1. A method of processing image data comprising: receiving 2D image data representing a pixellated image with a width and a height; receiving a depth map in respect of the 2D image data; processing the depth map to derive a modified depth map; generating left and right image data together forming a 3D image based on the modified depth map and the 2D image data, wherein processing the depth map comprises at least one of: analysing luminance information in respect of a set of pixels, and modifying the depth map using a function which correlates depth with pixel height in the pixellated image and which has a different correlation between depth and pixel height for different luminance values; and analysing chrominance information in respect of the set of pixels, and modifying the depth map using a function which correlates depth with pixel height in the pixellated image and which has a different correlation between depth and pixel height for different chrominance values.