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

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 8×8 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