (DCV data). Steps (a-d) are carried out with a first statistical pass through the image data, and step (e) is carried out in a second compression pass through the image data using the code allocation factor (AF) from step (d) and an estimation of unpredictable features for each block. Step (e) further includes comparing the estimation of unpredictable features to the accumulated actual code volume and adding extra bits to the allocation for at least the next block allocation and subtracting borrowed bits from at least the next block allocation.