Digital image compression in astronomy morphology or wavelets

A wide-field astronomical image is often considered as a set of quasi point-like sources spread on a slow-varying backgound. With this model, the image is described as a set of connected fields. We have to code the field positions, the field boundaries and their pixel values. It exists different methods for coding this information, they are mainly connected to the Mathematical Morphology. The fields may be coded from their contours, their binary skeletons or the grey-tone ones. The morphological skeleton transformation in general gives us the best results. The H-transform is a two-dimensional generalization of the Haar transform often used for compressing astronomical images. Blocking effects appear in the restored image. The quality of the restoration is improved by introducing an a priori knowledge on the solution. These two different approaches lead to high compression rates on classical astronomical images. The best compression technique is directly connected to the image modelling.