POV-Ray : Newsgroups : povray.advanced-users : Raytracing theory in Povray : Re: Raytracing theory in Povray Server Time
23 Jun 2024 23:24:51 EDT (-0400)
  Re: Raytracing theory in Povray  
From: muyu
Date: 16 Apr 2018 10:35:01
Message: <web.5ad4b44e948a4d2ed22d61e0@news.povray.org>
clipka <ano### [at] anonymousorg> wrote:
> Am 09.03.2018 um 15:45 schrieb muyu:
> > Hi,
> >
> > In Povray, the optical properties of each surface can be set by ambient, diffuse
> > and specular. This is very flexible but only single value is allowed over
> > color-channels. As a matter of fact, the optical properties of object often
> > differs over wavelength or colors. I am wondering what's the assumption in
> > Povary to deal with this. It will be great if you could recommend a reference to
> > describe the raytracing theory used in Povray. Thanks a lot in advance.
> Actually, in real life the wavelength-dependent effects are pretty few:
> (1) Different absorption and scattering coefficients in the material itself:
> In most materials (specifically dielectrics), light actually penetrates
> a tiny distance into the bulk of the object, scatters around a bit in
> there, and either exits again nearby or gets absorbed along the way.
> Three things can be wavelength-dependent here: (a) the distance a light
> ray travels on average before being absorbed, (b) The distance a light
> ray travels on average between scattering events, and (c) the
> directionality of those scattering events.
> At a large scale however, you'll only see the combination of all these
> effects at work, most dominantly the ratio between (a) and (b), which
> gives the material its apparent colour. This is modeled in POV-Ray via
> the pigment.
> For a more precise model suitable for realistic close-ups, POV-Ray
> provides SSLT, where besides the ratio (again via pigment) you also
> specify the effects' absolute values on a per-colour-channel basis. (c)
> is currently not modeled.
> (2) Different refractive indices:
> In transparent materials, the refractive indices can vary slightly
> between wavelengths, causing a change in the refraction angle. This is
> modeled in POV-Ray via the "dispersion" setting.
> Besides that, technically wavelength-dependent refractive indices also
> affect how much light enters the material vs. how much light is
> reflected (see Fresnel's law); however, this effect is miniscule for all
> real-life materials.
> (3) Interference in thin-film coatings:
> Some materials are coated with a thin layer of another material; if the
> thickness of the coating is reasonably uniform and close to the
> wavelength of visible light, destructive interference occurs for some
> wavelengths but constructive interference for others, affecting (a) how
> much light traverses the coating to reaches the bulk of the material,
> and (b) how much is reflected back.
> For single-layer coatings with slight variations in thickness, you'll
> get a rainbow effect (example: oil on water); in such cases, POV-Ray can
> approximately model (a) using the `irid` (iridescence) feature. (b) is
> currently not modeled.
> Some objects are coated with a series of extremely uniform layers, such
> as anti-reflex coating on camera lenses; in such cases, POV-Ray can
> approximately model (b) using a colour instead of a scalar as the
> reflection parameter. The effect on (a) is usually quite miniscule, but
> could be approximated by slightly tweaking the pigment colour.
> (4) Metals:
> Metals behave entirely differently: Here, light does not enter the bulk
> of the material, but rather some wavelengths are absorbed at the surface
> while all others are reflected.
> This can be modeled pretty well using the `metallic` setting for both
> highlights and reflections, and choosing a suitable pigment.
> (5) Diffraction:
> When a surface has a regular structure and the size of the structure is
> near the wavelength of visible light, this causes interference patterns,
> messing up the directions at which light is reflected or refracted. This
> effect also varies with wavelength, causing rainbow-like effects. (Think
> CD/DVD).
> Currently, this effect can not be modeled at all in POV-Ray.
> To my knowledge, that about wraps it up as far as wavelength-dependent
> effects on light are concerned. If you can think of any other, let me
> know and I can tell you if and how they can be modeled in POV-Ray.

Thanks for your answer. Actually I am simulating radiative tansfer in vegetation
canopy. Under this context, surface optical properties mainly means the
reflectance, tansmitance of leaves, stems etc. Regarding green leaves, their
optical properties differ a lot over r g and b. Now I am setting diffuse x, xx
to simulate double face effect, corresponding to reflectance and transmitance.
However this only allow me set constant value over three color channels. I am
wondering how to set differet value for different channels. Thanks.

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