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root/radiance/ray/doc/ray.1
Revision: 1.2
Committed: Tue Mar 11 19:29:04 2003 UTC (21 years ago) by greg
Branch: MAIN
Changes since 1.1: +2 -0 lines
Log Message:
Changed alias handling to allow tracking, fixed freeobjects() and do_irrad bugs

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