A method and apparatus are provided for generating image data for output to a display device such as an HUD or HMD defining pixels of an image for display. The image data for each pixel defines a predetermined background colour or a non-background colour. In an example implementation, the image generator comprises at least one image rendering module and a memory accessible by the at least one rendering module providing at least one frame buffer in which image data may be assembled. The at least one rendering module is arranged to process or to generate image data using a tile representation of an image for display, each tile comprising image data defining each of a predetermined number of pixels of the image. Furthermore, the at least one rendering module is arranged to store image data for the pixels of a tile in the at least one frame buffer only in the event that one or more pixels of the tile have respective image data defining a non-background colour or are enabled for display with a non-background colour, the pixels in the tile being otherwise assumed to be intended for display with the predetermined background colour. In this way the need to transfer data to and from the memory when generating and assembling image data is reduced, so reducing image rendering time.