A comparative test of different compression methods applied to solar images

In this work we conduct a comparative study on different data compression methods applied to high resolution images of the solar surface acquired at the Solar Dunn Telescope in Sacramento Peak with the IBIS (Interferometric Bidimensional Spectrometer) instrument. Our aim is to perform an estimation of the quality, efficiency and workload of the considered computing techniques both in the so-called lossless modality, where in the reconstruction phase there is no loss of information, and in lossy mode, where it should be possible to reach a high compression ratio at the expense of image information. In the latter case we quantify the quality with image analysis conventional methods and more specifically with the reconstruction of physical parameters through standard procedures used in this kind of observations. The considered methods constitute the most frequently adopted image compression procedures in a variety of fields of application; they exploit in different ways the properties of the Discrete Wavelet Transforms often coupled with standard entropy coders or similar coding procedures applied to the different bit planes in order to allow a progressive handling of the original image. In the lossless approach we found that all methods give a compression ratio around 2. For a lossy compression we reached a compression ratio of 8 (equivalent to a 2 bit per pixel) without any perceptual difference between original and reconstructed images, but with effects on the photometric accuracy. We also tested the performance of 3-D lossy methods for the compression of data-cubes. Maintaining the same data degradation level, those methods allows us to gain a 2x in the compression ratio over the 2-D methods.