This paper introduces a series of techniques aimed at facilitating the encoding of the upper bands of a subband decomposition. The fundamental idea behind these techniques is that the energy in the upper bands is concentrated around the discontinuities of the lower band. Therefore, for image compression the output of an edge detector applied on the lower band can be employed to locate the upper band pels with the largest amplitudes. Moreover, the percentage of pels declared as edges in each one of the upper bands can be interpreted as an additional degree of freedom in the definition of a subband compression scheme, in a similar way to the bit allocation between bands. The performance of the proposed techniques for various edge detectors and different filter banks is studied. We introduce an algorithm to distribute a given number of edge pels between the different upper bands in order to minimize the total distortion. A morphological operator is presented as a method to further increase the efficiency of the edge-based approach. The influence of the baseband distortion in the performance of the proposed techniques is also analyzed. Finally, we estimate the savings in transmitted bit rate of an edge based compression system with respect to traditional schemes.
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