Image data compression

A method and compression unit for compressing a block of image data to satisfy a target level of compression, wherein the block of image data comprises a plurality of image element values, each image element value comprising one or more data values relating to a respective channel. For each of the channels: (i) an origin value for the channel for the block is determined, (ii) difference values are determined representing differences between the data values and the determined origin value for the channel for the block, and (iii) a first number of bits for losslessly representing a maximum difference value of the difference values for the channel for the block is determined. The determined first number of bits for each of the channels is used to determine a respective second number of bits for each of the channels, the second number of bits being determined such that representing each of the difference values for the channels with the respective second number of bits satisfies the target level of compression for compressing the block of image data. Compressed data is formed, having for each of the one or more channels an indication of the determined origin value for the channel, an indication of the determined first number of bits for the channel, and representations of the determined difference values for the channel, wherein each of the representations of the determined difference values for the channel uses the determined second number of bits for the channel, such that the target level of compression is satisfied.