| 37 |
|
current color and current material. Initially, these contexts are |
| 38 |
|
unnamed, and have specific default values. The unnamed vertex is the |
| 39 |
|
origin. The unnamed color is neutral gray. The unnamed material is a |
| 40 |
< |
perfect absorber. The unnamed contexts may be modified, but those |
| 41 |
< |
modifications will not be saved. Thus, reestablishing an unnamed |
| 42 |
< |
context always gets its initial default value. To save a new context |
| 43 |
< |
or modify an old one, it must first be named. Entities associated with |
| 44 |
< |
named contexts (i.e. "v", "c" and "m") may be followed by an identifier |
| 45 |
< |
and an equals sign ('='), indicating a new context. If there is no |
| 46 |
< |
equals, then the context must already be defined, and the appearance of |
| 47 |
< |
the entity merely reestablishes this context. If the context id is |
| 48 |
< |
followed by an equals, then a new context is defined, destroying any |
| 49 |
< |
previous instance of that context name. Redefining or changing values |
| 50 |
< |
of a context does not affect earlier uses of the same name, however. |
| 51 |
< |
Contexts are always associated with a name id, which is any non-blank |
| 52 |
< |
sequence of printing ASCII characters. An optional template may be |
| 53 |
< |
given following the equals, which is a previously defined context to |
| 54 |
< |
use as a source of default values for this definition. If no template |
| 55 |
< |
is given, then the unnamed context of that type is used to set initial |
| 56 |
< |
values. Named contexts continue until the next context definition of |
| 57 |
< |
the same type. |
| 40 |
> |
perfect (two-sided) absorber. The unnamed contexts may be modified, |
| 41 |
> |
but those modifications will not be saved. Thus, reestablishing an |
| 42 |
> |
unnamed context always gets its initial default value. To save a new |
| 43 |
> |
context or modify an old one, it must first be named. Entities |
| 44 |
> |
associated with named contexts (i.e. "v", "c" and "m") may be followed |
| 45 |
> |
by an identifier and an equals sign ('='), indicating a new context. |
| 46 |
> |
If there is no equals, then the context must already be defined, and |
| 47 |
> |
the appearance of the entity merely reestablishes this context. If the |
| 48 |
> |
context id is followed by an equals, then a new context is defined, |
| 49 |
> |
destroying any previous instance of that context name. Redefining or |
| 50 |
> |
changing values of a context does not affect earlier uses of the same |
| 51 |
> |
name, however. Contexts are always associated with a name id, which is |
| 52 |
> |
any non-blank sequence of printing ASCII characters. An optional |
| 53 |
> |
template may be given following the equals, which is a previously |
| 54 |
> |
defined context to use as a source of default values for this |
| 55 |
> |
definition. If no template is given, then the unnamed context of that |
| 56 |
> |
type is used to set initial values. Named contexts continue until the |
| 57 |
> |
next context definition of the same type. |
| 58 |
|
|
| 59 |
|
Hierarchical Contexts |
| 60 |
|
===================== |
| 68 |
|
Note that this is strictly for ease of identification, and has no |
| 69 |
|
real meaning as far as the geometric description goes. In contrast, |
| 70 |
|
the transform entity is very significant as it determines how enclosing |
| 71 |
< |
objects are to be scaled and placed in the final description. |
| 71 |
> |
objects are to be scaled and placed in the final description. Hierarchical |
| 72 |
> |
contexts may be nested in any way, but should not overlap. |
| 73 |
|
|
| 74 |
|
Without further ado, here are the proposed entities and their interpretations: |
| 75 |
|
|
| 86 |
|
cspec l_min l_max v1 v2 .. set relative spectrum for current color |
| 87 |
|
cmix w1 c1 w2 c2 .. mix named colors to make current color |
| 88 |
|
m [id [= [template]]] get/set material context |
| 89 |
+ |
sides {1|2} set number of sides for current material |
| 90 |
|
rd rho_d set diffuse reflectance for current material |
| 91 |
|
td tau_d set diffuse transmittance for current material |
| 92 |
|
ed epsilon_d set diffuse emittance for current material |
| 97 |
|
sph vc radius sphere |
| 98 |
|
cyl v1 radius v2 truncated right cylinder (open-ended) |
| 99 |
|
cone v1 rad1 v2 rad2 truncated right cone (open-ended) |
| 100 |
< |
prism v1 v2 v3 .. length right prism (closed solid) |
| 100 |
> |
prism v1 v2 v3 .. length truncated right prism (closed solid) |
| 101 |
|
ring vc rmin rmax circular ring with inner and outer radii |
| 102 |
|
torus vc rmin rmax circular torus with inner and outer radii |
| 103 |
|
xf [xform] begin/end transformation context |
| 108 |
|
-------- -------- |
| 109 |
|
p, n vertex |
| 110 |
|
cxy, cspec, cmix color |
| 111 |
+ |
sides material |
| 112 |
|
rd, td, ed, rs, ts color, material |
| 113 |
|
f, sph, cyl, cone, ring, torus, prism material, object, transformation |
| 114 |
|
|
| 145 |
|
That is, with the thumb of the right hand pointing in the direction |
| 146 |
|
of the axis, rotation follows the curl of the fingers. |
| 147 |
|
|
| 148 |
< |
The transform command itself is also cumulative, and a transform |
| 149 |
< |
command with no arguments is used to return to the previous |
| 150 |
< |
condition. It is best if transforms and their end statements |
| 151 |
< |
("xf" by itself) are balanced in a file, so that later or enclosing |
| 152 |
< |
files are not affected. |
| 148 |
> |
The transform command itself is also cumulative, but in the reverse |
| 149 |
> |
order. That is, later transformations (i.e. enclosed transformations) |
| 150 |
> |
are prepended to existing (i.e. enclosing) ones. A transform command |
| 151 |
> |
with no arguments is used to return to the previous condition. It is |
| 152 |
> |
necessary that transforms and their end statements ("xf" by itself) be |
| 153 |
> |
balanced in a file, so that later or enclosing files are not affected. |
| 154 |
|
|
| 155 |
|
Transformations apply only to geometric types, e.g. polygons, spheres, etc. |
| 156 |
|
Vertices and the components that go into geometry are not directly affected. |
| 161 |
|
====== |
| 162 |
|
The -a N transform specification causes the following transform |
| 163 |
|
arguments to be repeated along with the contents of the included |
| 164 |
< |
file N times. (Note that this option is supported only for included |
| 165 |
< |
files.) The first instance of the geometry will be in its initial |
| 166 |
< |
location; the second instance will be repositioned according to the |
| 163 |
< |
named transformation; the third instance will be repositioned by |
| 164 |
> |
objects N times. The first instance of the geometry will be in its |
| 165 |
> |
initial location; the second instance will be repositioned according |
| 166 |
> |
to the named transformation; the third instance will be repositioned by |
| 167 |
|
applying this transformation twice, and so on up to N-1 applications. |
| 168 |
|
|
| 169 |
|
Multi-dimensional arrays may be specified with a single include |
| 210 |
|
included in the other material parameters. All colors are absolute, |
| 211 |
|
e.g. spectral reflectance or transmittance under uniform white light. |
| 212 |
|
|
| 213 |
< |
A CIE xy chromaticity pair is the most basic color specification. |
| 214 |
< |
A full spectrum is the most general specification, and the starting |
| 215 |
< |
(i.e. minimum) and ending (i.e. maximum) wavelengths are given along |
| 216 |
< |
with a set of evenly spaced values. Wavelengths are given in nanometers, |
| 217 |
< |
and must be within the range of 380-780. The spectral values themselves |
| 218 |
< |
are located starting at the first wavelength and proceeding at even |
| 219 |
< |
increments to the ending wavelength. The values in between will be |
| 220 |
< |
interpolated as necessary, so there must be at least two specified points. |
| 221 |
< |
The color mixing entity is intended not only for the mixing of named |
| 222 |
< |
colors, but also for color specifications using an arbitrary set |
| 223 |
< |
of basis functions. The actual totals for spectral and mixing |
| 224 |
< |
coefficients is irrelevant, since the results will be normalized. |
| 213 |
> |
A CIE xy chromaticity pair is the most basic color specification. A |
| 214 |
> |
full spectrum is the most general specification, and the starting (i.e. |
| 215 |
> |
minimum) and ending (i.e. maximum) wavelengths are given along with a |
| 216 |
> |
set of evenly spaced values. Wavelengths are given in nanometers, and |
| 217 |
> |
must be within the range of 380-780. The spectral values themselves, |
| 218 |
> |
which can be thought of as relative power density per nanometer, start |
| 219 |
> |
at the first wavelength and proceed at even increments to the last |
| 220 |
> |
wavelength. The values in between will be interpolated as necessary, |
| 221 |
> |
so there must be at least two specified points. The color mixing |
| 222 |
> |
entity is intended not only for the mixing of named colors, but also |
| 223 |
> |
for color specifications using an arbitrary set of basis functions. |
| 224 |
> |
The mixing coefficients are in effect relative luminances for each |
| 225 |
> |
color "primary." The actual total of the mixing coefficients or |
| 226 |
> |
spectral values is irrelevant, since the results will always be |
| 227 |
> |
normalized. |
| 228 |
|
|
| 229 |
|
Diffuse emittance is always given in SI units of lumens/meter^2. Note that |
| 230 |
|
this is emittance, not exitance, and does not include light reflected or |
| 243 |
|
to group the following surfaces up until an empty object statement |
| 244 |
|
under a descriptive name for improved file readability. |
| 245 |
|
|
| 246 |
< |
Surfaces are one-sided, and appear invisible when viewed from the |
| 247 |
< |
back side. This means that a transmitting object will affect the |
| 248 |
< |
light coming in through the front surface and ignore the characteristics |
| 249 |
< |
of the back surface. As long as the characteristics are the same, |
| 250 |
< |
the results should be correct. If the rendering technique does not |
| 251 |
< |
allow for one-sided surfaces, an approximately correct result can |
| 252 |
< |
be obtained for transmitting surfaces by using the square root of |
| 253 |
< |
the given tau_s and half the given alpha_t. |
| 246 |
> |
Surfaces are two-sided unless the "sides" entity is used to set the |
| 247 |
> |
number of sides for a material to one. If a surfaces is one-sided, |
| 248 |
> |
then it appears invisible when viewed from the back side. This means |
| 249 |
> |
that a transmitting object will affect the light coming in through the |
| 250 |
> |
front surface and ignore the characteristics of the back surface. As |
| 251 |
> |
long as the characteristics are the same, the results should be |
| 252 |
> |
correct. If the rendering technique does not allow for one-sided |
| 253 |
> |
surfaces, an approximately correct result can be obtained for one-sided |
| 254 |
> |
transmitting surfaces by using the square root of the given tau_s and |
| 255 |
> |
half the given alpha_t. If a rendering technique does not permit |
| 256 |
> |
two-sided surfaces, then each surface must be made into two for |
| 257 |
> |
full compliance if "sides" is set to 2 (the default). |
| 258 |
|
|
| 259 |
|
The surface normal of a face is oriented by the right-hand rule. |
| 260 |
|
Specifically, the surface normal faces towards the viewer when the |
| 266 |
|
and a length value. The prism will be extruded so that the end-face |
| 267 |
|
points outward, unless the length value is negative, in which case the |
| 268 |
|
object is extruded in the opposite direction, resulting in inward- |
| 269 |
< |
directed surface normals. |
| 269 |
> |
directed surface normals. If surface normals are specified for the |
| 270 |
> |
vertices, they will be applied to the side faces but not the end |
| 271 |
> |
faces, and they must generally point in the appropriate direction |
| 272 |
> |
(i.e. in or out depending on whether extrusion is negative or positive). |
| 273 |
|
|
| 274 |
|
A sphere, cylinder or cone with negative radii is interpreted as having |
| 275 |
|
an inward facing surface normal. Otherwise, the normal is assumed |
