Cabral et al. [17] describe a technique which combines image-based rendering (IBR) with spheremapping to provide view-independent rendering incorporating arbitrary surface BRDFs. Their approach involves two enhancements to rendering mirror reflections with spheremaps.
First, the technique uses a radiance spheremap, which encodes the total reflected radiance for a given surface orientation (i.e. normal), rather than just the light from a perfect reflector.
One can imagine that a spheremap constructed from six cube map views encodes light reflected off of a sphere whose BRDF has complete reflectance along the reflection direction and no reflectance in any other direction. Instead, their technique creates spheremaps using a custom raytracer that integrates the reflected radiance using a computed BRDF and the cube environment map. This ensures that the spheremap for a given view direction not only includes the environment reflected in the surface from that view direction but also the surface characteristics including Fresnel effects and backscatter. They recommend using high-fidelity images of the source environment with high dynamic range, as described by Debevec [23]. Some small number of these spheremaps are generated for various view directions.
Second, during interactive viewing, the technique creates new spheremaps for the current viewpoint by combining three spheremaps whose view directions are close to the view direction. Barycentric weights are calculated from the spherical triangle subtended by the three candidate view direction and the included triangles formed by the new view direction. Each of the original three spheremaps are drawn with a mesh warped to match the new view direction. These three new spheremaps are linearly interpolated using the accumulation buffer to form a new spheremap corresponding to the new view direction. The model is then rendered using this new spheremap.
Because the radiance map is computed offline over the course of several minutes, it is not feasible to regenerate the radiance maps in real time if the environment is changing. This technique, however, allows objects with complex BRDFs and complex reflection environments to be rendered with arbitrary orientations and locations.