Clusters of pixels are defined for use in image compression and decompression. The image information used to define the clusters may include pixel values at predetermined positions relative to a pixel or related motion vectors, gradients, texture etc. During compression of images the image information relative to pixels is examined to determine the cluster to which it belongs. Thus pixels can be classified according to the cluster for their image information. In an embodiment the definitions of the clusters are selected dynamically, dependent on image content. For each cluster a control parameter set is computed for a post-processing operation, such as filter coefficients for filtering or statistical data for locally generating texture. The control parameter set is selected dependent on the image content so that, when the post-processing operation is applied to the image after decompression it will improve image quality for the pixels that are classified as belonging to the cluster. The compressed image and the control parameter sets are transmitted to a decompression apparatus. Upon decompression, the image information that represents the decompressed image is examined to classify pixels according to the clusters and the different control parameter sets for the selected clusters are used to control post-processing at the locations of the pixels.