Improving a DWT-based compression algorithm for high image-quality requirement of satellite images
2011
Past and current optical Earth observation systems designed by CNES are using a fixed-rate data compression processing
performed at a high-rate in a pushbroom mode (also called scan-based mode). This process generates fixed-length data to
the mass memory and data downlink is performed at a fixed rate too. Because of on-board memory limitations and high data
rate processing needs, the rate allocation procedure is performed over a small image area called a "segment". For both
PLEIADES compression algorithm and CCSDS Image Data Compression recommendation, this rate allocation is realised
by truncating to the desired rate a hierarchical bitstream of coded and quantized wavelet coefficients for each segment.
Because the quantisation induced by truncation of the bit planes description is the same for the whole segment, some parts
of the segment have a poor image quality. These artefacts generally occur in low energy areas within a segment of higher
level of energy. In order to locally correct these areas, CNES has studied "exceptional processing" targeted for DWT-based
compression algorithms. According to a criteria computed for each part of the segment (called block), the wavelet
coefficients can be amplified before bit-plane encoding. As usual Region of Interest handling, these multiplied coefficients
will be processed earlier by the encoder than in the nominal case (without exceptional processing). The image quality
improvement brought by the exceptional processing has been confirmed by visual image analysis and fidelity criteria. The
complexity of the proposed improvement for on-board application has also been analysed.
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