Rendering with Radiance
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Glossary

absorption coefficient

The fraction of light absorbed per unit distance in
a participating medium. In SI units, this is specified as a
fraction per meter. In Radiance, the absorption is given
separately for red, green, and blue.

adaptive sampling

The automatic sending of additional samples in cases
where the default number of samples is deemed inadequate to achieve the
desired accuracy.

albedo

The unitless ratio between the scattering coefficient and the extinction
coefficient for a participating
medium. An albedo of 0 means that the particles do not scatter light.
An albedo of 1 means that the particles do not absorb light.

algorithm

A method that is fully described in a procedure or
computer program.

alias

In a Radiance scene description, an alias
associates a new identifier (and possibly a new modifier) with a
previously defined primitive.

altitude

The angle between a vector and the surface horizon,
which is equal to 90 degrees minus the polar
angle.

ambient (value, calculation)

An ambient value approximates the global average of
radiance over all directions and all points of interest in a scene. It
is specified in SI units of watts per steradian per square meter for
red, green, and blue. The ambient calculation in Radiance
replaces this global average with local averages based on indirect
irradiance computations. It is more accurately referred to as a diffuse interreflection calculation.

ambient file

A binary file in which indirect irradiance values
and their gradients are stored for reuse in other calculations. (See
the CDROM section on File Formats
for details.)

angular fisheye perspective

A perspective view projection in which the angle
from the central view direction is proportional to the distance from
the center of the image. (Compare to hemispherical fisheye perspective.)

animation (walkthrough,
scene animation)

A walkthrough animation is a sequence of
images rendered from the same scene description and lighting but from a
changing view point, direction, and so on. Scene animation
occurs when the actual scene geometry, materials, and/or lighting are
changing with each frame.

animation path

The sequence of view positions and directions in a
walkthrough animation. Also called the camera path. (See Chapter 9 for details.)

anisotropic

A reflection or transmission distribution function
(BRTDF) that varies with rotation about the surface normal. Examples of
anisotropic reflection include varnished wood with noticeable grain,
brushed metal, and combed hair.

antialiasing

Any technique that reduces sampling artifacts in the
final image, particularly "jaggies" caused by abrupt changes
in scene geometry. The usual antialiasing method used in Radiance
is called supersampling.

antimatter

A primitive type that provides a rudimentary CSG subtraction operation,
removing one solid from another.

array transform

A type of transform in which an object is repeated
at multiple positions. (See the xform manual page.)

ASCII

The American Standard for Character Information
Interchangethe standard encoding of alphanumeric and control
characters as integers between 0 and 127.

azimuth

The angle about the surface normal relative to some
standard direction, such as south.

Bspline

A cubic polynomial formulation with the potential
for C(2) continuity, which provides a visually smooth curve, even
though it does not generally pass through any of its control points.

Bezier curve

A cubic polynomial formulation whose parameters
specify four points near the desired curve shape. In three dimensions,
a Bezier surface is specified by sixteen points near the desired
surface patch.

boundary representation
(Brep)

A method for representing 3D objects by their
surface boundaries, which may be open or closed but do not necessarily
have any relationship to one another. (Compare with Constructive Solid Geometry.)

BRDF, BRTDF, BSDF

The bidirectional reflectance distribution function
(BRDF) is a mathematical function that describes the way light is
reflected from a point on a locally planar surface. The bidirectional
reflectancetransmittance distribution function (BRTDF) is also known
as the bidirectional scattering distribution function (BSDF), which
describes the way light is transmitted and reflected by a locally
planar surface. All are functions of four angles (two incident and two
scattered) and return units of 1/steradian.

CAD

Computeraided design or Computeraided drafting.
CAD tools are typically used to interactively create and describe
geometric entities.

candelas

The SI unit for the total visible light leaving a
light source in a certain direction, which may be written out as lumens
per steradian.

candlepower distribution

The light output distribution for a source, given as
a function of direction (two angles in the general case) that returns
candelas (in SI units). This is an approximation that assumes the
receiver is far from the source.

Cartesian coordinate
system

A system for locating points based on an origin and
two or more perpendicular axes passing through it. In Radiance,
3D coordinates are specified via a righthand
Cartesian coordinate system, where the zaxis is in the direction of
the thumb of the right hand when the fingers are curled in the
direction of the xaxis, and then the yaxis.

chromaticity

The exact color (but not the luminance) of a light
source or reflecting surface, given as two values, for example a CIE color (x,y)
coordinate. Also sometimes called chrominance.

CIE

The Commission International de l'Eclairage, which
sets international standards for lighting, daylight, and color
measurement and estimation.

CIE color

Several color systems are put forth by the CIE. The
most commonly applied system is the CIE 1931 1degree standard
observer, which specifies a perceived color as a tristimulus
value, that is, a coordinate triplet indicating the luminance
and chromaticity of a stimulus as it is perceived by a 1degree spot
about the foveal center. This may be given equivalently as an XYZ or
Yxy value, where x and y are equal to X/( X+Y+Z ) and Y/( X+Y+Z ),
respectively. (See the section titled Supplemental Information in Chapter10 for further information.)

CIE standard sky
distribution

Sky light is a continually varying and unpredictable
quantity, so for analysis and comparison purposes, the CIE recommends a
specific set of standard sky distributions that approximate average
skies. The most commonly applied are the CIE overcast sky
and the CIE clear sky. More recently, the CIE has proposed
a third, intermediate sky distribution that better approximates real
skies in many countries. (See Chapter 6
for details.)

client

A program or process that accesses the resources of
a server
executing locally or on a remote system over a network.

clipping plane

An imaginary plane before which or beyond which
nothing is visible. The fore clipping plane (vo option)
determines the closest visible surfaces, and the aft clipping plane (va
option) determines the furthest visible surfaces.

color temperature

A black body, whose radiation is solely the
result of thermal activity, emits a characteristic spectrum that is
determined exactly by its surface temperature. Light sources that
approximate such a spectrum, such as incandescent lamps, are often
characterized by their correlated color temperature, given
in degrees Kelvin.

command expansion

In a Radiance scene description, an inline command that begins with an exclamation point
(!) is executed by the system and the output is read in as more scene
input, which may also contain inline commands that are also executed.
(This is called a scene hierarchy.)

command line

In UNIX, a command line is generally a single line
passed to the user's shell for command execution. This typically involves
searching for an executable binary or shell script corresponding to the
first word in the line, and expanding any filematching wildcards in
the remaining (unquoted) arguments. Other substitutions may also take
place, such as variable or history substitution; you should consult the
manual page for your particular shell for details. Multiple commands
may appear on a single command line, joined either by a semicolon (;)
indicating sequential execution, or a pipe character () indicating parallel execution with
the standard output of the first command passed to the standard input
of the second command. Again, particular shells have additional syntax
for other interpretations.

computational
complexity

The limiting relationship of computation time to the
number of objects. It is often given in the form of "big O"
notation, which means "on the order of." For example, an
algorithm that has O(N^2) computational complexity has time
requirements proportional to the square of the number of objects as N
goes to infinity. (See also storage complexity.)

cone

In Radiance, the family of cones includes
rightangle truncated cones, cylinders, and rings (disks with optional
holes).

constructive solid geometry
(CSG)

A method for representing 3D objects as the union,
intersection, and subtraction of solid (volumetric) geometry. In
contrast to boundary representation, CSG representations are compact and make
better use of curved primitives, but place greater requirements on the
rendering software.

contrast

In lighting, contrast is defined as the difference
between the foreground luminance and the background luminance, divided
by the background luminance. Contrast is what the human eye relies on
to see object detail, and is the most important quantity to reproduce
faithfully in any rendering algorithm.

converter

See translator.

CPU

Central processing unit. The processor in a
computer, usually consisting of registers, an arithmetic logic unit
(ALU), memoryaddressing logic, and (in most cases) a floatingpoint
unit (FPU).

cylindrical perspective

A viewing projection that maps vertical angles the
same way as a linear perspective view, but maps horizontal angles such
that horizontal distance from the center of the image is proportional
to the angle from the center of view. Also called a panorama,
a cylindrical view can cover a full 360 degrees horizontally, which is
needed to generate QuickTime VR movies.

data file

A Radiance data file describes a scalar
function over Ndimensional space as a linearly interpolated table of
discrete values. (See the CDROM section on File Formats for file syntax and
semantics.)

data sharing

When multiple rendering or lighting simulation
processes are executed simultaneously on the same scene, data sharing
permits diffuse interreflection (ambient) values to be used by all
processes, avoiding unnecessary duplication of expensive raysampling
calculations.

daylight factor

The ratio of interior illuminance at the workplane
to the exterior illuminance at the ground plane. In the common case of
the standard CIE overcast sky distribution, this value is
usually less than 1, and is independent of the time of day or time of
year, which makes it very useful for comparison purposes. (See Chapter 6 for further details.)

deterministic

Any algorithm that consistently produces exactly the
same result for exactly the same input. (Compare to stochastic and Monte
Carlo.)

device driver

In rview, a device driver is a program or set of
subroutines that translates drawing commands and input requests into
the appropriate calls for the current display or window system.

dielectric

A participating
medium that refracts,
and may absorb, but does not scatter radiation. Examples include water,
wine, and glass. A dielectric is generally characterized by an index of refraction and
an absorption or transmission coefficient. In the case
of the dielectric primitive used in Radiance, an
additional Hartmann constant may be used to approximate the change in the
index of refraction with wavelength.

diffraction

The deviation from linear propagation that occurs
when light passes a small object or opening. This phenomenon is
significant only when the object or opening is on the order of the wavelength of
light, between 380 and 780 nanometers for human vision. For this
reason, diffraction effects are ignored in most rendering algorithms,
since most modeled geometry is on a much larger scale.

diffuse

Scattering light in all directions, as occurs in Lambertian
reflection, which is the ideal diffuse case. Diffuse transmission
means that light is scattered equally in all transmitted directions,
that is, in all directions on the opposite side of the surface.

diffuse
interreflection

The propagation of light by diffuse reflection and
transmission in an environment. The Radiance diffuse
interreflection calculation computes the diffuse component of
light bouncing around a scene. (See Chapter
12 for details.)

direct component

The illumination arriving at a surface point
directly from light sources.

directionaldiffuse

NonLambertian reflection or transmission, in which
light is scattered in all directions, but favors some directions over
others. (See also rough specular.)

dispersion

The tendency of light to scatter spatially when
refracted through a dielectric medium whose index
of refraction varies with wavelength.
(See also Hartmann constant.)

display

Any presentation method, such as a graphics monitor,
transparency, or print. The independent color channels available on a
display are called the display primaries.

emittance

The visible light emitted (but not reflected) by a
surface, expressed in SI units of lumens per square meter. (Radiant
emittance is given in radiometric units of watts per square meter.)

emitter

A surface that emits light. See light source.

exitance

The total visible light leaving a surface, including
reflections, expressed in SI units of lumens per square meter. (Radiant
exitance is given in radiometric units of watts per square
meter.)

exposure

A scaling factor used to map the computed or
measured world radiances or luminances to the appropriate range for
display, typically 0 to 1. This assumes a linear range for both input
and output. (See gamma correction.)

extinction coefficient

The fraction of light lost to scattering and
absorption per unit distance in a participating medium. A simple sum of
the absorption coefficient and the scattering
coefficient. In Radiance,
the extinction coefficient is divided into separate red, green, and
blue components. (See Chapter 14 for
details.)

fenestration

A window system, including vertical or horizontal
blinds, multiple glass panes, and any associated frames or obstructions.

filter

Any program that processes a Radiance picture
and produces a modified picture as its output. Programs that take more
than one input picture are sometimes included in this category, but
should probably be called something more general, such as image
processors.

fisheye perspective

See angular fisheye perspective and hemispherical
fisheye perspective.

font file

A Radiance font file describes the polygonal
outline of one or more character glyphs, which are used to produce text patterns or
mixtures (or by psign to make pictures directly).
(See the CDROM section on File
Formats for file syntax and semantics.)

fovea

The highresolution central region of the retina,
spanning about 1 degree of the visual field in humans.

fractal noise

A random function with a frequency spectrum that
follows a 1/f profile (where f is the frequency). In Radiance,
the fnoise3(x,y,z) function returns fractal noise values between 1 and
1 with an autocorrelation distance of 1, and is defined over all space.
(Contrast with Perlin noise. See Chapter 4
for examples.)

frozen octree

A Radiance octree that has been compiled with
the oconv f option, so that it contains a binary representation of all
the original scene description information. This type of octree is
quicker to load, and easier to transport to other locations because the
original scene files and their dependencies need not be copied.
(Auxiliary files, such as patterns and textures, will still need to be
accessible, however.)

function file

A file containing function and variable definitions
in the functional language of Radiance. These may be used to
define procedural textures, patterns, mixtures, and coordinate mappings
for the renderer, or numerous other functions for generators, filters,
and other utility programs. (See the CDROM section on File Formats
for file syntax and semantics.)

functional language

A language that replaces programming steps with
function definitions in no particular order. Usually, recursion
replaces looping as the primary construct for iteration, and
evaluation sequence is irrelevant because there are no explicit
temporary variables or side effects.

gamma correction

Most computer and television monitors exhibit a
response function that approximates a simple power law, and the
exponent is called the monitor's gamma. Typical displays have gamma
values between 1.5 and 3.0; 2.2 is a frequently used standard. Some
graphics hardware partially compensates for this response function,
bringing the effective system gamma down to 1.8 or 1.4. The correct
display of images, whether computergenerated or captured, requires
proper correction for the system's gamma response. (See Chapter 5 for details.)

Gaussian (distribution,
surface)

The normal distribution curve for a random variable
(i.e., the bellshaped curve). If the microfacet slope of a surface is
normally distributed, it is said to be a Gaussian surface.
(See also roughness.)

generator

A generator is a program that produces a geometric
description on its output for a specified shape, which may be
simple but is more often complex. A few generators, such as gensky,
produce nongeometric descriptions or combinations of materials and
geometry.

geometric
description

A geometric description contains primitives for the cones, polygons, and spheres that make up a scene's boundary
representation.

geometric model

This is a general term used for any finite model of
a scene's geometry. (The complexity of actual geometry is fundamentally
infinite in real environments.)

glare (discomfort, disability)

The result of bright sources in the field of view. Discomfort
glare refers to the pain a viewer experiences in trying to see
past bright sources; disability glare refers to the
associated loss of visibility.

glazing

The glass associated with a fenestration
system, which may be single, double, or triplepaned with various
coatings, films, and thermal barriers.

global
illumination

The general problem of light propagation in
simulated environments, accounting for the many interreflections
between scene surfaces.

glyph

A bounded 2D graphic typically used to represent
some character or other abstraction. A symbol. In a font, a glyph is
the name given to one character's geometric description.

ground plane

The horizontal surface outside a building
corresponding to the approximate ground level.

Hartmann constant

A parameter that approximates the change in the index of refraction of a particular dielectric material as a function of wavelength. Usually given in units of nanometers
(109 meters), this constant is divided by the wavelength
and added to a base index of refraction.

hemispherical
fisheye perspective

A viewing projection in which the distance from the
center of the image is proportional to the cosine of the angle from the
central view direction. (Compare to angular
fisheye perspective.)

hemispherical
reflectance

The total light reflected by a surface for a given
direction of parallel incident radiation.

hermite curve

A cubic polynomial formulation whose parameters
specify the starting point, ending point, starting tangent vector, and
ending tangent vector. In three dimensions, a hermite surface is
defined by the four corners with two tangent vectors plus one curl
vector at each vertex.

HeyneyGreenstein
parameter

The main parameter in a formula used to approximate
the anisotropic scattering of a participating medium, where a value of
0 indicates perfectly isotropic scattering, and a value approaching 1
indicates a strongly forwardscattering medium. (See Chapter 14 for formula and details.)

hierarchy

A tree of objects, such as a scene description
file that loads in other scene files that load other files in turn via inline commands. Hierarchy
allows one to animate a finger attached to a hand attached to an arm
attached to a torso, for example.

IES luminaire data

Candlepower distribution data together with other
miscellaneous information in a standard ASCII format proposed by the
IESNA [IES91].

IESNA

The Illuminating Engineering Society of North
America, a professional organization that sets standard practices for
lighting design in the U.S. and Canada.

illum

The basic secondary light
source type in Radiance. An
illum may also be called an impostor, since it is not really an emitter, but
plays as one in the direct calculation. (See Chapter 13 for details.)

illuminance

The integrated visible light arriving at a surface,
expressed in SI units of lumens per square meter, or lux. (Compare to irradiance.)

illuminance meter

A photometric device for measuring illuminance.

image plane

An imaginary plane on which a picture is projected.
The radiances arriving at this imaginary surface are written into a
picture as an array of color values.

image processing

Operations on images, such as scaling, inversion,
summation, warping, and so on. (See filter.)

image synthesis

The creation of a color or blackandwhite image
from a mathematical description of a scene. More commonly called rendering.

impostor

A surface with simplified properties that stands in
for a more complete description in the direct component calculation as
an efficiency measure. Also called an illum in Radiance. (See Chapter 13 for details.)

inline command

A command, embedded in a Radiance scene
description, which is preceded by an initial exclamation point (!). The
end of the command is indicated by a newline character, but if the
newline character is preceded by a backslash (\), then the command
continues on the next line. Inline commands may not read from their
standard input, and their standard output must contain only valid Radiance
scene description primitives, aliases, comments, and other inline
commands.

index of refraction

The ratio of the speed of light in a vacuum to the
speed of light in a particular participating medium. This value
determines how light is refracted (bent) as it passes from
one medium to another. The index of refraction of air is very close to
1, whereas the index of refraction of glass is about 1.52, meaning that
light travels over 1.5 times faster through air than it does through
glass.

indirect component

The component of light arriving at a surface
indirectly via bounces off other, nonemitting surfaces. This may
include specular and diffuse reflections. (See also indirect
irradiance and diffuse
interreflection.)

indirect
irradiance

A quantity equal to the integrated radiation
arriving at a surface point excluding light sources that are counted as
part of the direct component. It is expressed in SI units of watts per
steradian per square meter.

information header

A short section of ASCII text at the beginning of a Radiance
binary format file, which is terminated by an empty line (i.e., two
successive newline characters). This header usually begins with
#?RADIANCE and contains the commands that generated the file, along
with any pertinent variables indicating the software version, view,
exposure, format, and so on. This information may be conveniently read
with the getinfo command.

instance

A single example of an object, which may be repeated
many times in many places in a scene. The Radiance instance
primitive is used to replicate scene information that is compiled in a Radiance octree.

intersection point

The 3D point at which a ray intersects a surface. In
cases where multiple ray intersections are possible, the first is the
one that is used. (See ray tracing.)

irradiance

The integrated radiation arriving at a surface at
all wavelengths, expressed in SI units of watts per square meter. In Radiance,
only visible radiation is considered, but units of radiance and
irradiance are still used; they are divided into separate components
for the red, green, and blue spectral ranges. (Compare to illuminance.)

irradiance
gradient

The directional derivative of irradiance at a
surface point. In Radiance, this quantity is estimated for
changes in indirect irradiance as a function of surface
rotation and translation, and this is used to improve the accuracy of
interpolated indirect irradiance values.

isolux contour

A line corresponding to equal illuminance
values on a surface.

isotropic

Equal in all directions. Isotropic scattering occurs
when light is scattered equally over 4pi steradians. Isotropic
reflection, in contrast, usually refers to a surface that may have a
directional preference, but one that does not depend on the azimuth of the
incident vector.

jittered sampling

A stochastic process in which values are sampled
uniformly over a rectilinear subspace. For example, a ray may be
sampled at a random location on a square pixel by choosing random,
independent x and y offsets.

Lambertian

Ideal diffuse reflection, in which the luminance (or
radiance) reradiated by a surface is identical in all directions. Very
few surfaces exhibit this behavior, but some come close and many have a
component that is approximately Lambertian, which can be used to
accelerate interreflection calculations.

light source

A surface that originates light in a scene. In Radiance,
light sources are identified by their material type and geometry. In some cases, virtual light sources are created when reflecting geometry is
present in a scene, and these sources are used to direct rays back to
the original light sources. (See Chapter
11 for details.)

lighting
visualization

The creation of images, plots, and graphics for the
purpose of understanding lighting in a design space. (See physically based rendering.)

local illumination

The interaction of light with a surface,
disregarding the process by which light arrives at the surface.

lumens

A photometric unit related to the photopic stimulus
of the human fovea. It is computed by convolving the photopic response
function with the spectral power distribution function. Normalization is defined so that the
conversion from watts to lumens is 1/683 at the peak sensitivity
wavelength of 555 nanometers. (See Chapter10
for details.)

luminaire

A light fixture, including any and all lamps,
ballasts, reflectors, lenses, baffles, and so on.

luminance

The visible light passing through a point in a
certain direction, expressed in SI units of lumens per steradian (candelas) per
square meter, or nits.
(Compare to radiance.)

luminance meter

A photometric device for measuring luminance.

luminous flux

The visible light passing through an area, measured
in SI units of lux.

lux

Shorthand for lumens per square meter. (See illuminance.)

material

The substance from which an object is made. In the
context of Radiance, the characteristics of a surface that
describe its interaction with light.

material
description

A Radiance scene description fragment that defines a particular material's
properties, including its type, parameters, and any modifiers such as
textures, patterns, and mixtures.

memory overhead

The memory (RAM) required for a particular object or
operation above what is already in use by a process.

memory sharing

On a multiprocessing system, memory sharing means that
more than one processor is accessing the same RAM in parallel, thereby
avoiding duplicate memory requirements.

mesh

See tessellated surface.

metafile

An intermediate, deviceindependent binary file that
contains 2D vector graphics, used for plotting to various supported
output devices. (See the metafile(5) manual page on the CDROM.)

mirror direction

A reflected direction that lies in the plane defined
by the incident vector and the surface normal, at an equal but opposite
angle to the incident direction.

mist

A Radiance primitive type for participating
media.

mixture

A category of Radiance primitives
that provides for mixing two different patterns, textures,
materials, or mixtures.

modifier

A Radiance primitive whose purpose is to modify another Radiance
primitive. Specifically, any primitive that does not specify geometry
may be used to modify any other primitive.

Monte Carlo

Random (i.e., stochastic) sampling in the context of an integration
or averaging problem. Monte Carlo
inversion is the process of determining the appropriate random
variable for a desired weighting function so that the final sample
weights are equal. In the absence of better information or techniques,
this results in the most efficient sampling pattern.

multiprocessing

The application of multiple processors (CPUs) to the
same problem, running in parallel. Ideally, this would result in a
linear speedup proportional to the number of processors, but in
general, the improvement is somewhat less than this because of
overhead, communication delays, and data access contention.

nadir

Straight down, usually used for reference in the
output of a downwardfacing luminaire.

nanometer

The usual unit of measurement for the wavelength of
visible radiation, equal to 109 meters. Often abbreviated as nm.

network

A collection of computers connected by wires,
optical cables, and/or microwaves. Generally, a small number of
machines are connected together on a local area network at a
single site, which talks via a gateway machine to the global
network (i.e., the Internet).

network file system (NFS)

A network file access protocol that allows remote
disks to appear and act as local disks on a system. File servers provide disk
access to file clients.

network lock manager

A service that guarantees file consistency when the
same file is accessed by multiple processes over a network. This is a
standard part of the network file system on most UNIX
implementations, but some lock managers do not work efficiently and
others do not even operate correctly. The lock manager is a critical
part of the parallel processing algorithms in Radiance. (See
Chapter15 for details.)

nits

Shorthand for candelas per square meter. (See luminance.)

normal vector

A unit vector. (See surface
normal.)

NURBS

Acronym for NonUniform Rational BSpline, a
popular class of rational cubic patches used widely for the geometric modeling of smooth, curved surfaces.

octree

A data structure that subdivides space recursively
into eight cubes, each of which may be subdivided into eight subcubes,
and so on. An octree file is a special Radiance
binary format that contains this data structure, created by oconv to speed ray intersection
tests on the scene. A frozen octree file contains all of the
relevant scene information as well, providing for quick loading and
easy portability. (See the CDROM
section on File Formats for details.)

overture calculation

An initial lowresolution pass over an image to
collect indirect irradiance values into an ambient file
for later highresolution rendering. This generally improves the
quality of the final results at a modest expense.

panorama

See cylindrical perspective.

parallel
projection

A view type in which the distance from the center of
the image is proportional to the distance from the ray starting at the
view point and continuing in the view direction. Since all imaging rays
are parallel, this projection does not exhibit the foreshortening of a
normal perspective view.

parallel rendering

The simultaneous rendering of a single picture or
animated sequence on multiple processors (CPUs). (See multiprocessing.)

parallelepiped

A right angle, rectangular prism. That is, a closed,
convex, sixsided geometric solid, all of whose faces are rectangles.

parametric surface

A 3D surface described by a vector function of two
independent variables, each typically running from 0 to 1.

participating
medium

A solid, liquid, gas, or colloid that affects the
transport of light through its volume. Examples include glass, water,
wine, milk, fog, and smoke. Clear air is also a participating medium,
but one that has an effect only over great distances.

pattern

A variation in surface color, which can be described
in Radiance as a procedure, a picture or tabulated data.

penumbra

The partial shadow between the unshadowed region and
the umbra, or
fully shadowed region.

Perlin noise

A random function with a strong fundamental
frequency and few harmonics, first introduced by Ken Perlin [Per85]. In Radiance, the
noise3(x,y,z) function returns values between 1 and 1 with an
autocorrelation distance of 1, and is defined over all space. (Contrast
with fractal noise.
See Chapter 4 for examples.)

perspective
projection

See linear perspective projection, angular fisheye perspective, cylindrical
perspective, and hemispherical
fisheye perspective.

Phong shading

A simple model for rough
specular reflection (also known as directionaldiffuse), computed by raising the cosine
of the angle between the halfvector and the surface normal to a power.
(The halfvector bisects the incident and reflected directions.)

Phong smoothing

The interpolation of surface normals on a tessellated surface to simulate the appearance of a smoothly
curved object during rendering.

photometer

A device for measuring visible radiation. Typically
divided into two categories: illuminance meters and luminance
meters.

photometric

A quantity related to the human photopic response to
visible radiation. Examples of photometric units are lumens and
candelas. Examples of photometric quantities are luminance and
illuminance.

photometric data

The measured candlepower
distribution of a luminaire.

photometry

The measurement of visible radiation.

photopic response

The average spectral sensitivity of the human cone
system to radiation. Sensitivity begins near the far red wavelength of
780 nanometers, growing gradually to peak at about 555 nanometers, then
sloping gradually down into the ultraviolet at 350 nanometers. The
standard photopic response function is written as v(l). (See the
section titled Supplemental Information at the end of Chapter 10 for a plot of this function.)

photorealistic
rendering

A term used to describe rendering that appears
as real as a photograph.

physically based
rendering

A rendering technique that adheres to the physics of
light transport, producing images that are accurate predictions of the
appearance of a design space.

picture file

A Radiance floatingpoint color image, which
usually contains physical radiance values useful for
lighting analysis. (See the CDROM
section on File Formats for details.)

pipe

In UNIX, a "firstin, firstout" (FIFO)
interprocess communication channel, typically used to pass the standard
output of one command into the standard input of another command
running simultaneously. Sometimes, processes may open their own pipes
to other commands they start.

pipeline

Multiple UNIX commands connected together via pipes.

pixel

Short for picture element. The smallest
measured, calculated, or displayed point in an image.

planar

Lying in a plane. Completely flat.

polar angle

The angle between the surface normal and the
given vector direction. (Compate to altitude.)

polygon

An Nsided planar surface with a finite area. A convex
polygon has no interior angles greater than 180 degrees.

primitive

An element in a Radiance scene
description, which consists of a
modifier, type, identifier, and zero or more parameters. (See the Radiance
reference manual on the CDROM for syntax and semantics.)

prism

A polygon extruded into the third dimension. If the
extrusion vector parallels the surface normal, then the resulting solid
is called a right prism. (See the genprism manual page for details.)

process

A running program on a system.

processor

A computational unit. See CPU.

projected
hemisphere

A halfsphere, projected in parallel onto the
splitting plane along the surface normal such that each differential area
corresponds to the original area times the cosine of the polar angle.
A uniform sampling of luminance over the projected hemisphere times p (the
area of the unit circule) equals illuminance. The same relation holds for radiance and irradiance.

radiance

The radiation passing through a point in a specific
direction, expressed in SI units of watts per steradian per square
meter. In the Radiance system, only visible radiation is
considered, but units of radiance and irradiance are still used, which
are divided into separate components for the red, green, and blue
spectral ranges. (Compare to luminance.)

radiometric

A quantity related to radiation, usually measured in
terms of power or power/wavelength. Examples of radiometric units are
watts and watts/steradian. Exapmples of radiometric quantities are
radiance and irradiance.

radiosity

A physical quantity equal to the radiant exitance
of a Lambertian surface. (See also radiosity method.)

radiosity method

A formulation of the global illumination
problem based on the solution of radiosity for a finite
number of points in a 3D scene description. Also called finite
element flux transfer.

RAM

Random access memory, usually referred to simply as memory.

ray tracing

A method based on following onedimensional rays,
where each ray is defined by an origin point and a vector direction. In
a rendering algorithm, each ray is followed until it intersects a
visible surface, where new rays may be spawned in a recursive
process. In lightforwards ray tracing, light is followed from the
light sources to the final measurement areas. In lightbackwards ray tracing
(as in Radiance), each view ray is traced from the point of
measurement to the contributing light sources. In bidirectional
ray tracing, light is traced both from the measurement points
and from the light sources, meeting somewhere in between.

recursion

A function that calls a simpler version of itself to
arrive at a solution. For example, the factorial function (n!) can be
defined recursively as (n(n1)!), where (0!) is defined as equal to1.
If a function calls another function that calls back the original
function again, it is called mutual recursion; most
raytracing programs fall into this general category.
reflectance
The ratio of the light leaving a surface to the light
striking a surface. The general reflectance function is called the BRDF.
The averaged total reflection function is called the hemispherical reflectance. This may be a function of
wavelength (i.e., the spectral reflectance) or may be averaged
over the visible spectrum using the photopic
response function.

refraction

The bending of light as it enters a dielectric
medium at a nonzero polar angle. (See also index of
refraction.)

refractive index

See index of refraction.

rendering

The process of creating a 2D image from a 3D
representation. Also, the term given to the resulting synthetic image.

resolution

The density of discrete samples in a given region,
especially the total number of pixels in an image.

RGB color

Red, green, blue representation of color. Since
there is no defined standard for RGB in computer graphics, Radiance
defines its own canonical display primaries, which can be
found in the source header file src/common/color.h.

righthand rule

If the right hand points in the direction of the
first vector (i.e., the xaxis) and the fingers curl in the direction
of the second vector (i.e., the yaxis), the thumb points in the
direction of the third vector (i.e., the zaxis). (See also Cartesian coordinate system.)

roughness

The root mean squared (RMS) microfacet slope of a Gaussian surface.
This is equal to the standard deviation of the surface height over the
autocorrelation distance of this deviation. In the anisotropic material
model used in Radiance, this quantity may be given a
directional component.

rough specular

A directionaldiffuse component caused by specular
(surface interface) reflection or transmission from a rough surface.

runlength encoding

A simple datastream compression method whereby
repeated data values (runs) may be given as the value and its count.
This technique is used to reduce the size of Radiance picture files.

sampling

The process of determining an average or integral
value by evaluating a function at one or more positions. (See Monte Carlo.)

scattering coefficient

The fraction of light scattered per unit distance in
a participating medium, given in SI units as a fraction per meter.

scene description

An ASCII data stream that defines the surfaces,
materials, patterns, and textures of a Radiance model. (See
the Radiance Reference Manual and the section on File Formats on the CDROM
for format specifications.)

scotopic response

The average spectral sensitivity of the human rod
system, which dominates vision in dark environments. Compared to the
cones, with their photopic response function, the rods are more sensitive to
bluer (shorter) wavelengths. Scotopic lumens are defined in
a similar fashion to standard photopic lumens, with the same
conversion to 1/683 lumens/watt at 555 nanometers. However, since 555
is not the peak sensitivity wavelength for rods, the scotopic peak is
actually much larger than 1/683. It is closer to 1/329 at 505 nm.

scripting

Commandlevel programming. A sequence of commands in
a shellexecutable
file is called a script. Sometimes the execution of the
commands contained in the script requires other source files, which are
considered to be part of the same overall script. (See Chapter 4 for examples.)

secondary light source

A surface through which or by which large quantities
of light are directed. In Radiance, secondary light sources
are often made into impostors using the illum primitive type. (See Chapter
13 for details.)

server

A program or process that provides system resources
to clients
executing locally or on remote systems over a network.

shadow ray

A ray sent toward a light source to determine
whether a point is in shadow. If the ray intersects some other surface
between the origin (test) point and the light source, then it is (at
least partially) in shadow. To improve shadow and penumbra
accuracy, the light source may be subdivided and each sample point may
be jittered.
(See Chapter 11 for details.)

shadow testing

The process of determining whether points are in
shadow or not.

shell

A command interpreter, such as the Bourne shell or
the Cshell. The job of the shell is to interpret a user's command
line input or the commands read from a shell script.
(See also scripting.)

SI units

Standard International units. A specific recommended
set of metric units for physical quantities such as length, volume,
temperature, mass, force, power, energy, and luminous flux.

sky distribution

The luminance or radiance arriving at a specific point on the earth's
surface at a specific date and time as a function of altitude and azimuth. (See CIE standard sky distribution.)

solid

A region of 3D space, usually closed. (Compare to surface.)

solid angle

A solid slice of space starting from a point and
extending to infinity in a region of directions that can be indicated
by an area on the unit sphere. The size of this area is a measure of
the overall solid angle, which is given in steradians.

source

A Radiance prmitive describing an infinitely distant
light source, described by a direction vector and a solid angle.

spectral power distribution

The density of radiative energy as a function of
wavelength, usually over the visible spectrum. Depending on the
context, this may be given in watts per nanometer or watts per square
meter per nanometer or watts per steradian per square meter per
nanometer.

spectrophotometer

A reflectance meter that partitions the visible
spectrum into sections and measures the reflectance in each section
separately.

spectrum

A continuum or sequence, such as the spectrum of
visible radiation.

specular

Pertaining to highly directional reflection or
transmission. Specular reflection is usually centered about the mirror direction. Specular transmission may be refracted in
a new direction or unperturbed. Specular interactions usually take
place at the surface interface, as opposed to the interior of an object.

specularity

The amount of light reflected (or transmitted) by
specular mechanisms.

sphere

A round shell defined by a center point and a
surface that lies at a constant distance from this center.

spline

A sequence of curves, usually cubic polynomials,
joined to ensure C(2) continuity. This term is sometimes loosely
applied to any set of cubic curves.

steradian

The common unit used to measure solid angle,
equal to a unit area cut in the surface of the unit sphere. Since the
surface area of the unit sphere is 4pi, the maximum solid angle is 4pi
steradians.

stochastic

A stochastic method observes random occurrences
(samples) generated by some probability distribution function. In
practice, digital computers usually do not model ideal stochastic
processes, since most randomnumber generators actually generate a
fixed sequence of values, which may even repeat at some point. (Hence,
they are dubbed pseudorandom number generators.)
Nevertheless, they do a sufficient job for most Monte Carlo
simulation purposes; Monte Carlo is another term for stochastic
modeling. (Compare to deterministic.)

storage complexity

The limiting relationship of storage space (memory
and disk) requirements to the number of objects. It is often given in
the form of "big O" notation, which means "on the order
of." For example, an algorithm that has O(N3) storage complexity
requires storage space proportional to the cube of the number of
objects as N goes to infinity. (See also computational complexity.)

stratified sampling

The subdivision of a sample space into discrete
domains for separate (stochastic) sampling. This method improves
convergence when there is sufficient coherence in the sampled function.

STV (small target visibility)

STV is the weighted average of visibility for a
particular arrangement of view targets on a roadway. (See Chapter 7 for details.)

supersampling

Sending multiple samples to arrive at an average
value for a particular area, such as a pixel. The samples
may be regularly spaced, jittered,
or placed by some other stochastic
process.

surface

A 2D region in 3D space. (Technically, a surface may
reside in any space with two or more dimensions, but in the context of
computer graphics, it is almost always three.) In Radiance,
surfaces are modeled with cones, spheres, and polygons.

surface normal

A normal vector that is perpendicular to the tangent plane of
a surface. If the surface is planar, the surface normal is constant. If the
surface is curved, the normal varies over the surface. A texture is a
perturbation added to the surface normal. A tessellated surface may be smoothed by interpolating normals,
which is called Phong smoothing.

surface of
revolution

A surface defined by a 2D curve spun around a
coplanar axis. (See the genrev manual page on the CDROM
for details.)

synthetic image

A 2D image generated from a computermodeled
environment in a rendering process analogous to photography. See rendering.

tessellated surface

A surface mesh composed of many connected planar
facets, usually used as an approximation to a continuous curved
surface. This approximation may be improved during rendering using Phong
smoothing. (See the gensurf manual page on the CDROM
for details.)

texture

A perturbing function added to a surface normal to yield the
appearance of a macroscopically rough surface. The actual surface still
adheres to the original geometry, so silhouettes will not have the
correct appearance, selfshadowing is neglected, and some reflections
may not intersect the surface properly. Despite these shortcomings,
this approximation usually works very well.

transform

Short for transformation. A sequence of
translations (movements), scalings, and rotations applied to an object
or set of objects. In Radiance, only regular transformations
are allowed, since skewing and anisotropic scaling would fundamentally
change the nature of certain surface primitives. World coordinate unit
changes are implemented as simple scaling transforms.

translator (CAD, image)

A program that converts one data representation into
another. Radiance includes a few different geometric model
(i.e., CAD)
translators, and quite a few image translators. Image translators
usually translate both to and from Radiance picture format,
but CAD translators usually only import data into Radiance.
Sometimes also called a converter.

transmission
coefficient

The fraction of light that is passed per unit
length. This coefficient is given as part of a dielectric
primitive's arguments, and is equal to 1 minus the absorption coefficient.

transmissivity

The fraction of light that passes through the
interior of a glass pane at normal incidence. This does not consider
light lost to reflection by the front or back surface, or multiple
internal reflections. From the transmissivity and the index of refraction,
total transmittance and reflectance can be computed for any incident
angle via a simple formula. (See the CDROM section on materials for
details.)

transmittance

The total light transmitted by a system, usually
given for normal incidence. This is the quantity most easily measured,
although it is not the most convenient to work with mathematically.
(See Section 4.1.2, Calc and Rcalc, in Chapter
4 for information on computing transmissivity from
transmittance.)

umbra

The region in complete shadow, where no portion of
the light source is visible. (Compare to penumbra.)

vector

A direction and magnitude, specified in Cartesian
coordinates as three displacements, one for each axis.

veiling luminance

Light scattered in the lens of the eye that makes it
difficult to see areas immediately surrounding bright sources. (See glare, and the pcond manual page on the CDROM.)

view ray

A primary ray shot from the view point or
measurement origin into the scene to determine the value of a pixel or
to evaluate radiance.

virtual light source

An imaginary light source corresponding to a
reflection or redirection of a real light source in a mirror or prism.
(See Chapter 11 for details.)

void identifier

The word void is a special identifier used to
indicate no modifier for this primitive.

walkthrough

A camera moving through a static scene. See animation.

watt

The power equal to one joule/second. One horsepower
equals 746 watts.

wavelength

The longitudinal distance from one peak to another
on a wave form. For light, this corresponds to the speed of
lightdivided by the radiation frequency. The visible spectrum begins in
the infrared at 780 nanometers and continues into the ultraviolet at
350 nanometers.

workplane

An imaginary horizontal surface situated at the
nominal working height in an interior space.

world coordinates

The coordinate system used for defining points and
distances in a scene description file. It is defined by an origin and three
perpendicular coordinate axes. (The third is determined automatically
by the first two, using the righthand rule.) There is no default unit for world
coordinates in Radiance, though quantities of illumination
are given in SI units. Coordinate conversions are accomplished with the xform command.

worm

A 3D curve of varying thickness, represented by genworm
as a sequence of cones joined by spheres. (See the genworm maual page on the CDROM
for details.)

zbuffer file

An output file from rpict indicating the distance
traveled by each view ray in a rendered picture. This information is
used by pinterp to efficiently interpolate
frames in walkthrough animations. (See the CDROM section on File Formats
for details.)