Accurate analytic approximations for real-time specular area lighting

We introduce analytic approximations for accurate real-time rendering of specular surfaces lit by area light sources. Our solution leverages the Irradiance Tensors developed by Arvo for the rendering of Phong surfaces lit by a polygonal light source. Using a reformulation of the 1D boundary edge integral, we develop a general framework for approximating and evaluating the integral in constant time using simple peak shape functions. To overcome the Phong restriction, we propose a low cost edge splitting strategy that accounts for the spherical warp introduced by the half vector parametrization. Thanks to this novel extension, we accurately approximate common microfacet BRDFs, providing the first practical method producing specular stretches that closely match ground truth image references in real-time. Finally, using the same approximation framework, we introduce support for spherical and disc area light sources, based on an original polygon spinning method supporting non-uniform scaling operations and horizon clipping. Implemented on a GPU, our method achieves real-time performances without any assumption on area light shape nor surface roughness, with a quality close to the ground truth.