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MATERIALS AND GEOMETRY FORMAT |
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SCCSid "$SunId$ LBL" |
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RCSid "$Id$" |
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Introduction |
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============ |
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Each entity is given by a short keyword, followed by space- or tab- |
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delimited arguments on a single line. A single entity may be extended |
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over multiple lines using a backslash ('\') character right before the |
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end of line, though no extended line may exceed 512 characters in total |
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length. (Given the current set of entities, even approaching 80 |
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characters would be highly unusual.) |
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end of line, though no extended line may exceed 4096 characters in total |
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length. |
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Entities and Contexts |
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===================== |
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c [id [= [template]]] get/set color context |
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cxy x y set CIE (x,y) chromaticity for current color |
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cspec l_min l_max v1 v2 .. set relative spectrum for current color |
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cct temperature set spectrum based on black body temperature |
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cmix w1 c1 w2 c2 .. mix named colors to make current color |
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m [id [= [template]]] get/set material context |
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sides {1|2} set number of sides for current material |
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ed epsilon_d set diffuse emittance for current material |
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rs rho_s alpha_r set specular reflectance for current material |
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ts tau_s alpha_t set specular transmittance for current material |
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ir n_real n_imag set index of refraction for current material |
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o [name] begin/end object context |
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f v1 v2 v3 .. polygon using current material, spec. vertices |
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sph vc radius sphere |
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included in the other material parameters. All colors are absolute, |
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e.g. spectral reflectance or transmittance under uniform white light. |
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A CIE xy chromaticity pair is the most basic color specification. |
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A full spectrum is the most general specification, and the starting |
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(i.e. minimum) and ending (i.e. maximum) wavelengths are given along |
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with a set of evenly spaced values. Wavelengths are given in nanometers, |
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and must be within the range of 380-780. The spectral values themselves |
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are located starting at the first wavelength and proceeding at even |
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increments to the ending wavelength. The values in between will be |
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interpolated as necessary, so there must be at least two specified points. |
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The color mixing entity is intended not only for the mixing of named |
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colors, but also for color specifications using an arbitrary set |
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of basis functions. The actual totals for spectral and mixing |
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coefficients is irrelevant, since the results will be normalized. |
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A CIE xy chromaticity pair is the most basic color specification. A |
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full spectrum is the most general specification, and the starting (i.e. |
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minimum) and ending (i.e. maximum) wavelengths are given along with a |
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set of evenly spaced values. Wavelengths are given in nanometers, and |
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should be within the range of 380-780. The spectral values themselves, |
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which can be thought of as relative power density per nanometer, start |
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at the first wavelength and proceed at even increments to the last |
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wavelength. The values in between will be interpolated as necessary, |
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so there must be at least two specified points. The color temperature |
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entity corresponds to the spectrum of a black body at the specified |
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temperature (in degrees Kelvin). The color mixing entity is intended |
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not only for the mixing of named colors, but also for color |
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specifications using an arbitrary set of basis functions. The mixing |
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coefficients are in effect relative luminances for each color |
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"primary." The actual total of the mixing coefficients or spectral |
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values is irrelevant, since the results will always be normalized. |
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Diffuse emittance is always given in SI units of lumens/meter^2. Note that |
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this is emittance, not exitance, and does not include light reflected or |
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The sum of the diffuse and specular reflectances and transmittances |
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must be strictly less than one (with no negative values, obviously). |
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These values are assumed to be measured at normal incidence. If an |
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index of refraction is given, this may modify the balance between |
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diffuse and specular reflectance at other incident angles. If the |
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material is one-sided (see below), then it may be a dielectric interface. |
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In this case, the specular transmittance given is that which would be |
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measured at normal incidence for a pane of the material 5 mm thick. |
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This is important for figuring the actual transmittance for non-planar |
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geometries assuming a uniformly absorbing medium. If the index of |
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refraction has an imaginary part, then the surface is a metal and this |
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implies other properties according to physics. The default index of |
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refraction is that of a vacuum, i.e. (1,0). |
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The object entity establishes a hierarchical context, consisting of |
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this identifier and all those preceding. It has no real meaning except |
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under a descriptive name for improved file readability. |
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Surfaces are two-sided unless the "sides" entity is used to set the |
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number of sides for a material to one. If a surfaces is one-sided, |
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number of sides for a material to one. If a surface is one-sided, |
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then it appears invisible when viewed from the back side. This means |
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that a transmitting object will affect the light coming in through the |
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front surface and ignore the characteristics of the back surface. As |
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long as the characteristics are the same, the results should be |
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correct. If the rendering technique does not allow for one-sided |
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long as the transmission characteristics are the same, the results should |
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be correct. If the rendering technique does not allow for one-sided |
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surfaces, an approximately correct result can be obtained for one-sided |
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transmitting surfaces by using the square root of the given tau_s and |
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half the given alpha_t. If a rendering technique does not permit |
