Horizon-based screen-space ambient occlusion using mixture sampling

Ambient occlusion is a shading method which approximates the global illumination by taking into account attenuation of light due to occlusion. Screen-space ambient occlusion (SSAO) is a scene-independent improvement which performs the visibility test using depth buffer on GPU in real time. Shanmugam et.al [2007] efficiently reconstructs the surface using spheres that can be projected to screen, but has the overocclusion issue. Bavoil et.al [2008] approximates the ambient occlusion illumination A at a given surface point P by [EQUATION] where w is the sample directions at P within the unit normal-oriented hemisphere with azimuth angle θ and elevation angle α, the tangent angle t(θ) is the signed elevation angle of the surface tangent vector while the horizon angle h(θ) is the maximum elevation α ≥ t(θ) for which w is occluded for all α h(θ), and W is a linear attenuation function. It overcomes the overocclusion problem of [Shanmugam and Arikan 2007]. However, the rough approximation of the geometry occluder makes the shadows lack of rich details. In this paper we propose a new SSAO solution which overcomes the issues of both above methods and provides more realistic results, as shown in Figure 1.