An image processing apparatus (800) partitions (101) an image into a plurality of blocks. For a selected one of these blocks, the apparatus then uses a given available range of grayscale level values to determine (102) grayscale level values for image data in the selected block to provide corresponding determined grayscale level values. At least some of these determined grayscale level values are then transformed (103) to thereby use an altered amount of the given available range of grayscale level values to thereby provide rescaled grayscale level values for the selected block.