Method of processing image data for image display panel

A method of processing image data for an image display panel comprises, in a first mode: determining signal voltages to be applied to pixels of the image display panel from received image data constituting an image for display on the image display panel and from a secondary data value for the pixel thereby to generate luminance variations perceivable at a first viewing position 5 but substantially not perceivable at a second viewing position 3. The signal voltages to be applied to pixels in a group of pixels are determined such that the overall luminance of the pixels in the group is dependent on (for example, proportional to) the overall luminance specified for the pixels in the group by the image data. The number of pixels in a group is locally variable and is selected according to the image data and/or the secondary data values for pixels in a region of the image where the group is to be defined. The invention makes it possible to specify luminance redistribution over an increased pixel group size in local regions in which the main image content is relatively uniform, with no high spatial resolution image features, while specifying luminance redistribution over a reduced pixel group size for main image regions with sharp edges and other high resolution features.