Encyclopedia of Graphics File Formats Companion/GFF_CD.ISO/formats/radiance/spec/imaging.txt

Text File | 1994-06-01 | 40.1 KB | 1,172 lines

                        The RADIANCE 2.2
                    Synthetic Imaging System


                            Greg Ward
                  Lawrence Berkeley Laboratory
                         1 Cyclotron Rd.
                       Berkeley, CA 94720
                         [phone]


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.

     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.

     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. They can be made
from materials such as plastic, metal, and glass. Light sources
can be distant disks as well as local spheres, discs 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 PM 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"+. An
identifier can be any string (ie. sequence of non-blank
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). 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.
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.


2.1. Primitive Types

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

+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.


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 (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.

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 (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 local 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 prism 1 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 prism 1 id
     5+ coef dx dy dz functile 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+ coefl dx1 dy1 dz1 coef2 dx2 dy2 dz2 functile 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 O 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 functile. 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 trans. The only
specification required is the transmissivity at normal incidence.
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 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.

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+ refl 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 refl
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
     6+ red green blue rspec trans tspec A7 ..

The variables rrefl, grefl and brefl specify the color
coefficients for the ideal specular (mirror) reflection of the
surface. These values are not modified by rspec, thus the diffuse
reflectances red, green and blue implicitly exclude the mirrored
specular component and should be set accordingly. Similarly, the
variables rtrns, gtrns and btrns specify the color coefficients
for the transmitted direction. These values are modified by the
total transmittance, trans, but not by tspec. The functions
rbrtd, gbrtd and bbrtd take the direction to the incident light
and compute the color coefficient for the non-Lambertian part of
reflection and transmission. These functions are modified by
rspec and tspec appropriately, and they are expected to integrate
to 1 over each projected hemisphere.

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 a single variable, which is the interpolated value
from the n-dimensional data file.

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
     4+ 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.

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 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 runtime, 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]

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.

2.2.1. 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
     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.

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 are not allowed in data
files.

2.2.3. 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).

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. 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, rview
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 anti-aliasing.
Pcompos composites (cuts and pastes) pictures. Pvalue converts a
picture to and from alternate forms.

     Currently only a few graphics output devices are supported.
Tttyimage produces a crude character representation of an image
on a dumb terminal. Aedimage produces output on an AED 512
graphics terminal, and ximage produces an image on an X-window
server. Output is also available on certain dot-matrix printers
and the Dicomed film recorder. The list of supported output
devices is expected to grow as the system is made more widely
available.


4. 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.


5. References

Ward, G., "Measuring and Modeling Anisotropic Reflection,"
Computer Graphics, Chicago, July 1992.

Ward, G., P. Heckbert, "Irradiance Gradients," Third Annual
Eurographics Workshop on Rendering, to be published by
Springer-Verlag, held in Bristol, UK, May 1992.

Ward, G., "Adaptive Shadow Testing for Ray Tracing," Second
Annual Eurographics Workshop on Rendering, to be published by
Springer-Verlag, held in Barcelona, SPAIN, May 1991.

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.

ra_pict (1)    - convert Radiance pictures to Macintosh PICT
                   files
aedimage (1)   - RADIANCE driver for AED 512 color graphics
                   terminal
arch2rad (1)   - convert Architrion text file to RADIANCE
                   description
calc (1)       - calculator
cnt (1)        - index counter
dayfact (1)    - compute illuminance and daylight factor on
                   workplane
ev (1)         - evaluate expressions
falsecolor (1) - make a false color RADIANCE picture
findglare (1)  - locate glare sources in a RADIANCE scene
genbox (1)     - generate a RADIANCE description of a box
genprism (1)   - generate a RADIANCE description of a prism
genrev (1)     - generate a RADIANCE description  of surface of
                   revolution
gensky (1)     - generate a RADIANCE description of the sky
gensurf (1)    - generate a RADIANCE description of a curved
                   surface
genworm (1)    - generate a RADIANCE description of a functional
                   worm
getbbox (1)    - compute bounding box for RADIANCE scene
getinfo (1)    - get header information from a RADIANCE file
glare (1)      - perform glare and visual comfort calculations
glarendx (1)   - calculate glare index
ies2rad (1)    - convert IES luminaire data to RADIANCE
                   description
lam (1)        - laminate lines of multiple files
lampcolor (1)  - compute spectral radiance for diffuse emitter
lookamb (1)    - examine ambient file values
mkillum (1)    - compute illum sources for a RADIANCE scene
                   description
neat (1)       - neaten up output columns
normpat (1)    - normalize RADIANCE pictures for use as patterns.
oconv (1)      - create an octree from a RADIANCE scene
                   description
pcomb (1)      - combine RADIANCE pictures.
pcompos (1)    - composite RADIANCE pictures.
pextrem (1)    - find minimum and maximum values in RADIANCE
                   picture
pfilt (1)      - filter a RADIANCE picture
pflip (1)      - flip a RADIANCE picture.
pinterp (1)    - interpolate/extrapolate view from pictures
protate (1)    - rotate a RADIANCE picture.
psign (1)      - produce a RADIANCE picture from text.
pvalue (1)     - convert RADIANCE picture  to/from  alternate
                   formats
ra_bn (1)      - convert RADIANCE picture to/from Barneyscan
                   image
ra_pixar (1)   - convert RADIANCE picture to/from PIXAR picture
ra_ppm (1)     - convert RADIANCE picture to/from a Poskanzer
                   Portable Pix
ra_pr (1)      - convert RADIANCE picture to/from pixrect
                   rasterfile
ra_pr24 (1)    - convert RADIANCE picture to/from 24-bit
                   rasterfile
ra_ps (1)      - convert RADIANCE picture to a PostScript ASCII
                   file
ra-rgbe (1)    - change run-length encoding of a RADIANCE picture
ra_t16 (1)     - convert RADIANCE picture to/from Targa 16 or
                   24-bit image
ra_t8 (1)      - convert RADIANCE picture to/from Targa 8-bit
                   image file
ra_tiff (1)    - convert RADIANCE picture to/from a TIFF color or
                   greyscal
rcalc (1)      - record calculator
replmarks (1)  - replace triangular markers in a RADIANCE scene
                   description
rpict (1)      - generate a RADIANCE picture
rpiece (1)     - render pieces of a RADIANCE picture
rtrace (1)     - trace rays in RADIANCE scene
rview (1)      - generate RADIANCE images interactively
thf2rad (1)    - convert GDS things file to RADIANCE description
total (1)      - sum up columns
ttyimage (1)   - RADIANCE driver for X window system
vgaimage  (1)  - RADIANCE picture display program for VGA
xform (1)      - transform a RADIANCE scene description
xglaresrc (1)  - display glare sources under X11
ximage (1)     - RADIANCE driver for X window system
xshowtrace (1) - interactively show rays traced on RADIANCE image
                   under X1