1. A calculation method for an infinity homography based on parallelograms, comprising: transforming parallelograms in the three-dimensional space of images photographed by cameras to respective rectangles and calculating rectifying homographies of said respective images; calculating homographies of new images transformed by using said calculated rectifying homographies; calculating linear constraints for said infinity homography of said respective images by using said rectifying homographies and said calculated homographies of new images; calculating linear constraints for said infinity homography from vanishing points by using relationships between said infinity homography of said images and said vanishing points corresponding to two images; and calculating said infinity homography by using said calculated linear constraints.