A method of processing image data for display on a display device having a multi-primary image display panel (2) comprises receiving image data constituting an image for display on the image display panel. In a first mode, signal voltages to be applied to sub-pixels of the image display panel are determined from the received image data 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). This provides a private display mode, since the luminance variations perceivable at the first viewing position (5) obscure the image from an observer at the first viewing position, while an observer at the second viewing position (3) is able to perceive the image since the luminance variations are not perceivable (or are substantially not perceivable) at the second viewing position In a second mode, signal voltages to be applied to sub-pixels of the image display panel may be determined just from the received image data thereby to generate an image perceivable at the first viewing position (5) and at the second viewing position (3). This provides a public or wide-view display mode.