1. An image generating system for generating an image, comprising: a memory which stores a program and data for image generating; and at least one processor which is coupled with the memory and performs processing for image generating, the processor including: a mapping section which maps a texture on an object by a texel interpolation method; and a generating section which generates a defocused image for an original image by setting the original image as a texture and shifting a texture coordinate of the object when the texture is mapped on the object by the texel interpolation method.