Diffuse reflection: Difference between revisions

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The most general mechanism by which a surface gives diffuse reflection does not involve ''exactly'' the surface: most of the light is contributed by scattering centers beneath the surface,<ref>P.Hanrahan and W.Krueger (1993), ''Reflection from layered surfaces due to subsurface scattering'', in [http://www.cs.berkeley.edu/~ravir/6998/papers/p165-hanrahan.pdf SIGGRAPH ’93 Proceedings, J. T. Kajiya, Ed., vol.&nbsp;27, pp.&nbsp;165–174] {{webarchive|url=https://web.archive.org/web/20100727005751/http://www.cs.berkeley.edu/~ravir/6998/papers/p165-hanrahan.pdf |date=2010-07-27 }}.</ref><ref>H.W.Jensen et al. (2001), ''A practical model for subsurface light transport'', in '[http://www.cs.berkeley.edu/~ravir/6998/papers/p511-jensen.pdf Proceedings of ACM SIGGRAPH 2001', pp.&nbsp;511–518] {{webarchive|url=https://web.archive.org/web/20100727005456/http://www.cs.berkeley.edu/~ravir/6998/papers/p511-jensen.pdf |date=2010-07-27 }}</ref> as illustrated in Figure&nbsp;1.

If one were to imagine that the figure represents snow, and that the polygons are its (transparent) ice crystallites, an impinging ray is partially reflected (a few percent) by the first particle, enters in it, is again reflected by the interface with the second particle, enters in it, impinges on the third, and so on, generating a series of "primary" scattered rays in random directions, which, in turn, through the same mechanism, generate a large number of "secondary" scattered rays, which generate "tertiary" rays, and so forth.<ref>Only primary and secondary rays are represented in the figure.</ref> All these rays walk through the snow crystallytescrystallites, which do not absorb light, until they arrive at the surface and exit in random directions.<ref>Or, if the object is thin, it can exit from the opposite surface, giving diffuse transmitted light.</ref> The result is that the light that was sent out is returned in all directions, so that snow is white despite being made of transparent material (ice crystals).

For simplicity, "reflections" are spoken of here, but more generally the interface between the small particles that constitute many materials is irregular on a scale comparable with light wavelength, so diffuse light is generated at each interface, rather than a single reflected ray, but the story can be told the same way.