Method and apparatus for optimizing a JPEG image using regionally variable compression levels

摘要

A method of JPEG compression of an image frame divided up into a plurality of non-overlapping, tiled 8×8 pixel blocks X_i. A global quantization matrix Q is determined by either selecting a standard JPEG quantization table or selecting a quantization table such that the magnitude of each quantization matrix coefficient, Q[m,n] is inversely proportional to the aggregate visual importance in the image of the corresponding DCT basis vector. Next a linear scaling factor S_i is selected for each block, bounded by user selected values S_min and S_max. Transform coefficients, Y_i, obtained from a digital cosine transform of X_i, are quantized with global table S_min Q while emulated the effects of quantization with local table S_i Q and the quantized coefficients T_i[m,n] and global quantization table S_min Q are entropy encoded , where S_min is a user selected minimum scaling factor, to create a JPEG Part 1 image file. The algorithm is unique in that it allows for the effect of variable-quantization to be achieved while still producing a fully compliant JPEG Part 1 file.