1. An image generation system comprising: a memory that stores information; and a processor that operates based on the information stored in the memory, the processor comprising hardware, the processor being configured to implement: a vertex split count setting process that sets a vertex split count; a vertex split process that performs a vertex split process on a three-dimensional object in an object space using the vertex split count set by the vertex split count setting process; a vertex position calculation process that calculates an intersection position of a projection screen and a straight line that connects a vertex position of the three-dimensional object after the vertex split process and a position of a virtual camera, and calculates a vertex position of a drawing object corresponding to the three-dimensional object on a drawing buffer based on the calculated intersection position; and a drawing process that draws the drawing object on the drawing buffer based on the calculated vertex position of the drawing object to generate a projection image, wherein a first vertex and a second vertex form a line segment included in the three-dimensional object subjected to the vertex split process, wherein θ is an angle formed by a first straight line and a second straight line, the first straight line connecting the first vertex and the position of the virtual camera, the second straight line connecting the second vertex and the position of the virtual camera, wherein m is the vertex split count, wherein ε is used as a standard for setting the vertex split count, wherein δ is an angle under which a size of a pixel of the projection image on the screen is seen from the position of the virtual camera, wherein the vertex split count setting process sets the vertex split count m so that (θ/m)<ε is satisfied, and wherein the vertex split count setting process sets the vertex split count m using the angle ε that is set with respect to the angle δ.