RADIANCE scene description

Original by Greg Ward
Converted by Paul Bourke
June 1994


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 S3 .. 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 (ie. sequence of non-blank characters).

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

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. 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.

Blank 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.


Surfaces

x 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 (ie. 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.


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 (ie. 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/rad2/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, or becomes invisible. 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 (ie. 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 secondary 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.
        mod mirror id
        1 material
        0
        3 red green blue

Prism1
The prism1 material is for general light redirection from prismatic glazings, generating secondary light sources. It can only be used to modify a planar surface (ie. 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 secondary 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 direct1 id
        9+ coef1 dx1 dy1 dz1 coef2 dx2 dy2 dz2 funcfile transform
        0
        n A1 A2 .. An

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 (ie. 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 transmissivity in each wavelength band per unit length. Common glass has a index of refraction (n) around 1.5, and a transmissivity 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 transmissivities 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 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 must take three arguments, the x, y and z direction towards the incident light, and should integrate to 1 over the projected hemisphere. 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
        4+ 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 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. The data function (func) takes four variables, the interpolated value from the n-dimensional data file, followed by the x, y and z direction to the incident light. The light source direction 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 ..

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 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.

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

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).


Mixtures

A mixture is a blend of one or more 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 uniquely defined in the scene description. Vname is the coefficient defined in funcfile that determines the influence of foreground. The background coefficient is always (1-vname). Since the references are not resolved until run-time, the last definitions of the modifier id's will be used. This can result in modifier loops, which are detected by the renderer.

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

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]

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.

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
                Px, Py, Pz              - intersection point
                Nx, Ny, Nz              - surface normal at intersection point
                Rdot                    - cosine between ray and normal
                arg(0)                  - number of real arguments
                arg(i)                  - i'th real argument
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, although such expressions should be kept simple as they cannot contain any white space. Also, 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.


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 are not allowed in data files.

Font Files

A font file lists the polygons which make up a character set. There are no comments, and 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).