Compression of 2-D solar spectral images by adaptive piecewise linear approximation

Three published techniques for image compression by piecewise linear approximation are described and tested on 2D solar photographic UV spectra. The techniques allow the necessary image analysis for these particular images to be performed directly on the compressed data. The compressed/restored images have been evaluated quantitatively by measuring the preservation of Gaussian spectral line features. Tests indicate that conventional polygonization techniques based on invariant error tolerances can only give a limited compression of the present images before serious errors start to occur. Point-to-point adaptive control of the error tolerance through a continuous local activity measure does not achieve the desired stability and robustness. The authors find that introducing adaptivity through image segmentation allows a significant increase in the compression ratio, making these methods very useful for astronomical purposes. An algorithm for automatically finding spectral lines and fitting Gauss curves has also been developed.