Dielectric

Smooth dielectric material (dielectric)

This plugin models an interface between two dielectric materials having mismatched indices of refraction
(for instance, water and air). Exterior and interior IOR values can be specified independently,
where “exterior” refers to the side that contains the surface normal. When no parameters are given,
the plugin activates the defaults, which describe a borosilicate glass BK7/air interface.
In thismodel, the microscopic structure of the surface is assumed to be perfectly smooth, resulting
in a degenerate 10 BSDF described by a Dirac delta distribution. For a similar model that instead
describes a rough surface microstructure, take a look at the roughdielectric plugin.
When using this model, it is crucial that the scene contains meaningful and mutually compatible
indices of refraction changes—see Figure 7 for a description of what this entails.
In many cases, we will want to additionally describe the medium within a dielectric material.This
requires the use of a rendering technique that is aware of media (e.g. the volumetric path tracer). An
example of how one might describe a slightly absorbing piece of glass is shown below:

10 Meaning that for any given incoming ray of light, the model always scatters into a discrete set of directions, as opposed
to a continuum.

Remarks:
• Dispersion is currently unsupported but will be enabled in a future release.

Parameter

  • intIOR : float or string
    • Interior index of refraction specified numerically or using a known material name. (Default: bk7 / 1.5046)
  • extIOR : float or string
    • Exterior index of refraction specified numerically or using a known material name. (Default: air / 1.000277)
  • specular Reflectance : spectrum or texture
    • Optional factor that can be used to modulate the specular reflection component. Note that for physical realism, this parameter should never be touched. (Default: 1.0)
  • specular Transmittance : spectrum or texture
    • Optional factor that can be used to modulate the specular transmission component. Note that for physical realism, this parameter should never be touched. (Default: 1.0)


Name Value Name Value
vacuum 1.0 bromine 1.661
helium 1.00004 water ice 1.31
hydrogen 1.00013 fused quartz 1.458
air 1.00028 pyrex 1.470
carbon dioxide 1.00045 acrylic glass 1.49
water 1.3330 polypropylene 1.49
acetone 1.36 bk7 1.5046
ethanol 1.361 sodium chloride 1.544
carbon tetrachloride 1.461 amber 1.55
glycerol 1.4729 pet 1.575
benzene 1.501 diamond 2.419
silicone oil 1.52045




Thin dielectric material (thindielectric)

This plugin models a thin dielectric material that is embedded inside another dielectric—for instance,
glass surrounded by air.The interior of the material is assumed to be so thin that its effect on
transmitted rays is negligible, Hence, light exits such a material without any form of angular deflection
(though there is still specular reflection).
This model should be used for things like glass windows that were modeled using only a single
sheet of triangles or quads. On the other hand, when the window consists of proper closed geometry,
dielectric is the right choice.This is illustrated below:

The implementation correctly accounts formultiple internal reflections inside the thin dielectric at
no significant extra cost, i.e. paths of the type R, TRT, TR3T, .. for reflection and TT, TR2, TR4T, ..
for refraction, where T and R denote individual reflection and refraction events, respectively.

Parameter

  • intIOR : float or string
    • Interior index of refraction specified numerically or using a known material name. (Default: bk7 / 1.5046)
  • extIOR : float or string
    • Exterior index of refraction specified numerically or using a known material name. (Default: air / 1.000277)
  • specular Reflectance : spectrum or texture
    • Optional factor that can be used to modulate the specular reflection component. Note that for physical realism, this parameter should never be touched. (Default: 1.0)
  • specular Transmittance : spectrum or texture
    • Optional factor that can be used to modulate the specular transmission component. Note that for physical realism, this parameter should never be touched. (Default: 1.0)




Rough dielectric material (roughdielectric)

This plugin implements a realistic microfacet scattering model for rendering rough interfaces between
dielectric materials, such as a transition from air to ground glass. Microfacet theory describes
rough surfaces as an arrangement of unresolved and ideally specular facets, whose normal directions
are given by a specially chosen microfacet distribution. By accounting for shadowing and masking
effects between these facets, it is possible to reproduce the important off-specular reflections peaks
observed in real-world measurements of such materials.
This plugin is essentially the “roughened” equivalent of the (smooth) plugin dielectric. For very
low values of α, the two will be identical, though scenes using this plugin will take longer to render
due to the additional computational burden of tracking surface roughness.
The implementation is based on the paper “Microfacet Models for Refraction through Rough Surfaces”
by Walter et al. [48]. It supports several different types of microfacet distributions and has
a texturable roughness parameter. Exterior and interior IOR values can be specified independently,
where “exterior” refers to the side that contains the surface normal. Similar to the dielectric plugin,
IOR values can either be specified numerically, or based on a list of known materials (see Table 3 for
an overview). When no parameters are given, the plugin activates the default settings, which describe
a borosilicate glass BK7/air interface with a light amount of roughness modeled using a Beckmann
distribution.
To get an intuition about the effect of the surface roughness parameter α, consider the following
approximate classification: a value of α = 0.001 − 0.01 corresponds to a material with slight imperfections
on an otherwise smooth surface finish, α = 0.1 is relatively rough, and α = 0.3 − 0.7 is extremely
rough (e.g. an etched or ground finish).
Please note that when using this plugin, it is crucial that the scene contains meaningful and mutually
compatible index of refraction changes—see Figure 7 for an example of what this entails. Also,
note that the importance sampling implementation of this model is close, but not always a perfect a
perfect match to the underlying scattering distribution, particularly for high roughness values and
when the ggx microfacet distribution is used. Hence, such renderings may converge slowly.


Technical details
When rendering with the Ashikhmin-Shirley or Phong microfacet distributions, a conversion is used
to turn the specified α roughness value into the exponents of these distributions. This is done in a
way, such that the different distributions all produce a similar appearance for the same value of α.
The Ashikhmin-Shirley microfacet distribution allows the specification of two distinct roughness
values along the tangent and bitangent directions. This can be used to provide a material with a
“brushed” appearance. The alignment of the anisotropy will follow the UV parameterization of the
underlying mesh in this case.This also means that such an anisotropic material cannot be applied to
triangle meshes that are missing texture coordinates.

Parameter

  • distribution : string
    • Specifies the type of microfacet normal distribution used to model the surface roughness.
      • (i) beckmann: Physically-based distribution derived from Gaussian random surfaces.This is the default.
      • (ii) ggx: New distribution proposed byWalter et al. [48], which ismeant to better handle the long tails observed inmeasurements of ground surfaces. Renderingswith this distribution may converge slowly.
      • (iii) phong: Classical cosp θ distribution. Due to the underlying microfacet theory, the use of this distribution here leads to more realistic behavior than the separately available phong plugin.
      • (iv) as: Anisotropic Phong-style microfacet distribution proposed by Ashikhmin and Shirley [1].
  • alpha : float or texture
    • Specifies the roughness of the unresolved surface microgeometry. When the Beckmann distribution is used, this parameter is equal to the root mean square (RMS) slope of the microfacets. This parameter is only valid when distribution=beckmann/phong/ggx. (Default: 0.1).
  • alphaU, alphaV : float or texture
    • Specifies the anisotropic roughness values along the tangent and bitangent directions. These parameter are only valid when distribution=as. (Default: 0.1).
  • intIOR : float or string
    • Interior index of refraction specified numerically or using a known material name. (Default: bk7 / 1.5046)
  • extIOR : float or string
    • Exterior index of refraction specified numerically or using a known material name. (Default: air / 1.000277)
  • specular Reflectance : spectrum or texture
    • Optional factor that can be used to modulate the specular reflection component. Note that for physical realism, this parameter should never be touched. (Default: 1.0)
  • specular Transmittance : spectrum or texture
    • Optional factor that can be used to modulate the specular transmission component. Note that for physical realism, this parameter should never be touched. (Default: 1.0)




  • 最終更新:2014-05-20 16:15:48

このWIKIを編集するにはパスワード入力が必要です

認証パスワード