Volume-based indirect lighting with irradiance decomposition

High-quality indirect lighting at interactive speed is a difficult challenge. To fast approximate the indirect illumination, the volume-based rendering techniques were used. The Light Propagation Volume (LPV) [Kaplanyan et al. 2010] method departs scenes into coarse lattices and propagates spherical harmonics represented radiance on them. The LPV is able to render complex and dynamic scenes in real-time. But since the radiance transfer is highly approximated among the lattices, it fails to simulate the indirect lighting between the surfaces in the same lattice. On the other hand, the Voxel-based Global Illumination (VGI) [Kaplanyan et al. 2011] voxelizes the scene into fine voxels. It performs the voxel-based ray marching to find the reflected surface in the near-field. By adopting a 1/4x1/4 sampling, the VGI can render in a speed of 18~28 frame per second. But for complex scenes and real global illumination, it requires huge volume data and a large number of rays which degrades the rendering performance to a speed of 2.2s per frame.