1. A non-transitory computer-readable recording medium on which is recorded a program for causing a computer to, for a luminance computation performed based on a two-dimensional map indicating a space distribution, of a plurality of virtual lights determined for a global illumination of a scene, and a parameter of each virtual light, the luminance computation being performed to approximate the plurality of virtual lights to a smaller predetermined number of composite virtual lights, perform processing to decide a parameter of each of the predetermined number of composite virtual lights, wherein the program comprises: processing for obtaining an importance map of the scene that is generated to correspond to the two-dimensional map, the importance map having a data structure from which a summation of importances of a requested region can be obtained; processing for setting, in the importance map, clusters of virtual lights such that a summation of probability densities in each cluster is a reciprocal of a number of clusters, and a composite virtual light being generated for each cluster; and processing for deciding, for each composite virtual light of the predetermined number of composite virtual lights, a parameter of the composite virtual light based on a region of the two-dimensional map that corresponds to the set cluster, wherein a value obtained by totaling, for each composite virtual light of the predetermined number of the composite virtual lights, the summation of the probability densities for the cluster set for each composite virtual light, corresponds to a summation of the importances of an entire region of the importance map.