An image processor includes a readout arranged to read out an M-bit image data word from an image sensor pixel array and an adder arranged to add a noise contribution to the image data word to obtain a dithered M-bit word. A dither processor is arranged to derive correction data having a word size of M+1 bits from a combination of a plurality of M-bit reference words. The noise contribution are derived from said correction data, wherein different correction data are derived for different groups of pixels, each different group of pixels is associated with a specific pixel value DC shift.