Improved radiance gradient computation

We describe a new and accurate algorithm for computing translational gradients of incoming radiance in the context of a ray tracing-based global illumination solution. The gradient characterizes how the incoming directional radiance function changes with displacement on a surface. We use the gradient for a smoother radiance interpolation over glossy surfaces in the framework of the radiance caching algorithm. The proposed algorithm generalizes the irradiance gradient computation by [Ward and Heckbert 1992] to allow its use for non-diffuse, glossy, surfaces. Compared to previous method for radiance gradient computation, the new algorithm yields better gradient estimates in the presence of significant occlusion changes in the sampled environment, allowing a smoother indirect illumination interpolation.