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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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current color and current material. Initially, these contexts are |
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unnamed, and have specific default values. The unnamed vertex is the |
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origin. The unnamed color is neutral gray. The unnamed material is a |
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perfect absorber. The unnamed contexts may be modified, but those |
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modifications will not be saved. Thus, reestablishing an unnamed |
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context always gets its initial default value. To save a new context |
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or modify an old one, it must first be named. Entities associated with |
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named contexts (i.e. "v", "c" and "m") may be followed by an identifier |
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and an equals sign ('='), indicating a new context. If there is no |
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equals, then the context must already be defined, and the appearance of |
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the entity merely reestablishes this context. If the context id is |
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followed by an equals, then a new context is defined, destroying any |
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previous instance of that context name. Redefining or changing values |
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of a context does not affect earlier uses of the same name, however. |
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Contexts are always associated with a name id, which is any non-blank |
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sequence of printing ASCII characters. An optional template may be |
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given following the equals, which is a previously defined context to |
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use as a source of default values for this definition. If no template |
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is given, then the unnamed context of that type is used to set initial |
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values. Named contexts continue until the next context definition of |
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the same type. |
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perfect (two-sided) absorber. The unnamed contexts may be modified, |
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but those modifications will not be saved. Thus, reestablishing an |
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unnamed context always gets its initial default value. To save a new |
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context or modify an old one, it must first be named. Entities |
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associated with named contexts (i.e. "v", "c" and "m") may be followed |
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by an identifier and an equals sign ('='), indicating a new context. |
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If there is no equals, then the context must already be defined, and |
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the appearance of the entity merely reestablishes this context. If the |
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context id is followed by an equals, then a new context is defined, |
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destroying any previous instance of that context name. Redefining or |
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changing values of a context does not affect earlier uses of the same |
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name, however. Contexts are always associated with a name id, which is |
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any non-blank sequence of printing ASCII characters. An optional |
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template may be given following the equals, which is a previously |
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defined context to use as a source of default values for this |
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definition. If no template is given, then the unnamed context of that |
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type is used to set initial values. Named contexts continue until the |
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next context definition of the same type. |
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Hierarchical Contexts |
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===================== |
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Note that this is strictly for ease of identification, and has no |
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real meaning as far as the geometric description goes. In contrast, |
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the transform entity is very significant as it determines how enclosing |
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objects are to be scaled and placed in the final description. |
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objects are to be scaled and placed in the final description. Hierarchical |
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contexts may be nested in any way, but should not overlap. |
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Without further ado, here are the proposed entities and their interpretations: |
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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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rd rho_d set diffuse reflectance for current material |
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td tau_d set diffuse transmittance 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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cyl v1 radius v2 truncated right cylinder (open-ended) |
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cone v1 rad1 v2 rad2 truncated right cone (open-ended) |
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prism v1 v2 v3 .. length right prism (closed solid) |
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prism v1 v2 v3 .. length truncated right prism (closed solid) |
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ring vc rmin rmax circular ring with inner and outer radii |
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torus vc rmin rmax circular torus with inner and outer radii |
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xf [xform] begin/end transformation context |
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-------- -------- |
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p, n vertex |
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cxy, cspec, cmix color |
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sides material |
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rd, td, ed, rs, ts color, material |
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f, sph, cyl, cone, ring, torus, prism material, object, transformation |
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That is, with the thumb of the right hand pointing in the direction |
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of the axis, rotation follows the curl of the fingers. |
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The transform command itself is also cumulative, and a transform |
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command with no arguments is used to return to the previous |
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condition. It is best if transforms and their end statements |
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("xf" by itself) are balanced in a file, so that later or enclosing |
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files are not affected. |
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The transform command itself is also cumulative, but in the reverse |
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order. That is, later transformations (i.e. enclosed transformations) |
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are prepended to existing (i.e. enclosing) ones. A transform command |
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with no arguments is used to return to the previous condition. It is |
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necessary that transforms and their end statements ("xf" by itself) be |
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balanced in a file, so that later or enclosing files are not affected. |
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Transformations apply only to geometric types, e.g. polygons, spheres, etc. |
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Vertices and the components that go into geometry are not directly affected. |
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====== |
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The -a N transform specification causes the following transform |
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arguments to be repeated along with the contents of the included |
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file N times. (Note that this option is supported only for included |
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files.) The first instance of the geometry will be in its initial |
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location; the second instance will be repositioned according to the |
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named transformation; the third instance will be repositioned by |
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objects N times. The first instance of the geometry will be in its |
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initial location; the second instance will be repositioned according |
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to the named transformation; the third instance will be repositioned by |
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applying this transformation twice, and so on up to N-1 applications. |
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Multi-dimensional arrays may be specified with a single include |
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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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to group the following surfaces up until an empty object statement |
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under a descriptive name for improved file readability. |
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Surfaces are one-sided, and appear invisible when viewed from the |
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back side. This means that a transmitting object will affect the |
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light coming in through the front surface and ignore the characteristics |
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of the back surface. As long as the characteristics are the same, |
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the results should be correct. If the rendering technique does not |
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allow for one-sided surfaces, an approximately correct result can |
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be obtained for transmitting surfaces by using the square root of |
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the given tau_s and half the given alpha_t. |
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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 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 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 |
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two-sided surfaces, then each surface must be made into two for |
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full compliance if "sides" is set to 2 (the default). |
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The surface normal of a face is oriented by the right-hand rule. |
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Specifically, the surface normal faces towards the viewer when the |
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and a length value. The prism will be extruded so that the end-face |
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points outward, unless the length value is negative, in which case the |
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object is extruded in the opposite direction, resulting in inward- |
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directed surface normals. |
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directed surface normals. If surface normals are specified for the |
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vertices, they will be applied to the side faces but not the end |
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faces, and they must generally point in the appropriate direction |
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(i.e. in or out depending on whether extrusion is negative or positive). |
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A sphere, cylinder or cone with negative radii is interpreted as having |
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an inward facing surface normal. Otherwise, the normal is assumed |
