1. An image processor comprising a computer system configured to implement at least: an imaging parameter acquirer that acquires an imaging parameter which is used by a radiographic imaging apparatus in capturing a perspective image of a target; and a virtual perspective image generator that determines a method of generating a virtual perspective image from volume data of the target in accordance with the imaging parameter, the virtual perspective image generator generating the virtual perspective image from the volume data of the target in accordance with the imaging parameter, wherein the virtual perspective image generator determines a weighting coefficient for each voxel value of the volume data based on the imaging parameter, and the virtual perspective image generator generates the virtual perspective image from the volume data by using the weighting coefficients.