γ-ton Tracing (WONG Tien Tsin)

γ-ton tracing is a visual simulation framework for a wide variety of weathering effects. In this framework, blemishes (e.g. rust, stain, patina, etc) are formed by the cumulative propagation of an abstract weathering-catalysing agent called γ-ton. The propagation of γ-ton resembles the photon mapping. Therefore, once you know how to do photon mapping, you can simulate weathering. [paper]