1. An apparatus for generating stereoscopic display image data to be displayed on a stereoscopic display device, comprising: a stereoscopic display region calculator that calculates a stereoscopic display region to reproduce a three-dimensional positional relationship in the image data when the image data is displayed on the stereoscopic display device, based on two-dimensional or three-dimensional image data to be displayed on the stereoscopic display device, a position of a target of regard of a virtual camera set in processing of rendering the image data in a two-dimensional manner in response to light beams output from the stereoscopic display device, and orientations of the light beams output from the stereoscopic display device; an image processor that performs image processing on image data outside a region representing the outside of the stereoscopic display region calculated by the stereoscopic display region calculator, the image processing being different from image processing on image data inside a region representing the inside of the stereoscopic display region; and an image generator that generates the stereoscopic display image data from the two-dimensional or three-dimensional image data after processed by the image processor.