Efficient light scattering through thin semi-transparent objects

This paper concerns real-time rendering of thin semi-transparent objects. An object in this category could be a piece of cloth, eg. a curtain. Semi-transparent objects are visualized most correctly using volume rendering techniques. In general such techniques are, however, intractable for real-time applications. Surface rendering is more efficient, but also inadequate since semi-transparent objects should have a different appearance depending on whether they are front-lit or back-lit. The back-lit side of a curtain, for example, often seems quite transparent while the front-lit side seems brighter and almost opaque. To capture such visual effects in the standard rendering pipeline, Blinn [1982] proposed an efficient local illumination model based on radiative transfer theory. He assumed media of low density, hence, his equations can render media such as clouds, smoke, and dusty surfaces. Our observation is that Chandrasekhar [1960] has derived the same equations from a different set of assumptions. This alternative derivation makes the theory useful for realistic real-time rendering of dense, but thin, semi-transparent objects such as cloth. We demonstrate that application of the theory in this new area gives far better results than what is obtainable with a traditional real-time rendering scheme using a constant factor for alpha blending.