Optimal quantisation strategy for DCT image compression

The authors present a strategy for generating optimal quantisation tables for use in JPEG image compression and its extension to general block sizes. Directly optimised quantisation tables were obtained by simulated annealing. A composite cost function minimised the RMS error between original and recovered images while keeping the compression ratio dose to some desired value. Examination of these tables led to a simple model giving quantisation coefficients in terms of (x,y) position in the table and three model parameters. Annealing on the model parameters for several compressions yielded an expression for each parameter as a function of compression ratio. This approach was extended to general block sizes, and psychovisual evaluation determined the visually optimal block size for each compression ratio. The authors demonstrate significant improvements over JPEG coding due to the use of optimal quantisation rather than default tables. Use of general block size effectively extends the JPEG approach to higher compressions than are feasible with standard JPEG coding.