1. A method comprising: performing, by a computer, ray tracing for a scene to be rendered by the computer, wherein performing the ray tracing comprises calculating a ray spread value for a ray propagated through the scene, the ray spread value being calculated based on a reflection geometry of at least one surface within the scene, wherein calculating the ray spread value comprises computing a ray differential for the ray propagated through the scene, and calculating a ray divergence value for the ray propagated through the scene, the ray divergence value being calculated based on one or more material properties of the at least one surface, wherein calculating the ray divergence value comprises computing an estimate of divergence based on the one or more material properties and a geometric model of light diffusion; defining, by the computer, a filter using both the calculated ray spread value and the calculated ray divergence value; and rendering, by the computer, the scene using the filter.