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root/radiance/ray/doc/ray.1
Revision: 1.3
Committed: Fri Mar 14 21:27:45 2003 UTC (21 years, 7 months ago) by greg
Branch: MAIN
Changes since 1.2: +3 -0 lines
Log Message:
Added -a option to obj2mesh to incorporate materials in mesh

File Contents

# User Rev Content
1 greg 1.1 .\" RCSid "$Id"
2     .\" Print using the -ms macro package
3     .DA 1/20/99
4     .LP
5     .tl """Copyright \(co 1996 Regents, University of California
6     .sp 2
7     .TL
8     The
9     .so ../src/rt/VERSION
10     .br
11     Synthetic Imaging System
12     .AU
13     Greg Ward
14     .br
15     Lawrence Berkeley Laboratory
16     .br
17     1 Cyclotron Rd.
18     .br
19     Berkeley, CA 94720
20     .br
21     (510) 486-4757
22     .NH 1
23     Introduction
24     .PP
25     RADIANCE was developed as a research tool
26     for predicting the distribution of visible radiation in
27     illuminated spaces.
28     It takes as input a three-dimensional geometric model of
29     the physical environment, and produces a map of
30     spectral radiance values in a color image.
31     The technique of ray-tracing follows light backwards
32     from the image plane to the source(s).
33     Because it can produce realistic images from a simple description,
34     RADIANCE has a wide range of applications in graphic arts,
35     lighting design, computer-aided engineering and architecture.
36     .KF
37     .sp 25
38     .ce
39     .B "Figure 1."
40     .sp
41     .KE
42     .PP
43     The diagram in Figure 1 shows the flow between programs (boxes) and
44     data (ovals).
45     The central program is
46     .I rpict,
47     which produces a picture from a scene description.
48     .I Rview
49     is a variation of
50     .I rpict
51     that computes and displays images interactively.
52     .PP
53     A scene description file lists the surfaces and materials
54     that make up a specific environment.
55     The current surface types are spheres, polygons, cones,
56     and cylinders.
57     They can be made from materials such as plastic, metal,
58     and glass.
59     Light sources can be distant disks as well as local spheres, discs and
60     polygons.
61     .PP
62     From a three-dimensional scene description and a specified view,
63     .I rpict
64     produces a two-dimensional image.
65     A picture file is a compressed binary representation of the
66     pixels in the image.
67     This picture can be scaled in size and
68     brightness, anti-aliased, and sent to a graphics output device.
69     .PP
70     A header in each picture file lists the program(s) and
71     parameters that produced it.
72     This is useful for identifying a picture
73     without having to display it.
74     The information can be read by the program
75     .I getinfo.
76     .NH 1
77     Scene Description
78     .PP
79     A scene description file represents a
80     three-dimensional physical environment
81     in Cartesian (rectilinear) world coordinates.
82     It is stored as ASCII text, with the following basic format:
83     .DS
84     # comment
85    
86     modifier type identifier
87     n S1 S2 S3 .. Sn
88     0
89     m R1 R2 R3 .. Rm
90    
91     modifier alias identifier reference
92    
93     ! command
94    
95     ...
96     .DE
97     .PP
98     A comment line begins with a pound sign, `#'.
99     .PP
100     The scene description
101     .I primitives
102     all have the same general format, and can
103     be either surfaces or modifiers.
104     A primitive has a modifier, a type, and an identifier.
105     A modifier is either the identifier of a
106     .I "previously defined"
107     primitive, or "void"\(dg.
108     .FS
109     \(dgThe most recent definition of a modifier is the one used,
110     and later definitions do not cause relinking of loaded
111     primitives.
112     Thus, the same identifier may be used repeatedly, and each new
113     definition will apply to the primitives following it.
114     .FE
115     An identifier can be any string (i.e. sequence of non-blank
116     characters).
117     The
118     .I arguments
119     associated with a primitive can be strings or real numbers.
120     The first integer following the identifier is the number
121     of string arguments, and it is followed by the arguments themselves
122     (separated by white space).
123     The next integer is the number of integer arguments, and is followed
124     by the integer arguments.
125     (There are currently no primitives that use them, however.)
126     The next integer is the real argument count, and it is followed
127     by the real arguments.
128     .PP
129     An alias gets its type and arguments from a previously defined primitive.
130     This is useful when the same material is used with a different
131     modifier, or as a convenient naming mechanism.
132 greg 1.2 The reserved modifier name "inherit" may be used to specificy that
133     an alias will inherit its modifier from the original.
134 greg 1.1 Surfaces cannot be aliased.
135     .PP
136     A line beginning with an exclamation point, `!',
137     is interpreted as a command.
138     It is executed by the shell, and its output is read as input to
139     the program.
140     The command must not try to read from its standard input, or
141     confusion will result.
142     A command may be continued over multiple lines using a backslash, `\\',
143     to escape the newline.
144     .PP
145     Blank space is generally ignored, except as a separator.
146     The exception is the newline character after a command or comment.
147     Commands, comments and primitives may appear in any combination, so long
148     as they are not intermingled.
149     .NH 2
150     Primitive Types
151     .PP
152     Primitives can be surfaces, materials, textures or patterns.
153     Modifiers can be materials, textures or patterns.
154     Simple surfaces must have one material in their modifier list.
155     .NH 3
156     Surfaces
157     .PP
158     A scene description will consist mostly of surfaces.
159     The basic types are given below.
160     .LP
161     .UL Source
162     .PP
163     A source is not really a surface, but a solid angle.
164     It is used for specifying light sources that are very distant.
165     The direction to the center of the source and the number of degrees
166     subtended by its disk are given as follows:
167     .DS
168     mod source id
169     0
170     0
171     4 xdir ydir zdir angle
172     .DE
173     .LP
174     .UL Sphere
175     .PP
176     A sphere is given by its center and radius:
177     .DS
178     mod sphere id
179     0
180     0
181     4 xcent ycent zcent radius
182     .DE
183     .LP
184     .UL Bubble
185     .PP
186     A bubble is simply a sphere whose surface normal points inward.
187     .LP
188     .UL Polygon
189     .PP
190     A polygon is given by a list of three-dimensional vertices,
191     which are ordered counter-clockwise as viewed from
192     the front side (into the surface normal).
193     The last vertex is automatically connected to the first.
194     Holes are represented in polygons as interior vertices connected to
195     the outer perimeter by coincident edges (seams).
196     .DS
197     mod polygon id
198     0
199     0
200     3n
201     x1 y1 z1
202     x2 y2 z2
203     ...
204     xn yn zn
205     .DE
206     .LP
207     .UL Cone
208     .PP
209     A cone is a megaphone-shaped object.
210     It is truncated by two planes perpendicular to its axis,
211     and one of its ends may come to a point.
212     It is given as two axis endpoints, and the starting
213     and ending radii:
214     .DS
215     mod cone id
216     0
217     0
218     8
219     x0 y0 z0
220     x1 y1 z1
221     r0 r1
222     .DE
223     .LP
224     .UL Cup
225     .PP
226     A cup is an inverted cone (i.e. has an inward surface normal).
227     .LP
228     .UL Cylinder
229     .PP
230     A cylinder is like a cone, but its starting and ending radii are
231     equal.
232     .DS
233     mod cylinder id
234     0
235     0
236     7
237     x0 y0 z0
238     x1 y1 z1
239     rad
240     .DE
241     .LP
242     .UL Tube
243     .PP
244     A tube is an inverted cylinder.
245     .LP
246     .UL Ring
247     .PP
248     A ring is a circular disk given by its center, surface
249     normal, and inner and outer radii:
250     .DS
251     mod ring id
252     0
253     0
254     8
255     xcent ycent zcent
256     xdir ydir zdir
257     r0 r1
258     .DE
259     .LP
260     .UL Mesh
261     .PP
262     A mesh is a compound surface, made up of many triangles and
263     an octree data structure to accelerate ray intersection.
264     It is typically converted from a Wavefront .OBJ file using the
265     obj2mesh program.
266     .DS
267     mod mesh id
268     1+ meshfile transform
269     0
270     0
271     .DE
272 greg 1.3 If the modifier is "void", then surfaces will use the modifiers given
273     in the original mesh description.
274     Otherwise, the modifier specified is used in their place.
275 greg 1.1 The transform moves the mesh to the desired location in the scene.
276     Multiple instances using the same meshfile take little extra memory,
277     and the compiled mesh itself takes much less space than individual
278     polygons would.
279     In the case of an unsmoothed mesh, using the mesh primitive reduces
280     memory requirements by a factor of 30 relative to individual triangles.
281     If a mesh has smoothed surfaces, we save a factor of 50 or more,
282     permitting very detailed geometries that would otherwise exhaust the
283     available memory.
284     In addition, the mesh primitive can have associated (u,v) coordinates
285     for pattern and texture mapping.
286     These are made available to function files via the Lu and Lu variables.
287     .LP
288     .UL Instance
289     .PP
290     An instance is a compound surface, given by the contents of an
291     octree file (created by oconv).
292     .DS
293     mod instance id
294     1+ octree transform
295     0
296     0
297     .DE
298     If the modifier is "void", then surfaces will use the modifiers given
299     in the original description.
300     Otherwise, the modifier specified is used in their place.
301     The transform moves the octree to the desired location in the scene.
302     Multiple instances using the same octree take little extra memory,
303     hence very complex descriptions can be rendered using this primitive.
304     .PP
305     There are a number of important limitations to be aware of when using
306     instances.
307     First, the scene description used to generate the octree must stand on
308     its own, without referring to modifiers in the parent description.
309     This is necessary for oconv to create the octree.
310     Second, light sources in the octree will not be incorporated correctly
311     in the calculation, and they are not recommended.
312     Finally, there is no advantage (other than convenience) to
313     using a single instance of an octree, or an octree containing only a
314     few surfaces.
315     An xform command on the subordinate description is prefered in such cases.
316     .NH 3
317     Materials
318     .PP
319     A material defines the way light interacts with a surface.
320     The basic types are given below.
321     .LP
322     .UL Light
323     .PP
324     Light is the basic material for self-luminous surfaces (i.e. light
325     sources).
326     In addition to the source surface type, spheres, discs (rings with zero
327     inner radius), cylinders (provided they are long enough), and
328     polygons can act as light sources.
329     Polygons work best when they are rectangular.
330     Cones cannot be used at this time.
331     A pattern may be used to specify a light output distribution.
332     Light is defined simply as a RGB radiance value (watts/steradian/m2):
333     .DS
334     mod light id
335     0
336     0
337     3 red green blue
338     .DE
339     .LP
340     .UL Illum
341     .PP
342     Illum is used for secondary light sources with broad distributions.
343     A secondary light source is treated like any other
344     light source, except when viewed directly.
345     It then acts like it is made of a different material (indicated by
346     the string argument), or becomes invisible (if no string argument is given,
347     or the argument is "void").
348     Secondary sources are useful when modeling windows or
349     brightly illuminated surfaces.
350     .DS
351     mod illum id
352     1 material
353     0
354     3 red green blue
355     .DE
356     .LP
357     .UL Glow
358     .PP
359     Glow is used for surfaces that are self-luminous, but limited
360     in their effect.
361     In addition to the radiance value, a maximum radius for
362     shadow testing is given:
363     .DS
364     mod glow id
365     0
366     0
367     4 red green blue maxrad
368     .DE
369     If maxrad is zero, then the surface will never be tested
370     for shadow, although it may participate in an interreflection calculation.
371     If maxrad is negative, then the surface will never contribute to scene
372     illumination.
373     Glow sources will never illuminate objects on the other side of an
374     illum surface.
375     This provides a convenient way to illuminate local light fixture
376     geometry without overlighting nearby objects.
377     .LP
378     .UL Spotlight
379     .PP
380     Spotlight is used for self-luminous surfaces having directed output.
381     As well as radiance, the full cone angle (in degrees)
382     and orientation (output direction) vector are given.
383     The length of the orientation vector is the distance
384     of the effective focus behind the source center (i.e. the focal length).
385     .DS
386     mod spotlight id
387     0
388     0
389     7 red green blue angle xdir ydir zdir
390     .DE
391     .LP
392     .UL Mirror
393     .PP
394     Mirror is used for planar surfaces that produce secondary
395     source reflections.
396     This material should be used sparingly, as it may cause the light
397     source calculation to blow up if it is applied to many small surfaces.
398     This material is only supported for flat surfaces such as polygons
399     and rings.
400     The arguments are simply the RGB reflectance values, which should be
401     between 0 and 1.
402     An optional string argument may be used like the illum type to specify a
403     different material to be used for shading non-source rays.
404     If this alternate material is given as "void", then the mirror surface
405     will be invisible.
406     This is only appropriate if the surface hides other (more detailed)
407     geometry with the same overall reflectance.
408     .DS
409     mod mirror id
410     1 material
411     0
412     3 red green blue
413     .DE
414     .LP
415     .UL Prism1
416     .PP
417     The prism1 material is for general light redirection from prismatic
418     glazings, generating secondary light sources.
419     It can only be used to modify a planar surface (i.e. a polygon or disk)
420     and should not result in either light concentration or scattering.
421     The new direction of the ray can be on either side of the material,
422     and the definitions must have the correct bidirectional properties
423     to work properly with secondary light sources.
424     The arguments give the coefficient for the redirected light
425     and its direction.
426     .DS
427     mod prism1 id
428     5+ coef dx dy dz funcfile transform
429     0
430     n A1 A2 .. An
431     .DE
432     The new direction variables
433     .I "dx, dy"
434     and
435     .I dz
436     need not produce a normalized vector.
437     For convenience, the variables
438     .I "DxA, DyA"
439     and
440     .I DzA
441     are defined as the normalized direction to the target light source.
442     See section 2.2.1 on function files for further information.
443     .LP
444     .UL Prism2
445     .PP
446     The material prism2 is identical to prism1 except that
447     it provides for two ray redirections rather than one.
448     .DS
449     mod prism2 id
450     9+ coef1 dx1 dy1 dz1 coef2 dx2 dy2 dz2 funcfile transform
451     0
452     n A1 A2 .. An
453     .DE
454     .LP
455     .UL Mist
456     .PP
457     Mist is a virtual material used to delineate a volume
458     of participating atmosphere.
459     A list of important light sources may be given, along with an
460     extinction coefficient, scattering albedo and scattering eccentricity
461     parameter.
462     The light sources named by the string argument list
463     will be tested for scattering within the volume.
464     Sources are identified by name, and virtual light sources may be indicated
465     by giving the relaying object followed by '>' followed by the source, i.e:
466     .DS
467     3 source1 mirror1>source10 mirror2>mirror1>source3
468     .DE
469     Normally, only one source is given per mist material, and there is an
470     upper limit of 32 to the total number of active scattering sources.
471     The extinction coefficient, if given, is added to the global
472     coefficient set on the command line.
473     Extinction is in units of 1/distance (distance based on the world coordinates),
474     and indicates the proportional loss of radiance over one unit distance.
475     The scattering albedo, if present, will override the global setting within
476     the volume.
477     An albedo of 0\00\00 means a perfectly absorbing medium, and an albedo of
478     1\01\01\0 means
479     a perfectly scattering medium (no absorption).
480     The scattering eccentricity parameter will likewise override the global
481     setting if it is present.
482     Scattering eccentricity indicates how much scattered light favors the
483     forward direction, as fit by the Heyney-Greenstein function:
484     .DS
485     P(theta) = (1 - g*g) / (1 + g*g - 2*g*cos(theta))^1.5
486     .DE
487     A perfectly isotropic scattering medium has a g parameter of 0, and
488     a highly directional material has a g parameter close to 1.
489     Fits to the g parameter may be found along with typical extinction
490     coefficients and scattering albedos for various atmospheres and
491     cloud types in USGS meteorological tables.
492     (A pattern will be applied to the extinction values.)\0
493     .DS
494     mod mist id
495     N src1 src2 .. srcN
496     0
497     0|3|6|7 [ rext gext bext [ ralb galb balb [ g ] ] ]
498     .DE
499     There are two usual uses of the mist type.
500     One is to surround a beam from a spotlight or laser so that it is
501     visible during rendering.
502     For this application, it is important to use a cone (or cylinder) that
503     is long enough and wide enough to contain the important visible portion.
504     Light source photometry and intervening objects will have the desired
505     effect, and crossing beams will result in additive scattering.
506     For this application, it is best to leave off the real arguments, and
507     use the global rendering parameters to control the atmosphere.
508     The second application is to model clouds or other localized media.
509     Complex boundary geometry may be used to give shape to a uniform medium,
510     so long as the boundary encloses a proper volume.
511     Alternatively, a pattern may be used to set the line integral value
512     through the cloud for a ray entering or exiting a point in a given
513     direction.
514     For this application, it is best if cloud volumes do not overlap each other,
515     and opaque objects contained within them may not be illuminated correctly
516     unless the line integrals consider enclosed geometry.
517     .LP
518     .UL Plastic
519     .PP
520     Plastic is a material with uncolored highlights.
521     It is given by its RGB reflectance, its fraction of specularity,
522     and its roughness value.
523     Roughness is specified as the rms slope of surface facets.
524     A value of 0 corresponds to a perfectly smooth surface, and
525     a value of 1 would be a very rough surface.
526     Specularity fractions greater than 0.1 and
527     roughness values greater than 0.2 are not very
528     realistic.
529     (A pattern modifying plastic will affect the material color.)
530     .DS
531     mod plastic id
532     0
533     0
534     5 red green blue spec rough
535     .DE
536     .LP
537     .UL Metal
538     .PP
539     Metal is similar to plastic, but specular highlights
540     are modified by the material color.
541     Specularity of metals is usually .9 or greater.
542     As for plastic, roughness values above .2 are uncommon.
543     .LP
544     .UL Trans
545     .PP
546     Trans is a translucent material, similar to plastic.
547     The transmissivity is the fraction of penetrating light that
548     travels all the way through the material.
549     The transmitted specular component is the fraction of transmitted
550     light that is not diffusely scattered.
551     Transmitted and diffusely reflected light is modified by the material color.
552     Translucent objects are infinitely thin.
553     .DS
554     mod trans id
555     0
556     0
557     7 red green blue spec rough trans tspec
558     .DE
559     .LP
560     .UL Plastic2
561     .PP
562     Plastic2 is similar to plastic, but with anisotropic
563     roughness.
564     This means that highlights in the surface will appear elliptical rather
565     than round.
566     The orientation of the anisotropy is determined by the unnormalized
567     direction vector
568     .I "ux uy uz".
569     These three expressions (separated by white space) are evaluated in
570     the context of the function file
571     .I funcfile.
572     If no function file is required (i.e. no special variables or
573     functions are required), a period (`.') may be given in its
574     place.
575     (See the discussion of Function Files in the Auxiliary Files section).
576     The
577     .I urough
578     value defines the roughness along the
579     .B u
580     vector given projected onto the surface.
581     The
582     .I vrough
583     value defines the roughness perpendicular to this vector.
584     Note that the highlight will be narrower in the direction of the
585     smaller roughness value.
586     Roughness values of zero are not allowed for efficiency reasons
587     since the behavior would be the same as regular plastic in that
588     case.
589     .DS
590     mod plastic2 id
591     4+ ux uy uz funcfile transform
592     0
593     6 red green blue spec urough vrough
594     .DE
595     .LP
596     .UL Metal2
597     .PP
598     Metal2 is the same as plastic2, except that the highlights are
599     modified by the material color.
600     .LP
601     .UL Trans2
602     .PP
603     Trans2 is the anisotropic version of trans.
604     The string arguments are the same as for plastic2, and the real
605     arguments are the same as for trans but with an additional roughness
606     value.
607     .DS
608     mod trans2 id
609     4+ ux uy uz funcfile transform
610     0
611     8 red green blue spec urough vrough trans tspec
612     .DE
613     .LP
614     .UL Dielectric
615     .PP
616     A dielectric material is transparent, and it refracts light
617     as well as reflecting it.
618     Its behavior is determined by the index of refraction and
619     transmission coefficient in each wavelength band per unit length.
620     Common glass has a index of refraction (n) around 1.5,
621     and a transmission coefficient of roughly 0.92 over an inch.
622     An additional number, the Hartmann constant, describes how
623     the index of refraction changes as a function of wavelength.
624     It is usually zero.
625     (A pattern modifies only the refracted value.)
626     .DS
627     mod dielectric id
628     0
629     0
630     5 rtn gtn btn n hc
631     .DE
632     .LP
633     .UL Interface
634     .PP
635     An interface is a boundary between two dielectrics.
636     The first transmission coefficient and refractive index are for the inside;
637     the second ones are for the outside.
638     Ordinary dielectrics are surrounded by a vacuum (1 1 1 1).
639     .DS
640     mod interface id
641     0
642     0
643     8 rtn1 gtn1 btn1 n1 rtn2 gtn2 btn2 n2
644     .DE
645     .LP
646     .UL Glass
647     .PP
648     Glass is similar to dielectric, but it is optimized for thin glass
649     surfaces (n = 1.52).
650     One transmitted ray and one reflected ray is produced.
651     By using a single surface is in place of two, internal reflections
652     are avoided.
653     The surface orientation is irrelevant, as it is for plastic,
654     metal, and trans.
655     The only specification required is the transmissivity at normal
656     incidence.
657     (Transmissivity is the amount of light not absorbed in one traversal
658     of the material.
659     Transmittance -- the value usually measured -- is the total light
660     transmitted through the pane including multiple reflections.)\0
661     To compute transmissivity (tn) from transmittance (Tn) use:
662     .DS
663     tn = (sqrt(.8402528435+.0072522239*Tn*Tn)-.9166530661)/.0036261119/Tn
664     .DE
665     Standard 88% transmittance glass has a transmissivity of 0.96.
666     (A pattern modifying glass will affect the transmissivity.)
667     If a fourth real argument is given, it is interpreted as the index of
668     refraction to use instead of 1.52.
669     .DS
670     mod glass id
671     0
672     0
673     3 rtn gtn btn
674     .DE
675     .LP
676     .UL Plasfunc
677     .PP
678     Plasfunc in used for the procedural definition of plastic-like
679     materials with arbitrary bidirectional reflectance distribution
680     functions (BRDF's).
681     The arguments to this material include the color and specularity,
682     as well as the function defining the specular distribution and the
683     auxiliary file where it may be found.
684     .DS
685     mod plasfunc id
686     2+ refl funcfile transform
687     0
688     4+ red green blue spec A5 ..
689     .DE
690     The function
691     .I refl
692     takes four arguments, the x, y and z
693     direction towards the incident light, and the solid angle
694     subtended by the source.
695     The solid angle is provided to facilitate averaging, and is usually
696     ignored.
697     The
698     .I refl
699     function should integrate to 1 over
700     the projected hemisphere to maintain energy balance.
701     At least four real arguments must be given, and these are made
702     available along with any additional values to the reflectance
703     function.
704     Currently, only the contribution from direct light sources is
705     considered in the specular calculation.
706     As in most material types, the surface normal is always
707     altered to face the incoming ray.
708     .LP
709     .UL Metfunc
710     .PP
711     Metfunc is identical to plasfunc and takes the same arguments, but
712     the specular component is multiplied also by the material color.
713     .LP
714     .UL Transfunc
715     .PP
716     Transfunc is similar to plasfunc but with an arbitrary bidirectional
717     transmittance distribution as well as a reflectance distribution.
718     Both reflectance and transmittance are specified with the same function.
719     .DS
720     mod transfunc id
721     2+ brtd funcfile transform
722     0
723     6+ red green blue rspec trans tspec A7 ..
724     .DE
725     Where
726     .I trans
727     is the total light transmitted and
728     .I tspec
729     is the non-Lambertian fraction of transmitted light.
730     The function
731     .I brtd
732     should integrate to 1 over each projected hemisphere.
733     .LP
734     .UL BRTDfunc
735     .PP
736     The material BRTDfunc gives the maximum flexibility over surface
737     reflectance and transmittance, providing for spectrally-dependent
738     specular rays and reflectance and transmittance distribution functions.
739     .DS
740     mod BRTDfunc id
741     10+ rrefl grefl brefl
742     rtrns gtrns btrns
743     rbrtd gbrtd bbrtd
744     funcfile transform
745     0
746     9+ rfdif gfdif bfdif
747     rbdif gbdif bbdif
748     rtdif gtdif btdif
749     A10 ..
750     .DE
751     The variables
752     .I "rrefl, grefl"
753     and
754     .I brefl
755     specify the color coefficients for
756     the ideal specular (mirror) reflection of the surface.
757     The variables
758     .I "rtrns, gtrns"
759     and
760     .I btrns
761     specify the color coefficients for the ideal specular transmission.
762     The functions
763     .I "rbrtd, gbrtd"
764     and
765     .I bbrtd
766     take the direction to the incident light (and its solid angle)
767     and compute the color coefficients for the directional diffuse part of
768     reflection and transmission.
769     As a special case, three identical values of '0' may be given in place of
770     these function names to indicate no directional diffuse component.
771     .PP
772     Unlike most other material types, the surface normal is not altered to
773     face the incoming ray.
774     Thus, functions and variables must pay attention to the orientation of
775     the surface and make adjustments appropriately.
776     However, the special variables for the perturbed dot product and surface
777     normal,
778     .I "RdotP, NxP, NyP"
779     and
780     .I NzP
781     are reoriented as if the ray hit the front surface for convenience.
782     .PP
783     A diffuse reflection component may be given for the front side with
784     .I "rfdif, gfdif"
785     and
786     .I bfdif
787     for the front side of the surface or
788     .I "rbdif, gbdif"
789     and
790     .I bbdif
791     for the back side.
792     The diffuse transmittance (must be the same for both sides by physical law)
793     is given by
794     .I "rtdif, gtdif"
795     and
796     .I btdif.
797     A pattern will modify these diffuse scattering values,
798     and will be available through the special variables
799     .I "CrP, CgP"
800     and
801     .I CbP.
802     .PP
803     Care must be taken when using this material type to produce a physically
804     valid reflection model.
805     The reflectance functions should be bidirectional, and under no circumstances
806     should the sum of reflected diffuse, transmitted diffuse, reflected specular,
807     transmitted specular and the integrated directional diffuse component be
808     greater than one.
809     .LP
810     .UL Plasdata
811     .PP
812     Plasdata is used for arbitrary BRDF's that are most conveniently
813     given as interpolated data.
814     The arguments to this material are the data file and coordinate index
815     functions, as well as a function to optionally modify the data
816     values.
817     .DS
818     mod plasdata id
819     3+n+
820     func datafile
821     funcfile x1 x2 .. xn transform
822     0
823     4+ red green blue spec A5 ..
824     .DE
825     The coordinate indices
826     .I "(x1, x2,"
827     etc.) are themselves functions of
828     the x, y and z direction to the incident light, plus the solid angle
829     subtended by the light source (usually ignored).
830     The data function
831     .I (func)
832     takes five variables, the
833     interpolated value from the n-dimensional data file, followed by the
834     x, y and z direction to the incident light and the solid angle of the source.
835     The light source direction and size may of course be ignored by the function.
836     .LP
837     .UL Metdata
838     .PP
839     As metfunc is to plasfunc, metdata is to plasdata.
840     Metdata takes the same arguments as plasdata, but the specular
841     component is modified by the given material color.
842     .LP
843     .UL Transdata
844     .PP
845     Transdata is like plasdata but the specification includes transmittance
846     as well as reflectance.
847     The parameters are as follows.
848     .DS
849     mod transdata id
850     3+n+
851     func datafile
852     funcfile x1 x2 .. xn transform
853     0
854     6+ red green blue rspec trans tspec A7 ..
855     .DE
856     .LP
857     .UL Antimatter
858     .PP
859     Antimatter is a material that can "subtract" volumes from other volumes.
860     A ray passing into an antimatter object becomes blind to all the specified
861     modifiers:
862     .DS
863     mod antimatter id
864     N mod1 mod2 .. modN
865     0
866     0
867     .DE
868     The first modifier will also be used to shade the area leaving the
869     antimatter volume and entering the regular volume.
870     If mod1 is void, the antimatter volume is completely invisible.
871     Antimatter does not work properly with the material type "trans",
872     and multiple antimatter surfaces should be disjoint.
873     The viewpoint must be outside all volumes concerned for a correct
874     rendering.
875     .NH 3
876     Textures
877     .PP
878     A texture is a perturbation of the surface normal, and
879     is given by either a function or data.
880     .LP
881     .UL Texfunc
882     .PP
883     A texfunc uses an auxiliary function file
884     to specify a procedural texture:
885     .DS
886     mod texfunc id
887     4+ xpert ypert zpert funcfile transform
888     0
889     n A1 A2 .. An
890     .DE
891     .LP
892     .UL Texdata
893     .PP
894     A texdata texture uses three data files to get the surface
895     normal perturbations.
896     The variables
897     .I xfunc,
898     .I yfunc
899     and
900     .I zfunc
901     take three arguments
902     each from the interpolated values in
903     .I xdfname,
904     .I ydfname
905     and
906     .I zdfname.
907     .DS
908     mod texdata id
909     8+ xfunc yfunc zfunc xdfname ydfname zdfname vfname x0 x1 .. xf
910     0
911     n A1 A2 .. An
912     .DE
913     .NH 3
914     Patterns
915     .PP
916     Patterns are used to modify the reflectance of materials.
917     The basic types are given below.
918     .LP
919     .UL Colorfunc
920     .PP
921     A colorfunc is a procedurally defined color pattern.
922     It is specified as follows:
923     .DS
924     mod colorfunc id
925     4+ red green blue funcfile transform
926     0
927     n A1 A2 .. An
928     .DE
929     .LP
930     .UL Brightfunc
931     .PP
932     A brightfunc is the same as a colorfunc, except it is monochromatic.
933     .DS
934     mod brightfunc id
935     2+ refl funcfile transform
936     0
937     n A1 A2 .. An
938     .DE
939     .LP
940     .UL Colordata
941     .PP
942     Colordata uses an interpolated data map to modify a material's color.
943     The map is n-dimensional, and is stored in three
944     auxiliary files, one for each color.
945     The coordinates used to look up and interpolate the data are
946     defined in another auxiliary file.
947     The interpolated data values are modified by functions of
948     one or three variables.
949     If the functions are of one variable, then they are passed the
950     corresponding color component (red or green or blue).
951     If the functions are of three variables, then they are passed the
952     original red, green, and blue values as parameters.
953     .DS
954     mod colordata id
955     7+n+
956     rfunc gfunc bfunc rdatafile gdatafile bdatafile
957     funcfile x1 x2 .. xn transform
958     0
959     m A1 A2 .. Am
960     .DE
961     .LP
962     .UL Brightdata
963     .PP
964     Brightdata is like colordata, except monochromatic.
965     .DS
966     mod brightdata id
967     3+n+
968     func datafile
969     funcfile x1 x2 .. xn transform
970     0
971     m A1 A2 .. Am
972     .DE
973     .LP
974     .UL Colorpict
975     .PP
976     Colorpict is a special case of colordata, where the pattern is
977     a two-dimensional image stored in the RADIANCE picture format.
978     The dimensions of the image data are determined by the picture
979     such that the smaller dimension is always 1, and the other
980     is the ratio between the larger and the smaller.
981     For example, a 500x338 picture would have coordinates (u,v)
982     in the rectangle between (0,0) and (1.48,1).
983     .DS
984     mod colorpict id
985     7+
986     rfunc gfunc bfunc pictfile
987     funcfile u v transform
988     0
989     m A1 A2 .. Am
990     .DE
991     .LP
992     .UL Colortext
993     .PP
994     Colortext is dichromatic writing in a polygonal font.
995     The font is defined in an auxiliary file, such as
996     .I helvet.fnt.
997     The text itself is also specified in a separate file, or
998     can be part of the material arguments.
999     The character size, orientation, aspect ratio and slant is
1000     determined by right and down motion vectors.
1001     The upper left origin for the text block as well as
1002     the foreground and background colors
1003     must also be given.
1004     .DS
1005     mod colortext id
1006     2 fontfile textfile
1007     0
1008     15+
1009     Ox Oy Oz
1010     Rx Ry Rz
1011     Dx Dy Dz
1012     rfore gfore bfore
1013     rback gback bback
1014     [spacing]
1015     .DE
1016     or:
1017     .DS
1018     mod colortext id
1019     2+N fontfile . This is a line with N words ...
1020     0
1021     15+
1022     Ox Oy Oz
1023     Rx Ry Rz
1024     Dx Dy Dz
1025     rfore gfore bfore
1026     rback gback bback
1027     [spacing]
1028     .DE
1029     .LP
1030     .UL Brighttext
1031     .PP
1032     Brighttext is like colortext, but the writing is monochromatic.
1033     .DS
1034     mod brighttext id
1035     2 fontfile textfile
1036     0
1037     11+
1038     Ox Oy Oz
1039     Rx Ry Rz
1040     Dx Dy Dz
1041     foreground background
1042     [spacing]
1043     .DE
1044     or:
1045     .DS
1046     mod brighttext id
1047     2+N fontfile . This is a line with N words ...
1048     0
1049     11+
1050     Ox Oy Oz
1051     Rx Ry Rz
1052     Dx Dy Dz
1053     foreground background
1054     [spacing]
1055     .DE
1056     .LP
1057     By default, a uniform spacing algorithm is used that guarantees
1058     every character will appear in a precisely determined position.
1059     Unfortunately, such a scheme results in rather unattractive and difficult to
1060     read text with most fonts.
1061     The optional
1062     .I spacing
1063     value defines the distance between characters for proportional spacing.
1064     A positive value selects a spacing algorithm that preserves right margins and
1065     indentation, but does not provide the ultimate in proportionally spaced text.
1066     A negative value insures that characters are properly spaced, but the
1067     placement of words then varies unpredictably.
1068     The choice depends on the relative importance of spacing versus formatting.
1069     When presenting a section of formatted text, a positive spacing value is
1070     usually preferred.
1071     A single line of text will often be accompanied by a negative spacing value.
1072     A section of text meant to depict a picture, perhaps using a special purpose
1073     font such as hexbit4x1.fnt, calls for uniform spacing.
1074     Reasonable magnitudes for proportional spacing are
1075     between 0.1 (for tightly spaced characters) and 0.3 (for wide spacing).
1076     .NH 3
1077     Mixtures
1078     .PP
1079     A mixture is a blend of one or more materials or textures and patterns.
1080     The basic types are given below.
1081     .LP
1082     .UL Mixfunc
1083     .PP
1084     A mixfunc mixes two modifiers procedurally.
1085     It is specified as follows:
1086     .DS
1087     mod mixfunc id
1088     4+ foreground background vname funcfile transform
1089     0
1090     n A1 A2 .. An
1091     .DE
1092     Foreground and background are modifier names that must be
1093     defined earlier in the scene description.
1094     If one of these is a material, then
1095     the modifier of the mixfunc must be "void".
1096     (Either the foreground or background modifier may be "void",
1097     which serves as a form of opacity control when used with a material.)\0
1098     Vname is the coefficient defined in funcfile that determines the influence
1099     of foreground.
1100     The background coefficient is always (1-vname).
1101     Since the references are not resolved until runtime, the last
1102     definitions of the modifier id's will be used.
1103     This can result in modifier loops, which are detected by the
1104     renderer.
1105     .LP
1106     .UL Mixdata
1107     .PP
1108     Mixdata combines two modifiers using an auxiliary data file:
1109     .DS
1110     mod mixdata id
1111     5+n+
1112     foreground background func datafile
1113     funcfile x1 x2 .. xn transform
1114     0
1115     m A1 A2 .. Am
1116     .DE
1117     .LP
1118     .UL Mixpict
1119     .PP
1120     Mixpict combines two modifiers based on a picture:
1121     .DS
1122     mod mixpict id
1123     7+
1124     foreground background func pictfile
1125     funcfile u v transform
1126     0
1127     m A1 A2 .. Am
1128     .DE
1129     The mixing coefficient function "func" takes three
1130     arguments, the red, green and blue values
1131     corresponding to the pixel at (u,v).
1132     .LP
1133     .UL Mixtext
1134     .PP
1135     Mixtext uses one modifier for the text foreground, and one for the
1136     background:
1137     .DS
1138     mod mixtext id
1139     4 foreground background fontfile textfile
1140     0
1141     9+
1142     Ox Oy Oz
1143     Rx Ry Rz
1144     Dx Dy Dz
1145     [spacing]
1146     .DE
1147     or:
1148     .DS
1149     mod mixtext id
1150     4+N
1151     foreground background fontfile .
1152     This is a line with N words ...
1153     0
1154     9+
1155     Ox Oy Oz
1156     Rx Ry Rz
1157     Dx Dy Dz
1158     [spacing]
1159     .DE
1160     .NH 2
1161     Auxiliary Files
1162     .PP
1163     Auxiliary files used in textures and patterns
1164     are accessed by the programs during image generation.
1165     These files may be located in the working directory, or in
1166     a library directory.
1167     The environment variable
1168     .I RAYPATH
1169     can be assigned an alternate set of search directories.
1170     Following is a brief description of some common file types.
1171     .NH 3
1172     Function Files
1173     .PP
1174     A function file contains the definitions of variables, functions
1175     and constants used by a primitive.
1176     The transformation that accompanies the file name contains the necessary
1177     rotations, translations and scalings to bring the coordinates of
1178     the function file into agreement with the world coordinates.
1179     The transformation specification is the same as for the
1180     .I xform
1181     command.
1182     An example function file is given below:
1183     .DS
1184     {
1185     This is a comment, enclosed in curly braces.
1186     {Comments can be nested.}
1187     }
1188     { standard expressions use +,-,*,/,^,(,) }
1189     vname = Ny * func(A1) ;
1190     { constants are defined with a colon }
1191     const : sqrt(PI/2) ;
1192     { user-defined functions add to library }
1193     func(x) = 5 + A1*sin(x/3) ;
1194     { functions may be passed and recursive }
1195     rfunc(f,x) = if(x,f(x),f(-x)*rfunc(f,x+1)) ;
1196     { constant functions may also be defined }
1197     cfunc(x) : 10*x / sqrt(x) ;
1198     .DE
1199     Many variables and functions are already defined by the program,
1200     and they are listed in the file
1201     .I rayinit.cal.
1202     The following variables are particularly important:
1203     .DS
1204     Dx, Dy, Dz - incident ray direction
1205     Px, Py, Pz - intersection point
1206     Nx, Ny, Nz - surface normal at intersection point
1207     Rdot - cosine between ray and normal
1208     arg(0) - number of real arguments
1209     arg(i) - i'th real argument
1210     .DE
1211     For BRDF types, the following variables are defined as well:
1212     .DS
1213     NxP, NyP, NzP - perturbed surface normal
1214     RdotP - perturbed dot product
1215     CrP, CgP, CbP - perturbed material color
1216     .DE
1217     A unique context is set up for each file so that the same variable
1218     may appear in different function files without conflict.
1219     The variables listed above and any others defined in
1220     rayinit.cal are available globally.
1221     If no file is needed by a given primitive because all the required
1222     variables are global, a period (`.') can be given in
1223     place of the file name.
1224     It is also possible to give an expression instead of a straight
1225     variable name in a scene file, although such expressions should
1226     be kept simple as they cannot contain any white space.
1227     Also, functions (requiring parameters)
1228     must be given as names and not as expressions.
1229     .PP
1230     Constant expressions are used as an optimization in function
1231     files.
1232     They are replaced wherever they occur in an expression by their
1233     value.
1234     Constant expressions are evaluated only once, so they must not
1235     contain any variables or values that can change, such as the ray
1236     variables Px and Ny or the primitive argument function arg().
1237     All the math library functions such as sqrt() and cos() have the
1238     constant attribute, so they will be replaced by immediate values
1239     whenever they are given constant arguments.
1240     Thus, the subexpression cos(PI*sqrt(2)) is immediately replaced
1241     by its value, -.266255342, and does not cause any additional overhead
1242     in the calculation.
1243     .PP
1244     It is generally a good idea to define constants and variables before
1245     they are referred to in a function file.
1246     Although evaluation does not take place until later, the interpreter
1247     does variable scoping and constant subexpression evaluation based on
1248     what it has compiled already.
1249     For example, a variable that is defined globally in rayinit.cal then
1250     referenced in the local context of a function file cannot
1251     subsequently be redefined in the same file because the compiler
1252     has already determined the scope of the referenced variable as global.
1253     To avoid such conflicts, one can state the scope of a variable explicitly
1254     by preceding the variable name with a context mark (a back-quote) for
1255     a local variable, or following the name with a context mark for a global
1256     variable.
1257     .NH 3
1258     Data Files
1259     .PP
1260     Data files contain n-dimensional arrays of real numbers used
1261     for interpolation.
1262     Typically, definitions in a function file determine how
1263     to index and use interpolated data values.
1264     The basic data file format is as follows:
1265     .DS
1266     N
1267     beg1 end1 m1
1268     0 0 m2 x2.1 x2.2 x2.3 x2.4 .. x2.m2
1269     ...
1270     begN endN mN
1271     DATA, later dimensions changing faster.
1272     .DE
1273     N is the number of dimensions.
1274     For each dimension, the beginning and ending coordinate
1275     values and the dimension size is given.
1276     Alternatively, individual coordinate values can be given when
1277     the points are not evenly spaced.
1278     These values must either be increasing or decreasing monotonically.
1279     The data is m1*m2*...*mN real numbers in ASCII form.
1280     Comments may appear anywhere in the file, beginning with a pound
1281     sign ('#') and continuing to the end of line.
1282     .NH 3
1283     Font Files
1284     .PP
1285     A font file lists the polygons which make up a character set.
1286     Comments may appear anywhere in the file, beginning with a pound
1287     sign ('#') and continuing to the end of line.
1288     All numbers are decimal integers:
1289     .DS
1290     code n
1291     x0 y0
1292     x1 y1
1293     ...
1294     xn yn
1295     ...
1296     .DE
1297     The ASCII codes can appear in any order.
1298     N is the number of vertices, and the last is automatically
1299     connected to the first.
1300     Separate polygonal sections are joined by coincident sides.
1301     The character coordinate system is a square with lower left corner at
1302     (0,0), lower right at (255,0) and upper right at (255,255).
1303     .NH 2
1304     Generators
1305     .PP
1306     A generator is any program that produces a scene description
1307     as its output.
1308     They usually appear as commands in a scene description file.
1309     An example of a simple generator is
1310     .I genbox.
1311     .I Genbox
1312     takes the arguments of width, height and depth to produce
1313     a parallelepiped description.
1314     .I Genprism
1315     takes a list of 2-dimensional coordinates and extrudes them along a vector to
1316     produce a 3-dimensional prism.
1317     .I Genrev
1318     is a more sophisticated generator
1319     that produces an object of rotation from parametric functions
1320     for radius and axis position.
1321     .I Gensurf
1322     tessellates a surface defined by the
1323     parametric functions x(s,t), y(s,t), and z(s,t).
1324     .I Genworm
1325     links cylinders and spheres along a curve.
1326     .I Gensky
1327     produces a sun and sky distribution corresponding
1328     to a given time and date.
1329     .PP
1330     .I Xform
1331     is a program that transforms a scene description from one
1332     coordinate space to another.
1333     .I Xform
1334     does rotation, translation, scaling, and mirroring.
1335     .NH 1
1336     Image Generation
1337     .PP
1338     Once the scene has been described in three-dimensions, it
1339     is possible to generate a two-dimensional image from a
1340     given perspective.
1341     .PP
1342     The image generating programs use an
1343     .I octree
1344     to efficiently trace rays through the scene.
1345     An octree subdivides space into nested octants which
1346     contain sets of surfaces.
1347     In RADIANCE, an octree is created from a scene description by
1348     .I oconv.
1349     The details of this process are not important,
1350     but the octree will serve as input to the ray-tracing
1351     programs and directs the use of a scene description.
1352     .PP
1353     .I Rview
1354     is ray-tracing program for viewing a scene interactively.
1355     When the user specifies a new perspective,
1356     .I rview
1357     quickly displays a rough
1358     image on the terminal, then progressively
1359     increases the resolution as the user looks on.
1360     He can select a particular section of the image to improve,
1361     or move to a different view and start over.
1362     This mode of interaction is useful for debugging scenes
1363     as well as determining the best view for a final image.
1364     .PP
1365     .I Rpict
1366     produces a high-resolution picture of a scene from
1367     a particular perspective.
1368     This program features adaptive sampling, crash
1369     recovery and progress reporting, all of which are important
1370     for time-consuming images.
1371     .PP
1372     A number of filters are available for manipulating picture files.
1373     .I Pfilt
1374     sets the exposure and performs anti-aliasing.
1375     .I Pcompos
1376     composites (cuts and pastes) pictures.
1377     .I Pcond
1378     conditions a picture for a specific display device.
1379     .I Pcomb
1380     performs arbitrary math on one or more pictures.
1381     .I Protate
1382     rotates a picture 90 degrees clockwise.
1383     .I Pflip
1384     flips a picture horizontally, vertically, or both (180 degree rotation).
1385     .I Pvalue
1386     converts a picture to and from simpler formats.
1387     .PP
1388     Pictures may be displayed directly under X11 using the program
1389     .I ximage,
1390     or converted a standard image format.
1391     .I Ra_avs
1392     converts to and from AVS image format.
1393     .I Ra_pict
1394     converts to Macintosh 32-bit PICT2 format.
1395     .I Ra_ppm
1396     converts to and from Poskanzer Portable Pixmap formats.
1397     .I Ra_pr
1398     converts to and from Sun 8-bit rasterfile format.
1399     .I Ra_pr24
1400     converts to and from Sun 24-bit rasterfile format.
1401     .I Ra_ps
1402     converts to PostScript color and greyscale formats.
1403     .I Ra_rgbe
1404     converts to and from Radiance uncompressed picture format.
1405     .I Ra_t16
1406     converts to and from Targa 16 and 24-bit image formats.
1407     .I Ra_t8
1408     converts to and from Targa 8-bit image format.
1409     .I Ra_tiff
1410     converts to and from TIFF.
1411     .I Ra_xyze
1412     converts to and from Radiance CIE picture format.
1413     .NH 1
1414     License
1415     .PP
1416     Radiance is a registered copyright of The Regents of the University of
1417     California ("The Regents"). The Regents grant to you a nonexclusive,
1418     nontransferable license ("License") to use Radiance source code without fee.
1419     You may not sell or distribute Radiance to others without the prior express
1420     written permission of The Regents.
1421     You may compile and use this software on any machines to which you have
1422     personal access, and may share its use with others who have access to the
1423     same machines.
1424     .PP
1425     NEITHER THE UNITED STATES NOR THE UNITED STATES DEPARTMENT OF ENERGY, NOR ANY
1426     OF THEIR EMPLOYEES, MAKES ANY WARRANTY, EXPRESS OR IMPLIED, OR ASSUMES ANY
1427     LEGAL LIABILITY OR RESPONSIBILITY FOR THE ACCURACY, COMPLETENESS, OR
1428     USEFULNESS OF ANY INFORMATION, APPARATUS, PRODUCT, OR PROCESS DISCLOSED, OR
1429     REPRESENTS THAT ITS USE WOULD NOT INFRINGE PRIVATELY OWNED RIGHTS.
1430     By downloading, using or copying this software, you agree to abide by the
1431     intellectual property laws and all other applicable laws of the United
1432     States, and by the terms of this License Agreement. Ownership of the software
1433     shall remain solely in The Regents.
1434     The Regents shall have the right to terminate this License immediately by
1435     written notice upon your breach of, or noncompliance with, any of its terms.
1436     You shall be liable for any infringement or damages resulting from your
1437     failure to abide by the terms of this License Agreement.
1438     .PP
1439     NOTICE: The Government is granted for itself and others acting on its behalf
1440     a paid-up, nonexclusive irrevocable worldwide license in this data to
1441     reproduce, prepare derivative works, and perform publicly and display
1442     publicly. Beginning five (5) years after permission to assert copyright is
1443     granted, subject to two possible five year renewals, the Government is
1444     granted for itself and others acting on its behalf a paid-up, non-exclusive,
1445     irrevocable worldwide license in this data to reproduce, prepare derivative
1446     works, distribute copies to the public, perform publicly and display
1447     publicly, and to permit others to do so.
1448     .NH 1
1449     Acknowledgements
1450     .PP
1451     This work was supported by the Assistant Secretary of Conservation
1452     and Renewable Energy, Office of Building Energy Research and
1453     Development, Buildings Equipment Division of the U.S. Department of
1454     Energy under Contract No. DE-AC03-76SF00098.
1455     .PP
1456     Additional work was sponsored by the Swiss federal government
1457     under the Swiss LUMEN Project and was
1458     carried out in the Laboratoire d'Energie Solaire (LESO Group) at
1459     the Ecole Polytechnique Federale de Lausanne (EPFL University)
1460     in Lausanne, Switzerland.
1461     .NH 1
1462     References
1463     .LP
1464     Ward, G.,
1465     ``The Radiance Lighting Simulation and Rendering System,''
1466     .I "Computer Graphics",
1467     Orlando, July 1994.
1468     .LP
1469     Rushmeier, H., G. Ward,
1470     ``Energy-Preserving Non-Linear Filters,''
1471     .I "Computer Graphics",
1472     Orlando, July 1994.
1473     .LP
1474     Ward, G.,
1475     ``A Contrast-Based Scalefactor for Luminance Display,''
1476     .I "Graphics Gems IV",
1477     Edited by Paul Heckbert,
1478     Academic Press 1994.
1479     .LP
1480     Ward, G.,
1481     ``Measuring and Modeling Anisotropic Reflection,''
1482     .I "Computer Graphics",
1483     Chicago, July 1992.
1484     .LP
1485     Ward, G., P. Heckbert,
1486     ``Irradiance Gradients,''
1487     .I "Third Annual Eurographics Workshop on Rendering",
1488     to be published by Springer-Verlag, held in Bristol, UK, May 1992.
1489     .LP
1490     Ward, G.,
1491     ``Adaptive Shadow Testing for Ray Tracing,''
1492     .I "Second Annual Eurographics Workshop on Rendering",
1493     to be published by Springer-Verlag, held in Barcelona, SPAIN, May 1991.
1494     .LP
1495     Ward, G.,
1496     ``Visualization,''
1497     .I "Lighting Design and Application",
1498     Vol. 20, No. 6, June 1990.
1499     .LP
1500     Ward, G., F. Rubinstein, R. Clear,
1501     ``A Ray Tracing Solution for Diffuse Interreflection,''
1502     .I "Computer Graphics",
1503     Vol. 22, No. 4, August 1988.
1504     .LP
1505     Ward, G., F. Rubinstein,
1506     ``A New Technique for Computer Simulation of Illuminated Spaces,''
1507     .I "Journal of the Illuminating Engineering Society",
1508     Vol. 17, No. 1, Winter 1988.