Bit rate and local quality control for on-board satellite image compression

Abstract

The method we present is developed as part of an on-board satellite image compression with target ratios greater than six. In order to limit the ringing effects on these critical areas we have developed an adapted encoder based on a wavelet transform. Our method consists of first a classification of 8/spl times/8 blocks of coefficients and second of a coding which integrates a local quality constraint within a bit rate control. Actually, we perform coding and regulation in the same time by using an EZW encoder and also Shapiro's dependency protocol to define our blocks of coefficients, noted vectors. An energy-based threshold followed by an integration of the local context allows us to discriminate two classes of vectors. Once classification is set-up, we code the wavelet coefficients by using two EZW encoders, one for each class. Our encoder is quite different from Shapiro's because of the integration of a quality constraint. This constraint is established in order to privilege the ROI and is satisfied by an optimal choice of the proportionality between the two target rates.