A method and apparatus for diffuse indirect lighting computation in a scene is described. An irradiance caching algorithm may be implemented in an indirect lighting module that sparsely computes irradiance in the environment and then interpolates irradiance values between these sparsely computed points. The method for diffuse indirect lighting computation may use cosine weighting in determining nearby sample irradiance points to be used in interpolation. If an object detected by a ray is directly overhead, embodiments may assume a significant amount of occlusion or indirect illumination of that surface point, and thus may weight the ray more heavily. If an object is off to the side, and thus at a shallow angle to a normal of the surface point under test, the ray is given less weight. By using cosine weighting, the indirect lighting module tends to space out sample irradiance points better around edges and cracks than conventional techniques.