Plastic
Smooth plastic material (plastic)
This plugin describes a smooth plastic-like material with internal scattering. It uses the Fresnel
reflection and transmission coefficients to provide direction-dependent specular and diffuse components.
Since it is simple, realistic, and fast, this model is often a better choice than the phong, ward,
and roughplastic plugins when rendering smooth plastic-like materials.
For convenience, this model allows to specify IOR values either numerically, or based on a list of
known materials (see Table 3 for an overview).
Note that this plugin is quite similar to what one would get by applying the coating plugin to the
diffusematerial.Themain difference is that this plugin is significantly faster, while at the same time
causing less variance. Furthermore, it accounts for multiple interreflections inside the material (read
on for details), which avoids a serious energy loss problem of the aforementioned plugin combination.
Internal scattering
Internally, this is model simulates the interaction of light with a diffuse base surface coated by a thin
dielectric layer. This is a convenient abstraction rather than a restriction. In other words, there are
many materials that can be rendered with this model, even if they might not not fit this description
perfectly well.
Given illumination that is incident upon such a material, a portion of the illumination is specularly
reflected at the material boundary, which results in a sharp reflection in the mirror direction
(Figure 11a).The remaining illumination refracts into the material, where it scatters from the diffuse
base layer. (Figure 11b). While some of the diffusely scattered illumination is able to directly refract
outwards again, the remainder is reflected from the interior side of the dielectric boundary and will
in fact remain trapped inside the material for some number of internal scattering events until it is
finally able to escape (Figure 11c).
Due to the mathematical simplicity of this setup, it is possible to work out the correct form of the
model without actually having to simulate the potentially large number of internal scattering events.
Note that due to the internal scattering, the diffuse color of the material is in practice slightly different
from the color of the base layer on its own—in particular, the material color will tend to shift
towards darker colors with higher saturation. Since this can be counter-intuitive when using bitmap
textures, these color shifts are disabled by default. Specify the parameter nonlinear=true to enable
them. Figure 10 illustrates the resulting change.This effect is also seen in real life, for instance a piece
of wood will look slightly darker after coating it with a layer of varnish.
Parameter Type Description
- intIOR : float or string
- Interior index of refraction specified numerically or using a known material name. (Default: polypropylene / 1.49)
- 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)
- diffuse Reflectance : spectrum or texture
- Optional factor used tomodulate the diffuse reflection component (Default: 0.5)
- nonlinear : boolean
- Account for nonlinear color shifts due to internal scattering? See the main text for details. (Default: Don’t account for them and preserve the texture colors, i.e. false)
Rough plastic material (roughplastic)
This plugin implements a realistic microfacet scattering model for rendering rough dielectric materials
with internal scattering, such as plastic. It can be interpreted as a fancy version of the Cook-
Torrance model and should be preferred over heuristic models like phong and ward when possible.
Microfacet theory describes rough surfaces as an arrangement of unresolved and ideally specular
facets, whose normal directions are given by a specially chosenmicrofacet distribution. By accounting
for shadowing and masking effects between these facets, it is possible to reproduce the important offspecular
reflections peaks observed in real-world measurements of such materials.
This plugin is essentially the “roughened” equivalent of the (smooth) plugin plastic. 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.
For convenience, this model allows to specify IOR values either numerically, or based on a list of
knownmaterials (see Table 3 on page 58 for an overview). When no parameters are given, the plugin
activates the defaults, which describe a white polypropylene plastic material with a light amount of
roughness modeled using the Beckmann distribution.
Like the plastic material, this model internally simulates the interaction of light with a diffuse
base surface coated by a thin dielectric layer (where the coating layer is now rough). This is a convenient
abstraction rather than a restriction. In other words, there are many materials that can be
rendered with this model, even if they might not not fit this description perfectly well.
The simplicity of this setup makes it possible to account for interesting nonlinear effects due to
internal scattering, which is controlled by the nonlinear parameter. For more details, please refer
to the description of this parameter given in the the plastic plugin section on page 68.
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). Values significantly above that are probably not too realistic.
Technical details
The implementation of this model is partly based on the paper “Microfacet Models for Refraction
through Rough Surfaces” byWalter et al. [48]. Several different types of microfacet distributions are
supported. Note that the choices are slightly more restricted here—in comparison to other rough
scattering models in Mitsuba, anisotropic distributions are not allowed.
The implementation of this model makes heavy use of a rough Fresnel transmittance function,
which is a generalization of the usual Fresnel transmittion coefficient to microfacet surfaces. Unfortunately,
this function is normally prohibitively expensive, since each evaluation involves a numerical
integration over the sphere.
To avoid this performance issue,Mitsuba shipswith data files (contained in the data/microfacet
directory) containing precomputed values of this function over a large range of parameter values. At
runtime, the relevant parts are extracted using tricubic interpolation.
When rendering with the Phong microfacet distributions, a conversion is used to turn the specified
α roughness value into the Phong exponent.This is done in a way, such that the different distributions
all produce a similar appearance for the same value of α.
Parameter Type Description
- 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.
- Specifies the type of microfacet normal distribution used to model the surface roughness.
- 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. (Default: 0.1).
- intIOR : float or string
- Interior index of refraction specified numerically or using a known material name. (Default: polypropylene / 1.49)
- 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)
- diffuse Reflectance : spectrum or texture
- Optional factor used tomodulate the diffuse reflection component (Default: 0.5) nonlinear boolean Account for nonlinear color shifts due to internal scattering? See the plastic plugin for details. (Default: Don’t account for them and preserve the texture colors, i.e. false)
- 最終更新:2014-05-20 17:07:26
