# The RADIANCE 4.1 Synthetic Imaging System

Building Technologies Program
Lawrence Berkeley National Laboratory
1 Cyclotron Rd., 90-3111
Berkeley, CA 94720
http://radsite.lbl.gov/radiance

## Overview

1. Introduction
2. Scene Description
   1. Primitive Types
      1. Surfaces
      2. Materials
      3. Textures
      4. Patterns
      5. Mixtures
   2. Auxiliary Files
      1. Function Files
      2. Data Files
      3. Font Files
   3. Generators
3. Image Generation
4. License
5. Acknowledgements
6. References
7. Types Index

## 1. Introduction

RADIANCE was developed as a research tool for predicting the distribution of visible radiation in illuminated spaces. It takes as input a three-dimensional geometric model of the physical environment, and produces a map of spectral radiance values in a color image. The technique of ray-tracing follows light backwards from the image plane to the source(s). Because it can produce realistic images from a simple description, RADIANCE has a wide range of applications in graphic arts, lighting design, computer-aided engineering and architecture.

Figure 1

The diagram in Figure 1 shows the flow between programs (boxes) and data (ovals). The central program is rpict, which produces a picture from a scene description. Rview is a variation of rpict that computes and displays images interactively, and rtrace computes single ray values. Other programs (not shown) connect many of these elements together, such as the executive programs rad and ranimate, the interactive rendering program rholo, and the animation program ranimove. The program obj2mesh acts as both a converter and scene compiler, converting a Wavefront .OBJ file into a compiled mesh octree for efficient rendering.

A scene description file lists the surfaces and materials that make up a specific environment. The current surface types are spheres, polygons, cones, and cylinders. There is also a composite surface type, called mesh, and a pseudosurface type, called instance, which facilitates very complex geometries. Surfaces can be made from materials such as plastic, metal, and glass. Light sources can be distant disks as well as local spheres, disks and polygons.

From a three-dimensional scene description and a specified view, rpict produces a two-dimensional image. A picture file is a compressed binary representation of the pixels in the image. This picture can be scaled in size and brightness, anti-aliased, and sent to a graphics output device.

A header in each picture file lists the program(s) and parameters that produced it. This is useful for identifying a picture without having to display it. The information can be read by the program getinfo.

## 2. Scene Description

A scene description file represents a three-dimensional physical environment in Cartesian (rectilinear) world coordinates. It is stored as ASCII text, with the following basic format:

            # comment

            modifier type identifier
            n S1 S2 "S 3" .. Sn
            0
            m R1 R2 R3 .. Rm

            modifier alias identifier reference

            ! command

             ...

A comment line begins with a pound sign, `#'.

The scene description primitives all have the same general format, and can be either surfaces or modifiers. A primitive has a modifier, a type, and an identifier.

A modifier is either the identifier of a previously defined primitive, or "void".
[ The most recent definition of a modifier is the one used, and later definitions do not cause relinking of loaded primitives. Thus, the same identifier may be used repeatedly, and each new definition will apply to the primitives following it. ]

An identifier can be any string (i.e., any sequence of non-white characters).

The arguments associated with a primitive can be strings or real numbers.

- The first integer following the identifier is the number of string arguments, and it is followed by the arguments themselves (separated by white space or enclosed in quotes).
- The next integer is the number of integer arguments, and is followed by the integer arguments. (There are currently no primitives that use them, however.)
- The next integer is the real argument count, and it is followed by the real arguments.

An alias gets its type and arguments from a previously defined primitive. This is useful when the same material is used with a different modifier, or as a convenient naming mechanism. The reserved modifier name "inherit" may be used to specificy that an alias will inherit its modifier from the original. Surfaces cannot be aliased.

A line beginning with an exclamation point, `!', is interpreted as a command. It is executed by the shell, and its output is read as input to the program. The command must not try to read from its standard input, or confusion will result. A command may be continued over multiple lines using a backslash, `\', to escape the newline.

White space is generally ignored, except as a separator. The exception is the newline character after a command or comment. Commands, comments and primitives may appear in any combination, so long as they are not intermingled.

### 2.1. Primitive Types

Primitives can be surfaces, materials, textures or patterns. Modifiers can be materials, mixtures, textures or patterns. Simple surfaces must have one material in their modifier list.

#### 2.1.1. Surfaces

A scene description will consist mostly of surfaces. The basic types are given below.

Source
  A source is not really a surface, but a solid angle. It is used for specifying light sources that are very distant. The direction to the center of the source and the number of degrees subtended by its disk are given as follows:

      	mod source id
      	0
      	0
      	4 xdir ydir zdir angle

Sphere
  A sphere is given by its center and radius:

              mod sphere id
              0
              0
              4 xcent ycent zcent radius

Bubble
  A bubble is simply a sphere whose surface normal points inward.

Polygon
  A polygon is given by a list of three-dimensional vertices, which are ordered counter-clockwise as viewed from the front side (into the surface normal). The last vertex is automatically connected to the first. Holes are represented in polygons as interior vertices connected to the outer perimeter by coincident edges (seams).

              mod polygon id
              0
              0
              3n
                      x1      y1      z1
                      x2      y2      z2
                      ...
                      xn      yn      zn

Cone
  A cone is a megaphone-shaped object. It is truncated by two planes perpendicular to its axis, and one of its ends may come to a point. It is given as two axis endpoints, and the starting and ending radii:

              mod cone id
              0
              0
              8
                      x0      y0      z0
                      x1      y1      z1
                      r0      r1

Cup
  A cup is an inverted cone (i.e., has an inward surface normal).

Cylinder
  A cylinder is like a cone, but its starting and ending radii are equal.

              mod cylinder id
              0
              0
              7
                      x0      y0      z0
                      x1      y1      z1
                      rad

Tube
  A tube is an inverted cylinder.

Ring
  A ring is a circular disk given by its center, surface normal, and inner and outer radii:

              mod ring id
              0
              0
              8
                      xcent   ycent   zcent
                      xdir    ydir    zdir
                      r0      r1

Instance
  An instance is a compound surface, given by the contents of an octree file (created by oconv).

              mod instance id
              1+ octree transform
              0
              0

  If the modifier is "void", then surfaces will use the modifiers given in the original description. Otherwise, the modifier specified is used in their place. The transform moves the octree to the desired location in the scene. Multiple instances using the same octree take little extra memory, hence very complex descriptions can be rendered using this primitive.

  There are a number of important limitations to be aware of when using instances. First, the scene description used to generate the octree must stand on its own, without referring to modifiers in the parent description. This is necessary for oconv to create the octree. Second, light sources in the octree will not be incorporated correctly in the calculation, and they are not recommended. Finally, there is no advantage (other than convenience) to using a single instance of an octree, or an octree containing only a few surfaces. An xform command on the subordinate description is prefered in such cases.

Mesh
  A mesh is a compound surface, made up of many triangles and an octree data structure to accelerate ray intersection. It is typically converted from a Wavefront .OBJ file using the obj2mesh program.

              mod mesh id
              1+ meshfile transform
              0
              0

  If the modifier is "void", then surfaces will use the modifiers given in the original mesh description. Otherwise, the modifier specified is used in their place. The transform moves the mesh to the desired location in the scene. Multiple instances using the same meshfile take little extra memory, and the compiled mesh itself takes much less space than individual polygons would. In the case of an unsmoothed mesh, using the mesh primitive reduces memory requirements by a factor of 30 relative to individual triangles. If a mesh has smoothed surfaces, we save a factor of 50 or more, permitting very detailed geometries that would otherwise exhaust the available memory. In addition, the mesh primitive can have associated (u,v) coordinates for pattern and texture mapping. These are made available to function files via the Lu and Lv variables.

  #### 2.1.2. Materials

  A material defines the way light interacts with a surface. The basic types are given below.

  Light
    Light is the basic material for self-luminous surfaces (i.e., light sources). In addition to the source surface type, spheres, discs (rings with zero inner radius), cylinders (provided they are long enough), and polygons can act as light sources. Polygons work best when they are rectangular. Cones cannot be used at this time. A pattern may be used to specify a light output distribution. Light is defined simply as a RGB radiance value (watts/steradian/m2):

                mod light id
                0
                0
                3 red green blue

  Illum
    Illum is used for secondary light sources with broad distributions. A secondary light source is treated like any other light source, except when viewed directly. It then acts like it is made of a different material (indicated by the string argument), or becomes invisible (if no string argument is given, or the argument is "void"). Secondary sources are useful when modeling windows or brightly illuminated surfaces.

                mod illum id
                1 material
                0
                3 red green blue

  Glow
    Glow is used for surfaces that are self-luminous, but limited in their effect. In addition to the radiance value, a maximum radius for shadow testing is given:

                mod glow id
                0
                0
                4 red green blue maxrad

    If maxrad is zero, then the surface will never be tested for shadow, although it may participate in an interreflection calculation. If maxrad is negative, then the surface will never contribute to scene illumination. Glow sources will never illuminate objects on the other side of an illum surface. This provides a convenient way to illuminate local light fixture geometry without overlighting nearby objects.

  Spotlight
    Spotlight is used for self-luminous surfaces having directed output. As well as radiance, the full cone angle (in degrees) and orientation (output direction) vector are given. The length of the orientation vector is the distance of the effective focus behind the source center (i.e., the focal length).

                mod spotlight id
                0
                0
                7 red green blue angle xdir ydir zdir

  Mirror
    Mirror is used for planar surfaces that produce virtual source reflections. This material should be used sparingly, as it may cause the light source calculation to blow up if it is applied to many small surfaces. This material is only supported for flat surfaces such as polygons and rings. The arguments are simply the RGB reflectance values, which should be between 0 and 1. An optional string argument may be used like the illum type to specify a different material to be used for shading non-source rays. If this alternate material is given as "void", then the mirror surface will be invisible. This is only appropriate if the surface hides other (more detailed) geometry with the same overall reflectance.

                mod mirror id
                1 material
                0
                3 red green blue

  Prism1
    The prism1 material is for general light redirection from prismatic glazings, generating virtual light sources. It can only be used to modify a planar surface (i.e., a polygon or disk) and should not result in either light concentration or scattering. The new direction of the ray can be on either side of the material, and the definitions must have the correct bidirectional properties to work properly with virtual light sources. The arguments give the coefficient for the redirected light and its direction.

                mod prism1 id
                5+ coef dx dy dz funcfile transform
                0
                n A1 A2 .. An

    The new direction variables dx, dy and dz need not produce a normalized vector. For convenience, the variables DxA, DyA and DzA are defined as the normalized direction to the target light source. See section 2.2.1 on function files for further information.

  Prism2
    The material prism2 is identical to prism1 except that it provides for two ray redirections rather than one.

                mod prism2 id
                9+ coef1 dx1 dy1 dz1 coef2 dx2 dy2 dz2 funcfile transform
                0
                n A1 A2 .. An

  Mist
    Mist is a virtual material used to delineate a volume of participating atmosphere. A list of important light sources may be given, along with an extinction coefficient, scattering albedo and scattering eccentricity parameter. The light sources named by the string argument list will be tested for scattering within the volume. Sources are identified by name, and virtual light sources may be indicated by giving the relaying object followed by '>' followed by the source, i.e:

        	3  source1  mirror1>source10  mirror2>mirror1>source3

    Normally, only one source is given per mist material, and there is an upper limit of 32 to the total number of active scattering sources. The extinction coefficient, if given, is added the the global coefficient set on the command line. Extinction is in units of 1/distance (distance based on the world coordinates), and indicates the proportional loss of radiance over one unit distance. The scattering albedo, if present, will override the global setting within the volume. An albedo of 0 0 0 means a perfectly absorbing medium, and an albedo of 1 1 1 means a perfectly scattering medium (no absorption). The scattering eccentricity parameter will likewise override the global setting if it is present. Scattering eccentricity indicates how much scattered light favors the forward direction, as fit by the Henyey-Greenstein function:

        	P(theta) = (1 - g*g) / (1 + g*g - 2*g*cos(theta))^1.5

    A perfectly isotropic scattering medium has a g parameter of 0, and a highly directional material has a g parameter close to 1. Fits to the g parameter may be found along with typical extinction coefficients and scattering albedos for various atmospheres and cloud types in USGS meteorological tables. (A pattern will be applied to the extinction values.)

        	mod mist id
        	N src1 src2 .. srcN
        	0
        	0|3|6|7 [ rext gext bext [ ralb galb balb [ g ] ] ]

    There are two usual uses of the mist type. One is to surround a beam from a spotlight or laser so that it is visible during rendering. For this application, it is important to use a cone (or cylinder) that is long enough and wide enough to contain the important visible portion. Light source photometry and intervening objects will have the desired effect, and crossing beams will result in additive scattering. For this application, it is best to leave off the real arguments, and use the global rendering parameters to control the atmosphere. The second application is to model clouds or other localized media. Complex boundary geometry may be used to give shape to a uniform medium, so long as the boundary encloses a proper volume. Alternatively, a pattern may be used to set the line integral value through the cloud for a ray entering or exiting a point in a given direction. For this application, it is best if cloud volumes do not overlap each other, and opaque objects contained within them may not be illuminated correctly unless the line integrals consider enclosed geometry.
  Plastic
    Plastic is a material with uncolored highlights. It is given by its RGB reflectance, its fraction of specularity, and its roughness value. Roughness is specified as the rms slope of surface facets. A value of 0 corresponds to a perfectly smooth surface, and a value of 1 would be a very rough surface. Specularity fractions greater than 0.1 and roughness values greater than 0.2 are not very realistic. (A pattern modifying plastic will affect the material color.)

                mod plastic id
                0
                0
                5 red green blue spec rough

  Metal
    Metal is similar to plastic, but specular highlights are modified by the material color. Specularity of metals is usually .9 or greater. As for plastic, roughness values above .2 are uncommon.

  Trans
    Trans is a translucent material, similar to plastic. The transmissivity is the fraction of penetrating light that travels all the way through the material. The transmitted specular component is the fraction of transmitted light that is not diffusely scattered. Transmitted and diffusely reflected light is modified by the material color. Translucent objects are infinitely thin.

                mod trans id
                0
                0
                7 red green blue spec rough trans tspec

  Plastic2
    Plastic2 is similar to plastic, but with anisotropic roughness. This means that highlights in the surface will appear elliptical rather than round. The orientation of the anisotropy is determined by the unnormalized direction vector ux uy uz. These three expressions (separated by white space) are evaluated in the context of the function file funcfile. If no function file is required (i.e., no special variables or functions are required), a period (`.') may be given in its place. (See the discussion of Function Files in the Auxiliary Files section). The urough value defines the roughness along the u vector given projected onto the surface. The vrough value defines the roughness perpendicular to this vector. Note that the highlight will be narrower in the direction of the smaller roughness value. Roughness values of zero are not allowed for efficiency reasons since the behavior would be the same as regular plastic in that case.

                mod plastic2 id
                4+ ux uy uz funcfile transform
                0
                6 red green blue spec urough vrough

  Metal2
    Metal2 is the same as plastic2, except that the highlights are modified by the material color.

  Trans2
    Trans2 is the anisotropic version of trans. The string arguments are the same as for plastic2, and the real arguments are the same as for trans but with an additional roughness value.

                mod trans2 id
                4+ ux uy uz funcfile transform
                0
                8 red green blue spec urough vrough trans tspec

  Dielectric
    A dielectric material is transparent, and it refracts light as well as reflecting it. Its behavior is determined by the index of refraction and transmission coefficient in each wavelength band per unit length. Common glass has a index of refraction (n) around 1.5, and a transmission coefficient of roughly 0.92 over an inch. An additional number, the Hartmann constant, describes how the index of refraction changes as a function of wavelength. It is usually zero. (A pattern modifies only the refracted value.)

                mod dielectric id
                0
                0
                5 rtn gtn btn n hc

  Interface
    An interface is a boundary between two dielectrics. The first transmission coefficient and refractive index are for the inside; the second ones are for the outside. Ordinary dielectrics are surrounded by a vacuum (1 1 1 1).

                mod interface id
                0
                0
                8 rtn1 gtn1 btn1 n1 rtn2 gtn2 btn2 n2

  Glass
    Glass is similar to dielectric, but it is optimized for thin glass surfaces (n = 1.52). One transmitted ray and one reflected ray is produced. By using a single surface is in place of two, internal reflections are avoided. The surface orientation is irrelevant, as it is for plastic, metal, and trans. The only specification required is the transmissivity at normal incidence. (Transmissivity is the amount of light not absorbed in one traversal of the material. Transmittance -- the value usually measured -- is the total light transmitted through the pane including multiple reflections.) To compute transmissivity (tn) from transmittance (Tn) use:

                tn = (sqrt(.8402528435+.0072522239*Tn*Tn)-.9166530661)/.0036261119/Tn

    Standard 88% transmittance glass has a transmissivity of 0.96. (A pattern modifying glass will affect the transmissivity.) If a fourth real argument is given, it is interpreted as the index of refraction to use instead of 1.52.

                mod glass id
                0
                0
                3 rtn gtn btn

  Plasfunc
    Plasfunc in used for the procedural definition of plastic-like materials with arbitrary bidirectional reflectance distribution functions (BRDF's). The arguments to this material include the color and specularity, as well as the function defining the specular distribution and the auxiliary file where it may be found.

                mod plasfunc id
                2+ refl funcfile transform
                0
                4+ red green blue spec A5 ..

    The function refl takes four arguments, the x, y and z direction towards the incident light, and the solid angle subtended by the source. The solid angle is provided to facilitate averaging, and is usually ignored. The refl function should integrate to 1 over the projected hemisphere to maintain energy balance. At least four real arguments must be given, and these are made available along with any additional values to the reflectance function. Currently, only the contribution from direct light sources is considered in the specular calculation. As in most material types, the surface normal is always altered to face the incoming ray.

  Metfunc
    Metfunc is identical to plasfunc and takes the same arguments, but the specular component is multiplied also by the material color.

  Transfunc
    Transfunc is similar to plasfunc but with an arbitrary bidirectional transmittance distribution as well as a reflectance distribution. Both reflectance and transmittance are specified with the same function.

                mod transfunc id
                2+ brtd funcfile transform
                0
                6+ red green blue rspec trans tspec A7 ..

    Where trans is the total light transmitted and tspec is the non-Lambertian fraction of transmitted light. The function brtd should integrate to 1 over each projected hemisphere.

  BRTDfunc
    The material BRTDfunc gives the maximum flexibility over surface reflectance and transmittance, providing for spectrally-dependent specular rays and reflectance and transmittance distribution functions.

                mod BRTDfunc id
                10+  rrefl  grefl  brefl
                     rtrns  gtrns  btrns
                     rbrtd  gbrtd  bbrtd
                     funcfile  transform
                0
                9+   rfdif gfdif bfdif
                     rbdif gbdif bbdif
                     rtdif gtdif btdif
                     A10 ..

    The variables rrefl, grefl and brefl specify the color coefficients for the ideal specular (mirror) reflection of the surface. The variables rtrns, gtrns and btrns specify the color coefficients for the ideal specular transmission. The functions rbrtd, gbrtd and bbrtd take the direction to the incident light (and its solid angle) and compute the color coefficients for the directional diffuse part of reflection and transmission. As a special case, three identical values of '0' may be given in place of these function names to indicate no directional diffuse component.

    Unlike most other material types, the surface normal is not altered to face the incoming ray. Thus, functions and variables must pay attention to the orientation of the surface and make adjustments appropriately. However, the special variables for the perturbed dot product and surface normal, RdotP, NxP, NyP and NzP are reoriented as if the ray hit the front surface for convenience.

    A diffuse reflection component may be given for the front side with rfdif, gfdif and bfdif for the front side of the surface or rbdif, gbdif and bbdif for the back side. The diffuse transmittance (must be the same for both sides by physical law) is given by rtdif, gtdif and btdif. A pattern will modify these diffuse scattering values, and will be available through the special variables CrP, CgP and CbP.

    Care must be taken when using this material type to produce a physically valid reflection model. The reflectance functions should be bidirectional, and under no circumstances should the sum of reflected diffuse, transmitted diffuse, reflected specular, transmitted specular and the integrated directional diffuse component be greater than one.

  Plasdata
    Plasdata is used for arbitrary BRDF's that are most conveniently given as interpolated data. The arguments to this material are the data file and coordinate index functions, as well as a function to optionally modify the data values.

                mod plasdata id
                3+n+
                        func datafile
                        funcfile x1 x2 .. xn transform
                0
                4+ red green blue spec A5 ..

    The coordinate indices (x1, x2, etc.) are themselves functions of the x, y and z direction to the incident light, plus the solid angle subtended by the light source (usually ignored). The data function (func) takes five variables, the interpolated value from the n-dimensional data file, followed by the x, y and z direction to the incident light and the solid angle of the source. The light source direction and size may of course be ignored by the function.

  Metdata
    As metfunc is to plasfunc, metdata is to plasdata. Metdata takes the same arguments as plasdata, but the specular component is modified by the given material color.

  Transdata
    Transdata is like plasdata but the specification includes transmittance as well as reflectance. The parameters are as follows.

                mod transdata id
                3+n+
                        func datafile
                        funcfile x1 x2 .. xn transform
                0
                6+ red green blue rspec trans tspec A7 ..

  BSDF
    The BSDF material type loads an XML (eXtensible Markup Language) file describing a bidirectional scattering distribution function. Real arguments to this material may define additional diffuse components that augment the BSDF data. String arguments are used to define thickness for proxied surfaces and the "up" orientation for the material.

        	mod BSDF id
        	6+ thick BSDFfile ux uy uz funcfile transform
        	0
        	0|3|6|9
        		rfdif gfdif bfdif
        		rbdif gbdif bbdif
        		rtdif gtdif btdif

    The first string argument is a "thickness" parameter that may be used to hide detail geometry being proxied by an aggregate BSDF material. If a view or shadow ray hits a BSDF proxy with non-zero thickness, it will pass directly through as if the surface were not there. Similar to the illum type, this permits direct viewing and shadow testing of complex geometry. The BSDF is used when a scattered (indirect) ray hits the surface, and any transmitted sample rays will be offset by the thickness amount to avoid the hidden geometry and gather samples from the other side. In this manner, BSDF surfaces can improve the results for indirect scattering from complex systems without sacrificing appearance or shadow accuracy. If the BSDF has transmission and back-side reflection data, a parallel BSDF surface may be placed slightly less than the given thickness away from the front surface to enclose the complex geometry on both sides. The sign of the thickness is important, as it indicates whether the proxied geometry is behind the BSDF surface (when thickness is positive) or in front (when thickness is negative).

    The second string argument is the name of the BSDF file, which is found in the usual auxiliary locations. The following three string parameters name variables for an "up" vector, which together with the surface normal, define the local coordinate system that orients the BSDF. These variables, along with the thickness, are defined in a function file given as the next string argument. An optional transform is used to scale the thickness and reorient the up vector.

    If no real arguments are given, the BSDF is used by itself to determine reflection and transmission. If there are at least 3 real arguments, the first triplet is an additional diffuse reflectance for the front side. At least 6 real arguments adds diffuse reflectance to the rear side of the surface. If there are 9 real arguments, the final triplet will be taken as an additional diffuse transmittance. All diffuse components as well as the non-diffuse transmission are modified by patterns applied to this material. The non-diffuse reflection from either side are unaffected. Textures perturb the effective surface normal in the usual way.

    The surface normal of this type is not altered to face the incoming ray, so the front and back BSDF reflections may differ. (Transmission is identical front-to-back by physical law.) If back visibility is turned off during rendering and there is no transmission or back-side reflection, only then the surface will be invisible from behind. Unlike other data-driven material types, the BSDF type is fully supported and all parts of the distribution are properly sampled.

  Antimatter
    Antimatter is a material that can "subtract" volumes from other volumes. A ray passing into an antimatter object becomes blind to all the specified modifiers:

                mod antimatter id
                N mod1 mod2 .. modN
                0
                0

    The first modifier will also be used to shade the area leaving the antimatter volume and entering the regular volume. If mod1 is void, the antimatter volume is completely invisible. Antimatter does not work properly with the material type "trans", and multiple antimatter surfaces should be disjoint. The viewpoint must be outside all volumes concerned for a correct rendering.

  #### 2.1.3. Textures

  A texture is a perturbation of the surface normal, and is given by either a function or data.

  Texfunc
    A texfunc uses an auxiliary function file to specify a procedural texture:

                mod texfunc id
                4+ xpert ypert zpert funcfile transform
                0
                n A1 A2 .. An

  Texdata
    A texdata texture uses three data files to get the surface normal perturbations. The variables xfunc, yfunc and zfunc take three arguments each from the interpolated values in xdfname, ydfname and zdfname.

                mod texdata id
                8+ xfunc yfunc zfunc xdfname ydfname zdfname vfname x0 x1 .. xf
                0
                n A1 A2 .. An

  #### 2.1.4. Patterns

  Patterns are used to modify the reflectance of materials. The basic types are given below.

  Colorfunc
    A colorfunc is a procedurally defined color pattern. It is specified as follows:

                mod colorfunc id
                4+ red green blue funcfile transform
                0
                n A1 A2 .. An

  Brightfunc
    A brightfunc is the same as a colorfunc, except it is monochromatic.

                mod brightfunc id
                2+ refl funcfile transform
                0
                n A1 A2 .. An

  Colordata
    Colordata uses an interpolated data map to modify a material's color. The map is n-dimensional, and is stored in three auxiliary files, one for each color. The coordinates used to look up and interpolate the data are defined in another auxiliary file. The interpolated data values are modified by functions of one or three variables. If the functions are of one variable, then they are passed the corresponding color component (red or green or blue). If the functions are of three variables, then they are passed the original red, green, and blue values as parameters.

                mod colordata id
                7+n+
                        rfunc gfunc bfunc rdatafile gdatafile bdatafile
                        funcfile x1 x2 .. xn transform
                0
                m A1 A2 .. Am

  Brightdata
    Brightdata is like colordata, except monochromatic.

                mod brightdata id
                3+n+
                        func datafile
                        funcfile x1 x2 .. xn transform
                0
                m A1 A2 .. Am

  Colorpict
    Colorpict is a special case of colordata, where the pattern is a two-dimensional image stored in the RADIANCE picture format. The dimensions of the image data are determined by the picture such that the smaller dimension is always 1, and the other is the ratio between the larger and the smaller. For example, a 500x338 picture would have coordinates (u,v) in the rectangle between (0,0) and (1.48,1).

                mod colorpict id
                7+
                        rfunc gfunc bfunc pictfile
                        funcfile u v transform
                0
                m A1 A2 .. Am

  Colortext
    Colortext is dichromatic writing in a polygonal font. The font is defined in an auxiliary file, such as helvet.fnt. The text itself is also specified in a separate file, or can be part of the material arguments. The character size, orientation, aspect ratio and slant is determined by right and down motion vectors. The upper left origin for the text block as well as the foreground and background colors must also be given.

                mod colortext id
                2 fontfile textfile
                0
                15+
                        Ox Oy Oz
                        Rx Ry Rz
                        Dx Dy Dz
                        rfore gfore bfore
                        rback gback bback
                        [spacing]

    or:

                mod colortext id
                2+N fontfile . This is a line with N words ...
                0
                15+
                        Ox Oy Oz
                        Rx Ry Rz
                        Dx Dy Dz
                        rfore gfore bfore
                        rback gback bback
                        [spacing]

  Brighttext
    Brighttext is like colortext, but the writing is monochromatic.

                mod brighttext id
                2 fontfile textfile
                0
                11+
                        Ox Oy Oz
                        Rx Ry Rz
                        Dx Dy Dz
                        foreground background
                        [spacing]

    or:

                mod brighttext id
                2+N fontfile . This is a line with N words ...
                0
                11+
                        Ox Oy Oz
                        Rx Ry Rz
                        Dx Dy Dz
                        foreground background
                        [spacing]

    By default, a uniform spacing algorithm is used that guarantees every character will appear in a precisely determined position. Unfortunately, such a scheme results in rather unattractive and difficult to read text with most fonts. The optional spacing value defines the distance between characters for proportional spacing. A positive value selects a spacing algorithm that preserves right margins and indentation, but does not provide the ultimate in proportionally spaced text. A negative value insures that characters are properly spaced, but the placement of words then varies unpredictably. The choice depends on the relative importance of spacing versus formatting. When presenting a section of formatted text, a positive spacing value is usually preferred. A single line of text will often be accompanied by a negative spacing value. A section of text meant to depict a picture, perhaps using a special purpose font such as hexbit4x1.fnt, calls for uniform spacing. Reasonable magnitudes for proportional spacing are between 0.1 (for tightly spaced characters) and 0.3 (for wide spacing).

  #### 2.1.5. Mixtures

  A mixture is a blend of one or more materials or textures and patterns. The basic types are given below.

  Mixfunc
    A mixfunc mixes two modifiers procedurally. It is specified as follows:

                mod mixfunc id
                4+ foreground background vname funcfile transform
                0
                n A1 A2 .. An

    Foreground and background are modifier names that must be defined earlier in the scene description. If one of these is a material, then the modifier of the mixfunc must be "void". (Either the foreground or background modifier may be "void", which serves as a form of opacity control when used with a material.) Vname is the coefficient defined in funcfile that determines the influence of foreground. The background coefficient is always (1-vname).

  Mixdata
    Mixdata combines two modifiers using an auxiliary data file:

                mod mixdata id
                5+n+
                        foreground background func datafile
                        funcfile x1 x2 .. xn transform
                0
                m A1 A2 .. Am

  Mixpict
    Mixpict combines two modifiers based on a picture:

                mod mixpict id
                7+
                        foreground background func pictfile
                        funcfile u v transform
                0
                m A1 A2 .. Am

    The mixing coefficient function "func" takes three arguments, the red, green and blue values corresponding to the pixel at (u,v).

Mixtext
  Mixtext uses one modifier for the text foreground, and one for the background:

              mod mixtext id
              4 foreground background fontfile textfile
              0
              9+
                      Ox Oy Oz
                      Rx Ry Rz
                      Dx Dy Dz
                      [spacing]

  or:

              mod mixtext id
              4+N
                      foreground background fontfile .
                      This is a line with N words ...
              0
              9+
                      Ox Oy Oz
                      Rx Ry Rz
                      Dx Dy Dz
                      [spacing]

  ### 2.2. Auxiliary Files

  Auxiliary files used in textures and patterns are accessed by the programs during image generation. These files may be located in the working directory, or in a library directory. The environment variable RAYPATH can be assigned an alternate set of search directories. Following is a brief description of some common file types.

  #### 12.2.1. Function Files

  A function file contains the definitions of variables, functions and constants used by a primitive. The transformation that accompanies the file name contains the necessary rotations, translations and scalings to bring the coordinates of the function file into agreement with the world coordinates. The transformation specification is the same as for the xform command. An example function file is given below:

              {
                      This is a comment, enclosed in curly braces.
                      {Comments can be nested.}
              }
                                      { standard expressions use +,-,*,/,^,(,) }
              vname = Ny * func(A1) ;
                                      { constants are defined with a colon }
              const : sqrt(PI/2) ;
                                      { user-defined functions add to library }
              func(x) = 5 + A1*sin(x/3) ;
                                      { functions may be passed and recursive }
              rfunc(f,x) = if(x,f(x),f(-x)*rfunc(f,x+1)) ;
                                      { constant functions may also be defined }
              cfunc(x) : 10*x / sqrt(x) ;

  Many variables and functions are already defined by the program, and they are listed in the file rayinit.cal. The following variables are particularly important:

                      Dx, Dy, Dz              - incident ray direction
                      Nx, Ny, Nz              - surface normal at intersection point
                      Px, Py, Pz              - intersection point
      		T			- distance from start
      		Ts			- single ray (shadow) distance
                      Rdot                    - cosine between ray and normal
                      arg(0)                  - number of real arguments
                      arg(i)                  - i'th real argument

  For mesh objects, the local surface coordinates are available:

              	Lu, Lv                  - local (u,v) coordinates

  For BRDF types, the following variables are defined as well:

                      NxP, NyP, NzP           - perturbed surface normal
                      RdotP                   - perturbed dot product
                      CrP, CgP, CbP           - perturbed material color

  A unique context is set up for each file so that the same variable may appear in different function files without conflict. The variables listed above and any others defined in rayinit.cal are available globally. If no file is needed by a given primitive because all the required variables are global, a period (`.') can be given in place of the file name. It is also possible to give an expression instead of a straight variable name in a scene file. Functions (requiring parameters) must be given as names and not as expressions.

  Constant expressions are used as an optimization in function files. They are replaced wherever they occur in an expression by their value. Constant expressions are evaluated only once, so they must not contain any variables or values that can change, such as the ray variables Px and Ny or the primitive argument function arg(). All the math library functions such as sqrt() and cos() have the constant attribute, so they will be replaced by immediate values whenever they are given constant arguments. Thus, the subexpression cos(PI*sqrt(2)) is immediately replaced by its value, -.266255342, and does not cause any additional overhead in the calculation.

  It is generally a good idea to define constants and variables before they are referred to in a function file. Although evaluation does not take place until later, the interpreter does variable scoping and constant subexpression evaluation based on what it has compiled already. For example, a variable that is defined globally in rayinit.cal then referenced in the local context of a function file cannot subsequently be redefined in the same file because the compiler has already determined the scope of the referenced variable as global. To avoid such conflicts, one can state the scope of a variable explicitly by preceding the variable name with a context mark (a back-quote) for a local variable, or following the name with a context mark for a global variable.

  #### 2.2.2. Data Files

  Data files contain n-dimensional arrays of real numbers used for interpolation. Typically, definitions in a function file determine how to index and use interpolated data values. The basic data file format is as follows:

              N
              beg1 end1 m1
              0 0 m2 x2.1 x2.2 x2.3 x2.4 .. x2.m2
               ...
              begN endN mN
              DATA, later dimensions changing faster.

  N is the number of dimensions. For each dimension, the beginning and ending coordinate values and the dimension size is given. Alternatively, individual coordinate values can be given when the points are not evenly spaced. These values must either be increasing or decreasing monotonically. The data is m1*m2*...*mN real numbers in ASCII form. Comments may appear anywhere in the file, beginning with a pound sign ('#') and continuing to the end of line.

  #### 2.2.3. Font Files

  A font file lists the polygons which make up a character set. Comments may appear anywhere in the file, beginning with a pound sign ('#') and continuing to the end of line. All numbers are decimal integers:

              code n
                      x0 y0
                      x1 y1
                       ...
                      xn yn
               ...

  The ASCII codes can appear in any order. N is the number of vertices, and the last is automatically connected to the first. Separate polygonal sections are joined by coincident sides. The character coordinate system is a square with lower left corner at (0,0), lower right at (255,0) and upper right at (255,255).

  ### 2.3. Generators

  A generator is any program that produces a scene description as its output. They usually appear as commands in a scene description file. An example of a simple generator is genbox.
  - Genbox takes the arguments of width, height and depth to produce a parallelepiped description.
  - Genprism takes a list of 2-dimensional coordinates and extrudes them along a vector to produce a 3-dimensional prism.
  - Genrev is a more sophisticated generator that produces an object of rotation from parametric functions for radius and axis position.
  - Gensurf tessellates a surface defined by the parametric functions x(s,t), y(s,t), and z(s,t).
  - Genworm links cylinders and spheres along a curve.
  - Gensky produces a sun and sky distribution corresponding to a given time and date.
  - Xform is a program that transforms a scene description from one coordinate space to another. Xform does rotation, translation, scaling, and mirroring.

  ## 3. Image Generation

  Once the scene has been described in three-dimensions, it is possible to generate a two-dimensional image from a given perspective.

  The image generating programs use an octree to efficiently trace rays through the scene. An octree subdivides space into nested octants which contain sets of surfaces. In RADIANCE, an octree is created from a scene description by oconv. The details of this process are not important, but the octree will serve as input to the ray-tracing programs and directs the use of a scene description.

  - Rview is ray-tracing program for viewing a scene interactively. When the user specifies a new perspective, rvu quickly displays a rough image on the terminal, then progressively increases the resolution as the user looks on. He can select a particular section of the image to improve, or move to a different view and start over. This mode of interaction is useful for debugging scenes as well as determining the best view for a final image.
  - Rpict produces a high-resolution picture of a scene from a particular perspective. This program features adaptive sampling, crash recovery and progress reporting, all of which are important for time-consuming images.

  A number of filters are available for manipulating picture files:

  - Pfilt sets the exposure and performs antialiasing.
  - Pcompos composites (cuts and pastes) pictures.
  - Pcomb performs arbitrary math on one or more pictures.
  - Pcond conditions a picture for a specific display device.
  - Protate rotates a picture 90 degrees clockwise.
  - Pflip flips a picture horizontally, vertically, or both (180 degree rotation).
  - Pvalue converts a picture to and from simpler formats.

  Pictures may be displayed directly under X11 using the program ximage, or converted a standard image format using one of the following translators:

  - Ra_avs converts to and from AVS image format.
  - Ra_pict converts to Macintosh 32-bit PICT2 format.
  - Ra_ppm converts to and from Poskanzer Portable Pixmap formats.
  - Ra_pr converts to and from Sun 8-bit rasterfile format.
  - Ra_pr24 converts to and from Sun 24-bit rasterfile format.
  - Ra_ps converts to PostScript color and greyscale formats.
  - Ra_rgbe converts to and from Radiance uncompressed picture format.
  - Ra_t16 converts to and from Targa 16 and 24-bit image formats.
  - Ra_t8 converts to and from Targa 8-bit image format.
  - Ra_tiff converts to and from TIFF.
  - Ra_xyze converts to and from Radiance CIE picture format.

  ## 4. License

      The Radiance Software License, Version 1.0

      Copyright (c) 1990 - 2010 The Regents of the University of California,
      through Lawrence Berkeley National Laboratory.   All rights reserved.

      Redistribution and use in source and binary forms, with or without
      modification, are permitted provided that the following conditions
      are met:

      1. Redistributions of source code must retain the above copyright
              notice, this list of conditions and the following disclaimer.

      2. Redistributions in binary form must reproduce the above copyright
            notice, this list of conditions and the following disclaimer in
            the documentation and/or other materials provided with the
            distribution.

      3. The end-user documentation included with the redistribution,
                if any, must include the following acknowledgment:
                  "This product includes Radiance software
                      (http://radsite.lbl.gov/)
                      developed by the Lawrence Berkeley National Laboratory
                    (http://www.lbl.gov/)."
            Alternately, this acknowledgment may appear in the software itself,
            if and wherever such third-party acknowledgments normally appear.

      4. The names "Radiance," "Lawrence Berkeley National Laboratory"
            and "The Regents of the University of California" must
            not be used to endorse or promote products derived from this
            software without prior written permission. For written
            permission, please contact radiance@radsite.lbl.gov.

      5. Products derived from this software may not be called "Radiance",
            nor may "Radiance" appear in their name, without prior written
            permission of Lawrence Berkeley National Laboratory.

      THIS SOFTWARE IS PROVIDED ``AS IS" AND ANY EXPRESSED OR IMPLIED
      WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
      OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
      DISCLAIMED.   IN NO EVENT SHALL Lawrence Berkeley National Laboratory OR
      ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
      SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
      LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF
      USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
      ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
      OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
      OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
      SUCH DAMAGE.

  ## 5. Acknowledgements

  This work was supported by the Assistant Secretary of Conservation and Renewable Energy, Office of Building Energy Research and Development, Buildings Equipment Division of the U.S. Department of Energy under Contract No. DE-AC03-76SF00098.

  Additional work was sponsored by the Swiss federal government under the Swiss LUMEN Project and was carried out in the Laboratoire d'Energie Solaire (LESO Group) at the Ecole Polytechnique Federale de Lausanne (EPFL University) in Lausanne, Switzerland.

  ## 6. References

  - Ward, G., R. Mistrick, E.S. Lee, A. McNeil, J. Jonsson, "Simulating the Daylight Performance of Complex Fenestration Systems Using Bidirectional Scattering Distribution Functions within Radiance," Leukos, 7(4) April 2011.
  - Cater, Kirsten, Alan Chalmers, Greg Ward, "Detail to Attention: Exploiting Visual Tasks for Selective Rendering," Eurographics Symposium on Rendering 2003, June 2003.
  - Ward, Greg, Elena Eydelberg-Vileshin, "Picture Perfect RGB Rendering Using Spectral Prefiltering and Sharp Color Primaries," Thirteenth Eurographics Workshop on Rendering (2002), P. Debevec and S. Gibson (Editors), June 2002.
  - Ward, Gregory, "High Dynamic Range Imaging," Proceedings of the Ninth Color Imaging Conference, November 2001.
  - Ward, Gregory and Maryann Simmons, " The Holodeck Ray Cache: An Interactive Rendering System for Global Illumination in Nondiffuse Environments," ACM Transactions on Graphics, 18(4):361-98, October 1999.
  - Larson, G.W., "The Holodeck: A Parallel Ray-caching Rendering System," Proceedings of the Second Eurographics Workshop on Parallel Graphics and Visualisation, September 1998.
  - Larson, G.W. and R.A. Shakespeare, Rendering with Radiance: the Art and Science of Lighting Visualization, Morgan Kaufmann Publishers, 1998.
  - Larson, G.W., H. Rushmeier, C. Piatko, "A Visibility Matching Tone Reproduction Operator for High Dynamic Range Scenes," LBNL Technical Report 39882, January 1997.
  - Ward, G., "Making Global Illumination User-Friendly," Sixth Eurographics Workshop on Rendering, Springer-Verlag, Dublin, Ireland, June 1995.
  - Rushmeier, H., G. Ward, C. Piatko, P. Sanders, B. Rust, " Comparing Real and Synthetic Images: Some Ideas about Metrics," Sixth Eurographics Workshop on Rendering, Springer-Verlag, Dublin, Ireland, June 1995.
  - Ward, G., "The RADIANCE Lighting Simulation and Rendering System," Computer Graphics, July 1994.
  - Rushmeier, H., G. Ward, "Energy Preserving Non-Linear Filters," Computer Graphics, July 1994.
  - Ward, G., "A Contrast-Based Scalefactor for Luminance Display," Graphics Gems IV, Edited by Paul Heckbert, Academic Press 1994.
  - Ward, G., "Measuring and Modeling Anisotropic Reflection," Computer Graphics, Vol. 26, No. 2, July 1992.
  - Ward, G., P. Heckbert, "Irradiance Gradients," Third Annual Eurographics Workshop on Rendering, Springer-Verlag, May 1992.
  - Ward, G., "Adaptive Shadow Testing for Ray Tracing" Photorealistic Rendering in Computer Graphics, proceedings of 1991 Eurographics Rendering Workshop, edited by P. Brunet and F.W. Jansen, Springer-Verlag.
  - Ward, G., "Visualization," Lighting Design and Application, Vol. 20, No. 6, June 1990.
  - Ward, G., F. Rubinstein, R. Clear, "A Ray Tracing Solution for Diffuse Interreflection," Computer Graphics, Vol. 22, No. 4, August 1988.
  - Ward, G., F. Rubinstein, "A New Technique for Computer Simulation of Illuminated Spaces," Journal of the Illuminating Engineering Society, Vol. 17, No. 1, Winter 1988.

  See the RADIANCE Reference Materials page for additional information.

  ## 7. Types Index

  ####

      SURFACES	MATERIALS	TEXTURES	PATTERNS	MIXTURES
      Source		Light		Texfunc		Colorfunc	Mixfunc
      Sphere		Illum		Texdata		Brightfunc	Mixdata
      Bubble		Glow				Colordata	Mixtext
      Polygon		Spotlight			Brightdata
      Cone		Mirror				Colorpict
      Cup		Prism1				Colortext
      Cylinder	Prism2				Brighttext
      Tube		Plastic
      Ring		Metal
      Instance	Trans
      Mesh		Plastic2
      		Metal2
      		Trans2
      		Mist
      		Dielectric
      		Interface
      		Glass
      		Plasfunc
      		Metfunc
      		Transfunc
      		BRTDfunc
      		Plasdata
      		Metdata
      		Transdata
      		BSDF
      		Antimatter

  Last Update: October 22, 1997