1. A correction information calculator used in an image display system in which a plurality of projectors each including an image forming element collaboratively displays one image on a projection surface, comprising: a processor configured to calculate correction information, wherein based on predetermined correction information which is set in advance as a correspondence between the coordinates of pixels on the image forming element and the coordinates of pixels on a first effective projection region of the projection surface on which the image is presently displayed and a second correspondence which is a correspondence between the coordinates of the pixels on the first effective projection region and the coordinates of pixels on a second effective projection region of the projection surface, which is specified as a region on which the image is to be displayed, the correction information is calculated based on a correspondence between the coordinates of pixels on the image forming element and the coordinates of pixels on the second effective projection region.