1. A display method using a display device operable to display an image to a viewer using a head-mountable display, the display method comprising: detecting at least one of an initial position or orientation of the viewer's head; rendering, by one or more processors, an image for display according to the at least one of the detected initial position or orientation to define a viewpoint of the viewer that is correct at a time that the rendering is initiated; detecting at least one of a current position or orientation of the viewer's head depending upon a display time at which the rendered image is to be displayed, the at least one of the current position or orientation being used to define a viewpoint of the viewer appropriate to the display time; re-projecting, by one or more processors, the rendered image according to any differences between the at least one of the detected initial position and orientation and the at least one of the current position or orientation of the viewer's head, to perform a viewpoint adjustment of the rendered image to the viewpoint of the viewer appropriate to the display time; and displaying the re-projected image using the head-mountable display at the display time; in which the step of re-projecting comprises detecting an overlapping portion between the rendered image and the re-projected image, and reproducing the overlapping portion as part of the re-projected image, wherein the method further comprises filling portions of the re-projected image other than the overlapping portion with image material from a further image source.