A program for calculating a radiance at a shading point in a 3D scene that causes one or more computers to execute: processing for translating an indirect light source defined virtually, which is caused by a direct light source being reflected by a predetermined surface, into a spherical light source having a predetermined spread; processing for approximating by a spherical Gaussian, SG approximating, an incident radiance of the spherical light source and a bidirectional reflection distribution function, BRDF, at a position of the spherical light source; processing for SG approximating a function that indicates a cone shape of the spherical light source from the perspective of the shading point based on the opening angle of the cone; processing for obtaining an incident radiance at the shading point by calculating a product of an SG approximated light emission radiance of the spherical light source, the SG approximated function that indicates the cone shape, and the SG approximated BRDF at the position of the spherical light source; processing for SG approximating a BRDF at the shading point; and processing for obtaining the radiance at the shading point towards a viewpoint at which the shading point is to be rendered, by calculating a product of the obtained incident radiance at the shading point and the SG approximated BRDF at the shading point, wherein, in a case where a sharpness of the SG approximated function that indicates the cone shape of the spherical light source is sufficiently larger than a sharpness of a product of the SG approximated light emission radiance of the spherical light source and the SG approximated BRDF at the shading point, the program causes the one or more computers not to execute the processing for SG approximating the BRDF at the shading point and the BRDF at the position of the spherical light source. processing for translating an indirect light source defined virtually, which is caused by a direct light source being reflected by a predetermined surface, into a spherical light source having a predetermined spread; processing for approximating by a spherical Gaussian, SG approximating, an incident radiance of the spherical light source and a bidirectional reflection distribution function, BRDF, at a position of the spherical light source; processing for SG approximating a function that indicates a cone shape of the spherical light source from the perspective of the shading point based on the opening angle of the cone; processing for obtaining an incident radiance at the shading point by calculating a product of an SG approximated light emission radiance of the spherical light source, the SG approximated function that indicates the cone shape, and the SG approximated BRDF at the position of the spherical light source; processing for SG approximating a BRDF at the shading point; and processing for obtaining the radiance at the shading point towards a viewpoint at which the shading point is to be rendered, by calculating a product of the obtained incident radiance at the shading point and the SG approximated BRDF at the shading point, wherein, in a case where a sharpness of the SG approximated function that indicates the cone shape of the spherical light source is sufficiently larger than a sharpness of a product of the SG approximated light emission radiance of the spherical light source and the SG approximated BRDF at the shading point, the program causes the one or more computers not to execute the processing for SG approximating the BRDF at the shading point and the BRDF at the position of the spherical light source.