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Fast subsurface scattering

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Fig.1 - Fast Subsurface scattering of Stanford Bunny
Based on the implementation of three.js. It provides a cheap, fast, and convincing approach to do ray-tracing in translucent surfaces. It refers the sharing in GDC 2011 [1], and the approach is used by Frostbite 2 and Unity engines [1][2][3]. Traditionally, when a ray intersects with surfaces, it needs to calculate the bouncing result after intersections. Materials can be divided into three types roughly. Opaque, lights can't go through its geometry and the ray will be bounced back. Transparency, the ray passes and allow it through the surface totally, it probably would loose a little energy after leaving. Translucency, the ray after entering the surface will be bounced internally like below Fig. 2.

Fig.2 - BSSRDF [1]
In the case of translucency, we have several subsurface scattering approaches to solve our problem. When a light is traveling inside the shape, that needs to consider the diffuse value influence according the vary…