Interpolation-Friendly Soft Shadow Maps

We present Penumbra Limit Maps, a technique for rendering soft shadows from a modified shadow map. The shadow representation used by our method has excellent interpolation properties, allowing the shadow boundary to be rendered with sub shadowmap-pixel accuracy, which partially mitigates the resolution problems common to shadow map methods. Unlike similar shadow map methods, our method includes both inner and outer penumbrae, and is in fact physically correct for the simplest case of a straight object edge and infinitely distant extended light source. At object corners or where multiple object edges overlap, our method is no longer physically exact, but still gives plausible results. Finally, we show the method can be implemented naturally and efficiently on programmable graphics hardware.