A method of processing image data for display by a display panel of a display device is provided. The method comprises receiving main image pixel data representing a main and side image pixel data representing a side image. In a first processing step, a mapping is performed of the pixel data to signals used to drive the display panel. The mapping is arranged to produce an average on-axis luminance which is dependent mainly on the main image pixel data and an average off-axis luminance which is dependent at least to some extent on the side image pixel data. In a second processing step, the received side image pixel data are processed to emphasise at least one feature of the side image which might otherwise be perceived by a viewer as being de-emphasised in the side image displayed off axis as a result of the first processing step.