1. A computer-readable non-transitory information storage medium having stored thereon a program for controlling a computer to execute the processing of: changing a parameter relating to gravity of each of a plurality of rigid bodies which are constrained to one another and included in a first object based on a positional relationship between each of the plurality of rigid bodies and a second object; physically calculating a motion of each of the plurality of rigid bodies included in the first object based on the changed parameter; and rendering an image representing a surface of the first object based on the motions of the plurality of rigid bodies included in the first object.