Fixed-quality/variable bit-rate on-board image compression for future CNES missions

摘要

The huge improvements in resolution and dynamic range of current [1][2] and future CNES remote sensing missions(from 5m/2.5m in Spot5 to 70cm in Pleiades) illustrate the increasing need of efficient on-board image compressors.Many techniques have been considered by CNES during the last years in order to go beyond usual compression ratios:new image transforms or post-transforms [3][4], exceptional processing [5], selective compression [6].However, even if significant improvements have been obtained, none of those techniques has ever contested an essentialdrawback in current on-board compression schemes: fixed-rate (or compression ratio).This classical assumption provides highly-predictable data volumes that simplify storage and transmission. But on theother hand, it demands to compress every image-segment (strip) of the scene within the same amount of data. Therefore,this fixed bit-rate is dimensioned on the worst case assessments to guarantee the quality requirements in all areas of theimage. This is obviously not the most economical way of achieving the required image quality for every single segment.Thus, CNES has started a study to re-use existing compressors [7] in a Fixed-Quality/Variable bit-rate mode. The mainidea is to compute a local complexity metric in order to assign the optimum bit-rate to comply with quality requirements.Consequently, complex areas are less compressed than simple ones, offering a better image quality for an equivalentglobal bit-rate.“Near-lossless bit-rate” of image segments has revealed as an efficient image complexity estimator. It links qualitycriteria and bit-rates through a single theoretical relationship. Compression parameters are thus automatically computedin accordance with the quality requirements. In addition, this complexity estimator could be implemented in a one-passcompression and truncation scheme.© (2012) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.