Rendering with Radiance

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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.
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.
A method that is fully described in a procedure or computer program.
In a Radiance scene description, an alias associates a new identifier (and possibly a new modifier) with a previously defined primitive.
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 CD-ROM 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 (walk-through, scene animation)
A walk-through 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 walk-through animation. Also called the camera path. (See Chapter 9 for details.)
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.
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.
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.)
The American Standard for Character Information Interchange-the standard encoding of alphanumeric and control characters as integers between 0 and 127.
The angle about the surface normal relative to some standard direction, such as south.
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 (B-rep)
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.)
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 reflectance-transmittance 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.
Computer-aided design or Computer-aided drafting. CAD tools are typically used to interactively create and describe geometric entities.
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 right-hand Cartesian coordinate system, where the z-axis is in the direction of the thumb of the right hand when the fingers are curled in the direction of the x-axis, and then the y-axis.
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.
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 1-degree 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 1-degree 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.)
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 in-line 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 in-line 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 file-matching 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.)
In Radiance, the family of cones includes right-angle 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.
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.
See translator.
Central processing unit. The processor in a computer, usually consisting of registers, an arithmetic logic unit (ALU), memory-addressing logic, and (in most cases) a floating-point 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 N-dimensional space as a linearly interpolated table of discrete values. (See the CD-ROM 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 ray-sampling 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.)
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.
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.
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.
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.
Non-Lambertian reflection or transmission, in which light is scattered in all directions, but favors some directions over others. (See also rough specular.)
The tendency of light to scatter spatially when refracted through a dielectric medium whose index of refraction varies with wavelength. (See also Hartmann constant.)
Any presentation method, such as a graphics monitor, transparency, or print. The independent color channels available on a display are called the display primaries.
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.)
A surface that emits light. See light source.
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.)
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.)
A window system, including vertical or horizontal blinds, multiple glass panes, and any associated frames or obstructions.
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 CD-ROM section on File Formats for file syntax and semantics.)
The high-resolution 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 CD-ROM 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 computer-generated 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 bell-shaped curve). If the microfacet slope of a surface is normally distributed, it is said to be a Gaussian surface. (See also roughness.)
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.
The glass associated with a fenestration system, which may be single-, double-, or triple-paned 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.
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 (10-9 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.
Heyney-Greenstein 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 forward-scattering medium. (See Chapter 14 for formula and details.)
A tree of objects, such as a scene description file that loads in other scene files that load other files in turn via in-line 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].
The Illuminating Engineering Society of North America, a professional organization that sets standard practices for lighting design in the U.S. and Canada.
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.)
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 black-and-white image from a mathematical description of a scene. More commonly called rendering.
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.)
in-line 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 new-line character, but if the new-line character is preceded by a backslash (\), then the command continues on the next line. In-line commands may not read from their standard input, and their standard output must contain only valid Radiance scene description primitives, aliases, comments, and other in-line 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 new-line 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.
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.)
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.
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.
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.
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.)
A light fixture, including any and all lamps, ballasts, reflectors, lenses, baffles, and so on.
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.
Shorthand for lumens per square meter. (See illuminance.)
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.
See tessellated surface.
An intermediate, device-independent binary file that contains 2D vector graphics, used for plotting to various supported output devices. (See the metafile(5) manual page on the CD-ROM.)
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.
A Radiance primitive type for participating media.
A category of Radiance primitives that provides for mixing two different patterns, textures, materials, or mixtures.
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.
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.
Straight down, usually used for reference in the output of a downward-facing luminaire.
The usual unit of measurement for the wavelength of visible radiation, equal to 10-9 meters. Often abbreviated as nm.
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.)
Shorthand for candelas per square meter. (See luminance.)
normal vector
A unit vector. (See surface normal.)
Acronym for Non-Uniform Rational B-Spline, a popular class of rational cubic patches used widely for the geometric modeling of smooth, curved surfaces.
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 CD-ROM section on File Formats for details.)
overture calculation
An initial low-resolution pass over an image to collect indirect irradiance values into an ambient file for later high-resolution rendering. This generally improves the quality of the final results at a modest expense.
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.)
A right angle, rectangular prism. That is, a closed, convex, six-sided 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.
A variation in surface color, which can be described in Radiance as a procedure, a picture or tabulated data.
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 directional-diffuse), computed by raising the cosine of the angle between the half-vector and the surface normal to a power. (The half-vector 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.
A device for measuring visible radiation. Typically divided into two categories: illuminance meters and luminance meters.
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.
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 floating-point color image, which usually contains physical radiance values useful for lighting analysis. (See the CD-ROM section on File Formats for details.)
In UNIX, a "first-in, first-out" (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.
Multiple UNIX commands connected together via pipes.
Short for picture element. The smallest measured, calculated, or displayed point in an image.
Lying in a plane. Completely flat.
polar angle
The angle between the surface normal and the given vector direction. (Compate to altitude.)
An N-sided planar surface with a finite area. A convex polygon has no interior angles greater than 180 degrees.
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 CD-ROM for syntax and semantics.)
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.)
A running program on a system.
A computational unit. See CPU.
projected hemisphere
A half-sphere, 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.
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.)
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.
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.
Random access memory, usually referred to simply as memory.
ray tracing
A method based on following one-dimensional 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 light-forwards ray tracing, light is followed from the light sources to the final measurement areas. In light-backwards 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.
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(n-1)!), 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 ray-tracing programs fall into this general category.


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.
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.
The process of creating a 2D image from a 3D representation. Also, the term given to the resulting synthetic image.
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.
right-hand rule
If the right hand points in the direction of the first vector (i.e., the x-axis) and the fingers curl in the direction of the second vector (i.e., the y-axis), the thumb points in the direction of the third vector (i.e., the z-axis). (See also Cartesian coordinate system.)
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 directional-diffuse component caused by specular (surface interface) reflection or transmission from a rough surface.
run-length encoding
A simple data-stream 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.
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 CD-ROM 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.
Command-level programming. A sequence of commands in a shell-executable 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.)
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.
A command interpreter, such as the Bourne shell or the C-shell. 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.)
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.
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.
A reflectance meter that partitions the visible spectrum into sections and measures the reflectance in each section separately.
A continuum or sequence, such as the spectrum of visible radiation.
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.
The amount of light reflected (or transmitted) by specular mechanisms.
A round shell defined by a center point and a surface that lies at a constant distance from this center.
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.
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.
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 random-number generators actually generate a fixed sequence of values, which may even repeat at some point. (Hence, they are dubbed pseudo-random 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.)
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.
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 CD-ROM for details.)
synthetic image
A 2D image generated from a computer-modeled 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 CD-ROM for details.)
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, self-shadowing is neglected, and some reflections may not intersect the surface properly. Despite these shortcomings, this approximation usually works very well.
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.
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 CD-ROM section on materials for details.)
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.)
The region in complete shadow, where no portion of the light source is visible. (Compare to penumbra.)
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 CD-ROM.)
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.
A camera moving through a static scene. See animation.
The power equal to one joule/second. One horsepower equals 746 watts.
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.
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 right-hand 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.
A 3D curve of varying thickness, represented by genworm as a sequence of cones joined by spheres. (See the genworm maual page on the CD-ROM for details.)
z-buffer 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 walk-through animations. (See the CD-ROM section on File Formats for details.)