Glindex - Combined Glossary and Index

Copyright 1997, Olin Lathrop.  See the copyright notice on the main page for details. 

The glindex in the book contains both definitions and page numbers, but the page references were removed for this web version. 

Click on any of the following letters to jump to the section for that letter. 

2D

Two dimensional. 

2D display controller

A type of display controller that is intended for 2D operations, like windows, popup menus, text, etc.  This kind of display controller can usually not interpolate color values within a primitive, and therefore lacks support for 3D drawing. 

2 1/2 D display controller

A 2D display controller that is able to perform the 2D operations required to display 3D primitives.  This usually means color interpolation and Z buffering. 

3D

Three dimensional. 

3D display controller

A type of display controller that fully supports 3D operations and primitives.  At a minimum, such a display controller can accept a 3D triangle, apply a 3D transform, perform the apparent color determination at the vertices, project it onto the 2D bitmap, and draw it with hidden surfaces suppressed.  Today's 3D display controllers use the Z buffer algorithm for hidden surface suppression. 

3D texture

A texture map that is a function of three variables.  This is also called a "solid texture", because the texture map is a volume.  Solid textures have been used in diffuse color texture mapping to simulate things like marble and wood grain.  Sometimes specifying the color in a volume is simpler than specifying it on the surface of an object, especially if the object has a complex shape. 

3D transform

The act of converting the coordinates of a point, vector, etc., from one coordinate space to another.  The term can also refer collectively to the numbers used to describe the mapping of one space on another, which are also called a "3D transformation matrix."

3D transformation matrix

See "3D transform."

- - - - A - - - -

adaptive space subdivision

Space subdivision refers to the process of breaking up the scene space into many small regions.  Adaptive means this is only done where and when needed, instead of in a fixed way up front.  Adaptive space subdivision is often used by ray tracers.  Octrees and BSP trees are examples of subdivision algorithms that can be adaptive. 

addition, vector

See "vector addition."

aliasing

The phenomenon that makes smooth lines and edges appear stair stepped or jagged.  Aliasing is also called the "jaggies". 

aliasing, temporal

See "temporal aliasing."

alpha buffer

The collective name for the alpha values for every pixel of an image or bitmap. 

alpha buffered rendering

Using an alpha buffer for rendering, as opposed to for image compositing or matting.  Alpha buffered rendering implies the ability to render semi-transparent primitives. 

alpha buffering

The process of rendering or compositing images using an alpha buffer.  An alpha buffer supplies an opacity fraction for every pixel. 

alpha value

The alpha buffer value for a single pixel.  An alpha value is a value indicating the pixels opacity.  Zero usually represents totally transparent (invisible) and the maximum value represents completely opaque.  Alpha values are commonly represented in 8 bits, in which case transparent to opaque ranges from 0 to 255. 

ambient light

A light source that shines equally on everything.  This is a hack used to give some illumination to areas that are not in direct view of any light source.  In the real world, such areas are illuminated indirectly by light bouncing off other objects.  Ambient illumination is commonly used except in radiosity rendering, because radiosity actually computes light bouncing between objects. 

AND operator

A constructive solid geometry (CSG) modeling operation on two objects.  The resulting object exists only where both input objects existed.  This operation is also call INTERSECTION. 

animation

Any method that can make an image appear to change over time.  In computer graphics, this is done by showing many still images in rapid succession.  This produces the illusion of a moving, or animated, image. 

animation, inverse dynamics

See "inverse dynamics."

animation, inverse kinematics

See "inverse kinematics."

animation, key frame

See "key frame animation."

animation, parametric

See "parametric animation."

anti-aliasing

The process of reducing aliasing, or jaggies, in creating an image. 

anti-aliasing filter, box

A box filter used in anti-aliasing averages all the samples of a high resolution image within each resulting pixel.  All the samples are weighted equally over a rectangular region, usually the resulting anti-aliased pixel.  Box filtering provides fair to medium quality results, but is much less complex than higher quality methods. 

anti-aliasing filter, good quality

A good quality anti-aliasing filter blends values from a high resolution image such that samples near the resulting pixel center are weighted more than others.  Sample weights smoothly approach zero at a distance of about 1¼ pixels. 

anti-aliasing filter, simple

See "anti-aliasing filter, box."

apparent color determination

See "color determination."

apparent surface orientation

The orientation (which direction it's facing) a surface appears to have in an image.  This is controlled by the shading normal vector, which is not necessarily the same as the normal vector of the primitive as it's actually drawn (the geometric normal vector). 

- - - - B - - - -

B spline

A particular type of spline, the mathematical details of which are beyond the scope of this book. 

back end

See "video back end". 

basis vector

A vector that defines one axis of a coordinate system.  Three basis vectors, one each for the X, Y and Z axis are needed to define a 3D coordinate system.  A basis vector indicates the length and direction of a +1 increment in its axis. 

beam current

The current of an electron beam in a cathode ray tube.  The current is the number of electrons per unit time, which is usually measured in milliamperes.  A higher beam current produces a brighter phosphor dot. 

beta spline

A particular type of spline, the mathematical details of which are beyond the scope of this book. 

bi-cubic patch

A particular type of surface patch.  Bi-cubic surface patches can have curved edges, as opposed to polygons, which always have straight edges.  The mathematical details of bi-cubic patches are beyond the scope of this book. 

bi-quadratic patch

A particular type of surface patch.  Bi-quadratic surface patches can have curved edges, as opposed to polygons, which always have straight edges.  The mathematical details of bi-quadratic patches are beyond the scope of this book. 

bi-linear interpolation

Interpolation is the process of determining plausible in-between values, given explicit values at particular points.  Linear means that the values fall along a line from one known point to the next.  This means the value changes a fixed amount for a fixed-sized step.  Bi-linear means this process is carried out in two dimensions.  In computer graphics, bi-linear interpolation is often applied to find color values at the interior pixels of a primitive.  The apparent color values are computed explicitly at the vertices of a polygon and are bi-linearly interpolated in the polygon's interior.  Bi-linear interpolation of pixel color values is also called Gouraud shading. 

binary space partition

See "BSP tree."

bitmap

The collective name for all the stored pixels in a display controller.  The bitmap is also the interface between the display controller's drawing front end and its video back end.  The term bitmap is also used in general to refer to any 2D array of pixels. 

blobbies

A name sometimes applied to potential functions used in modeling objects. 

blue screen

A background often used for photographs or video that are to be matted, or composited, over other images.  A blue background is almost universally used in chroma keying video compositing. 

boolean operator

A mathematical operator that works on true or false values.  These are also called logical operators.  In computer graphics, constructive solid geometry (CSG) operators may also be called boolean operators.  Some common CSG operators are AND, OR, NOT, and XOR. 

bounding volume

A volume that encloses multiple 3D primitives.  If the bounding volume doesn't intersect an object of interest (such as a ray in ray tracing), then the objects within the bounding volume are guaranteed to also not intersect the object, eliminating the need to check explicitly. 

box filter

See "anti-aliasing filter, box."

BSP tree

A hierarchical method of subdividing a volume.  BSP stands for "binary space partition." In this method, the volume is originally represented as one whole.  If more detail is needed, the volume is subdivided into two volumes.  Each of these are further subdivided into two volumes if more detail is needed.  The process is repeated until the desired level of detail is achieved, or an arbitrary subdivision limit is reached. 

BSP tree, regular

A special form of BSP tree where all the volume elements are rectangular solids that are always subdivided exactly in half along one of their three major axes. 

bump mapping

A form of texture mapping where the texture supplies small perturbations of the shading normal vector. 

- - - - C - - - -

calligraphic

An early type of computer graphics display that could only draw lines, not filled in areas.  Calligraphic displays are rarely used today, and have mostly been replaced by raster displays. 

camera point

See "eye point."

cathode ray tube

A type of vacuum tube that is commonly used as a computer graphics output device.  A thin beam of electrons is shot at a spot on the inside of the tube's face.  The inside of the face is coated with phosphors that emit light when the beam hits them.  The beam is swept in a raster pattern to hit every spot on the screen.  The beam current is modulated to make light and dark areas on the phosphors, forming an image.  Cathode ray tube is usually abbreviated as CRT. 

chroma keying

An image compositing technique commonly used on video signals.  An overlay video signal is selected instead of a background video signal whenever the overlay isn't a particular preset hue, usually blue.  Action shot in front of a blue screen can thereby appear on top of the background signal. 

circular reasoning

See "reasoning, circular."

color determination

The process of figuring out what the apparent color of a particular point on a particular primitive is.  This process is used to answer the question "What color is the object at this pixel?"

color, diffuse

See "diffuse color."

color, emissive

See "emissive color."

color, specular

See "specular color."

color index value

The input value to a color lookup table (LUT) of a display controller's video back end operating in pseudo color mode.  Color index values are also referred to as pseudo colors. 

color lookup table

A table of color values in a display controller's video back end.  In pseudo color mode, it translates the pseudo color values into RGB color values.  In true color mode it becomes three separate tables, one for each red, green, and blue component.  It then translates the red, green, and blue pixel component values to the final displayed red, green and blue component values.  The color lookup table is usually just called the LUT. 

color purity

The degree to which a color CRT can display just one of its three red, green, or blue primary colors without displaying any portion of the other two.  This is a measure of how much each electron gun can only hit the phosphor dots of its color. 

color space

A scheme for describing different shades or colors.  The RGB color space defines a shade as a mixture of specific quantities of red, green, and blue. 

color space, RGB

See "RGB."

color space, IHS

See "IHS."

color wheel

A circular diagram of colors.  The hue varies as the angle within the disc.  Saturation increases from the center outward.  The entire disc is usually shown at the same intensity. 

compositing

The process of combining two images to yield a resulting, or composite, image.  Alpha buffering is a common compositing technique in computer graphics. 

compression

As used in this book, the process of encoding an image that contains redundant information such that it requires less storage.  Runlength and LZW encoding are examples of lossless compression techniques.  JPEG and MPEG are examples of lossy compression techniques. 

compression, JPEG

See "JPEG."

compression, lossless

See "lossless compression."

compression, lossy

See "lossy compression."

compression, LZW

See "LZW compression."

compression, MPEG

See "MPEG."

compression, runlength encoding

See "runlength encoding."

concave

A property of a polygon that has at least one vertex bulge inward instead of outward.  See the text for a more rigorous definition. 

constraint

A rule of an inverse kinematics or inverse dynamics animation system that must be adhered to in solving the motion of all the objects.  For example, a constraint in animating a walking human might be "the lower end of the leg always remains joined to the foot at the ankle."

constructive solid geometry

A modeling technique where complex shapes are built by combinations of simple shapes.  Shapes are combined with boolean operators such as AND, OR, XOR, MINUS, and NOT.  For example, a box with a hole thru it could be defined as box minus cylinder. 

control point

A point used in defining a spline. 

convergence

The degree to which all three electron beams of a color CRT meet at the same spot on the screen.  A poorly converged CRT will show the red, green, and blue components of the image slightly offset from each other.  This makes the image look blurry and produces color fringes around the edges of objects. 

convex

A property of a polygon that bulges outward at all vertices.  See the text for a more rigorous definition. 

convolution

A mathematical operation that applies a weighted average defined by one function onto another function.  This is a very loose definition.  A rigorous detailed one is beyond the scope of this book.  Anti-aliasing is often done with convolutions.  coordinate space Error! Bookmark not defined.  A reference frame that defines numerical values, or coordinates, over an area (2D) or volume (3D). 

cross product

A mathematical operation performed on two vectors, yielding a third vector.  The third vector is always at right angles to the first two. 

CRT

See "cathode ray tube."

CSG

See "constructive solid geometry."

CSG operator

See "boolean operator."

CSG AND operator

See "AND operator."

CSG EXCLUSIVE OR operator

See "XOR operator."

CSG INTERSECTION operator

See "AND operator."

CSG MINUS operator

See "MINUS operator."

CSG NOT operator

See "NOT operator."

CSG OR operator

See "OR operator."

CSG UNION operator

See "OR operator."

CSG XOR operator

See "XOR operator."

curved patch

A primitive used to model a small piece, or patch, of a surface.  A curved patch, as opposed to a polygon, can have edges that are not straight. 

- - - - D - - - -

de-Gauss

An action performed on a CRT monitor to de-magnetize the CRT and any material near it.  CRTs are very sensitive to external magnetic fields.  Such fields can cause alignment, convergence, purity, and image distortion problems.  Most CRT monitors automatically perform de-Gaussing when they are first switched on. 

deflection yoke

Coils mounted on a CRT that are used to steer the electron beam. 

depth buffer

See "Z buffer."

depth buffer rendering

See "Z buffer rendering."

depth value

See "Z value."

diffuse color

An object surface property defined in the Phong lighting model.  An object's diffuse color is reflected equally in all directions.  Visually, this is the not-shiny or dull color. 

diffuse reflection

The light reflected from an object that is reflected equally in all directions.  These are the kind of inter- object reflections that are modeled well by radiosity. 

direct evaluation (of object color)

The process of determining the apparent color of an object at a particular point by direct evaluation of the lighting model, as opposed to interpolating from previously determined values. 

directional light

A light source that shines from the same direction at every point in the scene.  This is a handy shortcut for modeling far away light sources. 

displacement

A distance in a particular direction.  A vector exactly describes a displacement. 

displacement vector

One of the vectors that defines a new coordinate system in terms of an old.  The displacement vector is the vector from the old coordinate system's origin to the new coordinate system's origin. 

display adapter

See "display controller."

display controller

A piece of computer hardware that receives drawing commands from the processor and drives the display.  Some display controllers are commonly called the "video card", "display adapter", "graphics card", or something similar. 

display controller, 2D

See "2D display controller."

display controller, 2 1/2 D

See "2 1/2 D display controller."

display controller, 3D

See "3D display controller."

display controller, GUI engine

See "2D display controller."

dither pattern

The particular pattern of threshold values used in dithering.  Some dither patterns are random, while others are applied as repeating tiles across the whole image. 

dithering

A technique for increasing an image's apparent color (or gray scale) resolution without increasing the number of color (or gray) levels actually used, at the cost of adding a "grainy" look to the image. 

dot pitch

A measure of how closely spaced the phosphor triads are on the face of a color CRT.  The triads are arranged in a hexagonal pattern, and the dot pitch is the distance from the center of one triad to the center of any of its six neighbors.  Dot pitch usually specified in millimeters.  Typical values are from .2 to .3 mm. 

dot product

A mathematical operation of two vectors that produces a scalar.  If both vectors have a length of one (unit vectors), then the dot product is the perpendicular projection of one vector onto the other. 

dot width

The width of a point, or dot, primitive.  Unlike mathematical points, computer graphics point primitives must have finite width to be visible.  Many graphics subsystems allow the application to specify a width for such point primitives. 

DPI

Dots per inch.  This is a common measure of how close individual dots are on some output devices, such as inkjet printers. 

DRAM

Dynamic random access memory.  Most computer main memories are implemented with DRAM. 

drawing front end

See "front end."

dye sublimation printer

See "printer, dye sublimation."

- - - - E - - - -

electron gun

The part of a cathode ray tube (CRT) that emits the electron beam. 

emissive color

An object surface property sometimes used with the Phong lighting model.  An object's emissive color is independent of any illumination.  It therefore appears as if the object were emitting the color. 

even field

See "field, even."

exclusive or

See "XOR operator."

explicit surface modeling

A class of modeling techniques that define objects by providing an explicit list of patches that cover their surfaces.  These surface patches can be either polygons or any of a number of curved surface patch types.  Other modeling techniques work on volumes, or only define an object's surface implicitly. 

eye point

The point, or coordinate, a scene is being viewed from.  This is also called the "eye point" or "camera point". 

eye ray

A ray in ray tracing that originated at the eye point.  All recursively generated rays have an eye ray as their original ancestor. 

eye vector

A vector from anywhere in the scene to the eye point.  Eye vectors are usually unitized before use.  The eye vector is needed in computing the apparent color when the object's surface properties include specular reflection. 

- - - - F - - - -

facet shading

A shading method where the shading normal vector is taken from the geometric normal of the surface actually drawn.  This makes the surface patches visible, especially if they are planar. 

field

Half of a complete video frame when interlacing is used.  The two fields are referred to as the odd and the even.  Each field contains only every other scan line. 

field, even

The first of the two fields that make up a frame in interlaced video. 

field, odd

The second of the two fields that make up a frame in interlaced video. 

film recorder

An computer output device that can write images to film. 

filter, anti-aliasing, box

See "anti-aliasing filter, box."

filter, anti-aliasing, good quality

See "anti-aliasing filter, good quality."

filter, box

See "anti-aliasing filter, box."

filter kernel

The function that defines the relative weight of a point depending on its position.  The relative weight is used in computing a weighted average.  This is actually a convolution operation, which is commonly used in anti-aliasing. 

filtering

This is a broad word which can mean the removal of coffee grinds from the coffee.  However, within the narrow usage of this book, a filtering operation is the same as a convolution operation (see "convolution").  Anti-aliasing is usually done by filtering. 

flat projection

A method of projecting a 3D scene onto a 2D image such that the resulting object sizes are not dependent on their position.  Flat projection can be useful when a constant scale is needed throughout an image, such as in some mechanical drawings. 

flat shading

A shading method where each pixel of a primitive is drawn with the same color. 

form factors

The name for the illumination coupling factors between polygons used in radiosity.  Each form factor indicates how much light from one polygon will reach another polygon. 

fractal

Something that has infinite detail.  You will always see more detail as you magnify a small portion of a fractal.  Mandelbrot set functions are examples of 2D fractals. 

frame

One complete video image.  When interlacing is used, a frame is composed of two fields, each containing only half the scan lines. 

front end

The part of a display controller that receives drawing commands from the processor and writes the primitives into the bitmap. 

- - - - G - - - -

gaze direction

The direction the virtual camera is pointed in the scene description.  The center of the image will display whatever is along the gaze direction from the eye point. 

geometric normal vector

A normal vector of the primitives as they are actually drawn.  This often differs from the normal vector of the surface that was being modeled.  Facet shading results when the shading normal vector is taken from the geometric normal vector. 

GIF

A file format for storing images.  GIF stands for Graphics Interchange format, and is owned by Compuserve, Inc. 

Gouraud shading

See "interpolation, bi-linear."

graftal

A modeling technique where complex shapes are defined as relatively simple, recursive procedures. 

graphic primitive

An object that the graphics system is capable of drawing into the bitmap.  Examples are lines, points, and some polygons. 

graphics card

See "display controller."

GUI engine

See "2D display controller."

- - - - H - - - -

hard copy

A copy of computer data that is directly readable by a human without using the computer.  A printout is a good example of a hard copy. 

HLS

Another name frequently used for the same thing as IHS.  See "IHS."

homogeneous

Constant throughout. 

horizontal refresh rate

See "refresh rate, horizontal."

HSV

Another name frequently used for the same thing as IHS.  See "IHS."

HTML

HyperText Markup Language.  The text formatting language used by documents on the world wide web. 

- - - - I - - - -

IHS

A color space where colors are defined by their intensity, hue, and saturation attributes.  This is sometimes referred to as HSV, which stands for "hue, saturation, value."

image

A two dimensional array of pixels that together form a picture. 

image file

A computer file that contains an image. 

implicit surface modeling

A class of modeling techniques that define an object by its surface, but without explicitly providing primitives that make up the surface.  Examples are potential functions and iso-surfaces. 

ink jet printer

See "printer, ink jet."

internet

The name for the global network connecting many computers to each other. 

interpolation

The mathematical process of determining plausible in-between values, given explicit values at particular points.  Pixel values of polygons are often interpolated from values explicitly calculated at the vertices.  Interpolation is usually much faster than explicitly calculating values. 

inverse dynamics

A method for specifying motion in an animation.  Linkages and other constraints are defined for the objects.  A final state is then specified for some of the objects, and the computer calculates the motion of all the objects so that the final state is reached.  Unlike in inverse kinematics, dynamic properties are taken into account, such as momentum, friction, energy loss in collisions, etc. 

inverse kinematics

A method for specifying motion in an animation.  Linkages and other constraints are defined for the objects.  A final state is then specified for some of the objects, and the computer calculates the motion of all the objects so that the final state is reached. 

iso-surface

An implicit surface that exists wherever a continuous scalar field in a volume is at a particular value (the iso-value). 

interpolation, bi-linear

See "bi-linear interpolation."

INTERSECTION operator

See "AND operator."

- - - - J - - - -

jaggies

See "aliasing."

JAVA

A platform-independent way of defining procedures for use with world wide web documents. 

JPEG

A lossy image file compression technique for still images. 

- - - - K - - - -

key frame

A selected frame of an animation at which all the scene state is defined.  In the key frame animation method, the scene state at key frames is interpolated to create the scene state at the in-between frames. 

key frame animation

An animation control method that works by specifying the complete scene state at selected, or key, frames.  The scene state for the remaining frames is interpolated from the state at the key frames. 

key frame interpolation, linear

- - - - L - - - -

laser printer

See "printer, laser."

left vector

A vector sometimes used to define the virtual camera orientation in a scene description.  This is more typically done with an up vector. 

light, ambient

See "ambient light."

light attenuation

The property of light to get dimmer with distance from the light source.  Real light intensity is proportional to 1/R**2, where R is the distance from the light source. 

light, directional

See "directional light."

light, point

See "point light source."

light, point with 1/R**2 falloff

See "point light source with 1/R**2 falloff."

light ray

A ray in ray tracing that is launched from a point on an object towards a light source.  The ray is used to determine whether the light source is illuminating the point.  If the ray reaches the light source without hitting anything, then the light source is illuminating the point. 

light source

A scene object that illuminates other objects. 

light, spot

See "spot light source."

light vector

A vector from a point on an object towards a light source.  Light vectors are usually unitized before use.  A light vector is needed for each light source in computing the apparent color when the object's surface properties include diffuse or specular reflection. 

linear interpolation

Interpolation is the process of determining plausible in-between values, given explicit values at particular points.  Linear means that the values fall along a line from one known point to the next.  This means the value changes a fixed amount for a fixed-sized step.  Sometimes the term "linear interpolation" is used to refer to "bi-linear interpolation". 

linear shading

See "bi-linear interpolation."

logical operator

See "boolean operator."

lookat point

A point in the scene that will project to the center of the image.  The gaze vector points from the eye point towards a lookat point. 

lossless compression

A compression scheme (see "compression") where all data is preserved.  The data may be compressed and de-compressed any number of times without causing any changes.  The compression ratio of lossless compression schemes is generally lower than that of lossy schemes.  Runlength and LZW encoding are examples of lossless compression schemes. 

lossy compression

A compression scheme (see "compression") where some data may be irreversibly lost, in favor of a high compression ratio.  Many lossy schemes can trade off between amount of loss and the compression ratio.  JPEG and MPEG are examples of lossy compression schemes for images. 

lookup table

See "color lookup table."

LUT

See "color lookup table."

LZW compression

A digital data compression scheme that works by identifying and compressing recurring patterns in the data.  LZW stands for Lempel-Ziv and Welch.  Unisys corporation now claims that LZW compression is covered by its U.S. patent number 4,558,302. 

- - - - M - - - -

mach bands

An optical illusion caused by a sudden change in the rate of change of the brightness (discontinuities in the brightness' second derivative).  This can give the appearance of a light or dark line at the sudden change. 

mandelbrot set

A popular mathematical function that exhibits fractal characteristics. 

material properties

See "surface properties."

MINUS operator

A constructive solid geometry (CSG) modeling operation on two objects.  For the operation "a MINUS b", the resulting object exists wherever A existed without being coincident with B.  This operation can also be expresses "a AND (NOT b)."

modeling

As used in this book, the process of creating a description of an object or scene for the purpose of subsequent rendering. 

modeling, explicit surface

See "explicit surface modeling."

modeling, implicit surface

See "implicit surface modeling"

modeling, level of detail

modeling, polygon

See "polygon modeling."

modeling, potential functions

See "potential function."

modeling, procedural

See "procedural modeling."

modeling, space subdivision

See "space subdivision modeling."

monitor

A piece of computer hardware for the live display of output.  A monitor is different from a CRT, in that a monitor is the complete user-accessible unit that includes a case, power supply, knobs, etc.  Many monitors use a CRT as the main imaging component, but other technologies are also used, such as flat panels. 

MPEG

A lossy image file compression technique for motion pictures or animation sequences. 

- - - - N - - - -

normal vector

A vector pointing straight our from (at right angles to) a surface. 

normal vector, geometric

See "geometric normal vector."

NOT operator

A constructive solid geometry (CSG) modeling operation on one object.  The resulting object exists only where the original object did not. 

NURBS

Non Uniform Rational B-Splines.  A particular type of surface spline for making curved surface patches in modeling complex shapes.  These type of splines are supported by many computer aided design (CAD) systems. 

- - - - O - - - -

object hierarchy ray tracing

See "ray tracing, object hierarchy."

octree

A hierarchical method for subdividing a volume.  In this method, the volume is originally represented as one rectangular box (parallelpiped, to be more precise).  If more detail is needed, the box is split exactly in half along each of its three major dimensions, yielding eight smaller boxes.  This process is repeated on each sub-box until the desired level of detail is achieved, or an arbitrary subdivision limit is reached.  Octrees are the 3D equivalent of quadtrees. 

odd field

See "field, odd."

OpenGL

A 3D graphics procedural interface.  It was developed by Silicon Graphics, Inc., based on their earlier proprietary GL graphics library. 

OR operator

A constructive solid geometry (CSG) modeling operator on two objects.  The resulting object exists where either or both input objects exist.  This operation is also call UNION. 

origin

The point in a coordinate space where all coordinates are zero. 

orthographic projection

See "flat projection."

overlay where not zero

A simple image compositing method where the overlay image results wherever it's not zero, and the background image results wherever the overlay image is zero.  This method is rather simplistic, but is sometimes supported in low-end hardware.  It can be useful in overlaying text, for example.  Alpha buffering is a more general compositing method. 

- - - - P - - - -

palette image format

A format for storing an image that works much like pseudo color in a display controller.  Each pixel contains a color ID instead of the actual color value.  The true color represented by each color ID is defined in a table called the palette, which must also be stored with the image.  A palette is much like a LUT in a pseudo color display controller. 

parametric animation

An animation control method where scene state is determined by mathematical functions or computer procedures that take animation time as an input parameter. 

particle system

A modeling technique where objects are defined by the collective tracks of many individual particles.  Randomness is usually used to determine the details for each particle automatically, although overall guidance is supplied by the user. 

persistence of vision

The property of the human visual system to continue seeing an image a short time (fraction of a second) after it has gone away. 

perspective projection

A method of projecting a 3D scene onto a 2D image such that distant objects are displayed smaller than near ones.  A normal camera produces images using perspective projection. 

phong lighting model

A particular method for computing the apparent color of an object at a particular point. 

phong shading

A shading method where the shading normal vector is interpolated for each pixel, then used in a separate apparent color calculation for that pixel. 

phosphor (or phosphors)

The material coating the inside of a CRT face.  Phosphors have the special property that they emit light when struck by an electron beam. 

phosphor persistence

The property of phosphors to stay lit a short time (fraction of a second) after the electron beam is cut off or moved away. 

phosphor triad

One red, green, and blue phosphor dot on the face of a color CRT.  The dots are arranged in an equilateral triangle.  Triads are arranged in a hexagonal pattern, such that each triad is the same distance from each of its six neighbors. 

pixel

The smallest indivisible unit of a digital image.  A pixel is always the same color throughout.  An image is a two dimensional array of pixels. 

point light source

A light source where all the light comes from one point.  Although real light gets dimmer farther from a light source, a computer graphics point light source shouldn't be assumed to work that way unless explicitly stated. 

point light source with 1/R**2 falloff

A point light source where the light gets dimmer farther from the source.  The light intensity is proportional to 1/R**2, where R is the distance to the light source.  This formula is used because that's how real light works. 

point primitive

A graphic primitive that looks like a dot, or point. 

polygon primitive

A graphic primitive that is an area enclosed by a loop of straight edges.  Triangles and squares are examples of polygon primitives. 

polygon modeling

A modeling method where surfaces are approximated with abutting polygons. 

polyline primitive

A graphic primitive of end-to-end line segments.  A polyline primitive is more efficient than each of the line segments as separate vector primitives. 

potential function

A type of implicit surface.  See the text for details. 

primitive

See "graphics primitive."

primitive, point

See "point primitive."

primitive, polygon

See "polygon primitive."

primitive, polyline

See "polyline primitive."

primitive, quad mesh

See "quad mesh primitive."

primitive, vector

See "vector primitive."

printer

Computer peripherals for producing hard copy on paper and other similar media. 

printer, dye sublimation

A type of printer that works by evaporating controlled amounts of dye from a ribbon onto the page.  The amount of dye can be accurately controlled, yielding continuous color resolution.  Dye sublimation printers are relatively expensive, but produce output comparable in quality to the traditional wet silver photographic process. 

printer, ink jet

A type of printer that shoots tiny droplets of ink onto the page.  Ink jet printers are a relatively low cost way to get computer graphics output. 

printer, laser

A type of printer that works almost like a photocopier.  The image is created by a laser under computer control, instead of coming from an original document as in a photocopier. 

printer, thermal wax

See "printer, wax transfer."

printer, wax transfer

A type of printer that works by depositing small specs of wax from a ribbon onto the page.  The ribbon is pressed against the page, and wax is transferred wherever the ribbon is heated. This type of printer is also called "thermal wax."

procedural model

An object model defined implicitly by a procedure that can produce volume or surface elements. 

projection, flat

See "flat projection."

projection method

A scheme for mapping the 3D scene geometry onto the 2D image. 

projection, orthographic

See "flat projection."

projection, perspective

See "perspective projection."

pseudo color system

A graphics system that stores pseudo colors, instead of true colors, in its bitmap.  Pseudo colors are translated to true colors by the color lookup table (LUT). 

pseudo color value

See "color index value."

- - - - Q - - - -

quad mesh primitive

A graphic primitive that contains a grid of quadrilaterals.  A quad mesh primitive is more efficient than the equivalent separate quadrilateral primitives. 

quadtree

A hierarchical method for subdividing an area.  In this method, the area is originally represented as one box (parallelogram, to be precise).  If more detail is needed, the box is split exactly in half along each of its two major dimensions, yielding four smaller boxes.  This process is repeated on each sub-box until the desired level of detail is achieved, or an arbitrary subdivision limit is reached.  Quadtrees are the 2D equivalent of octrees. 

- - - - R - - - -

radiosity

A rendering method that takes into account diffuse reflection between objects. 

raster scan

The name for the pattern the electron beam sweeps out on a CRT face.  The image is made of closely spaced scan lines, or horizontal sweeps. 

ray casting

A term sometimes used to denote non-recursive ray tracing. 

ray, eye

See "eye ray."

ray, light

See "light ray."

ray tracing

A rendering method that follows rays of light backwards to eventually find what color they are.  Rays may be launched recursively when the color of a point on an object depends on incoming light from specific known directions.  For example, when a ray hits a reflective object, a recursive ray is launched in the direction the reflected light is coming from. 

ray tracing, object hierarchy

A ray tracing speedup method where objects are kept track of in groups.  These groups can be further grouped in a hierarchy.  A bounding volume is maintained for each group in the hierarchy.  If a ray doesn't intersect a bounding volume, then it definitely doesn't intersect any subordinate object in the hierarchy. 

ray tracing, space subdivision

A ray tracing speedup method where the scene space is subdivided into blocks.  A list is kept for each block indicating which objects a ray could hit in that block.  As a ray is traced, it is walked thru the blocks, checking for intersection only with the objects listed in each block. 

ray tracing speed issues

reasoning, circular

See "circular reasoning."

recursive ray tracing

See "ray tracing."

reflection vector

A vector used in the phong lighting model to compute the specular reflection.  It is usually unitized, and points in the direction light is reflecting off the object. 

refresh rate

The rate at which parts of the image on a CRT are re-painted, or refreshed.  The horizontal refresh rate is the rate at which individual scan lines are drawn.  The vertical refresh rate is the rate at which fields are drawn in interlaced mode, or whole frames are drawn in non-interlaced mode. 

refresh rate, horizontal

The rate at which scan lines are drawn when the image on a CRT is re-drawn, or refreshed. 

refresh rate, vertical

The rate at which fields are re-drawn on a CRT when in interlaced mode, or the rate at which the whole image is re-drawn when in non-interlaced mode. 

regular BSP tree

See "BSP tree, regular."

rendering

The process of deriving an 2D image from the 3D scene description.  This is basically the "drawing" step. 

resolution

The measure of how closely spaced the pixels are in a displayed image.  For example, if 1,024 pixels are displayed across a screen that is 12 inches wide, then the image has a resolution of 85 pixels per inch. 

RGB

A color space where colors are defined as mixtures of the three additive primary colors red, green, and blue. 

right vector

A vector sometimes used to define the virtual camera orientation in a scene description.  This is more typically done with an up vector. 

runlength encoding

A lossless digital data compression scheme.  It identifies identical consecutive values, and replaces them with just one copy of the value and a repeat count. 

- - - - S - - - -

scalar

A regular, single number, as opposed to a vector. 

scan line

One line in a raster scan.  Also used to mean one horizontal row of pixels. 

scan line order

A way of arranging image data so that all the pixels for one scan line are stored or transmitted before the pixels for the next scan line. 

scan rate

See "refresh rate."

scattered light

See "secondary scatter."

scene

The complete 3D description of everything needed to render an image.  This includes all the object models, the light sources, and the viewing geometry. 

secondary scatter

Light that is reflected from a non-emitting object that illuminates other objects.  This is the kind of inter- object illumination that is computed by radiosity. 

shading

This term is used several ways in computer graphics.  However, shading always has something to do with figuring out pixel color values, as opposed to figuring out the geometry or which pixels are to be drawn. 

shading, bi-linear

See "bi-linear interpolation."

shading, facet

See "facet shading."

shading, flat

See "flat shading."

shading, Gouraud

See "bi-linear interpolation."

shading, linear

See "bi-linear interpolation."

shading normal vector

The normal vector used in determining the apparent color of an object.  In facet shading, the shading normal is the geometric normal of the primitives used to model the surface.  In smooth shading, the shading normal is the normal vector of the surface that was modeled.  The shading normal vector may be further modified by bump mapping. 

shadow mask

A thin layer in a color CRT that the electron beams can not penetrate.  It is suspended just in front of (from the electron beam's point of view) the phosphor screen.  The shadow mask has one hole for each phosphor color triad.  Due to the position of the phosphor triads, the shadow mask holes, and the angle of the three electron beams, each electron beam can only hit the phosphor dots of its assigned color. 

SIGGRAPH

The Special Interest Group on Graphics of the Association for Computing Machinery (ACM).  SIGGRAPH is the premier professional association for computer graphics. 

smooth shading

Any shading technique that attempts to make the rendered surface look as close as possible to what was modeled, instead of the primitives used to approximate that model.  This is usually done by taking the shading normal vector from modeled surface, instead of from the geometric normal vector. 

solid texture

See "3D texture."

space subdivision, adaptive

See "adaptive space subdivision."

space subdivision modeling

A modeling technique where the scene is broken into small regions of space.  A list of the objects present is kept for each region.  Examples of space subdivision modeling are BSP trees and octrees. 

space subdivision ray tracing

See "ray tracing, space subdivision."

specular color

The color of an object's shiny highlights.  Specular reflection also depends on the specular exponent.  Both these parameters are part of the object's surface properties. 

specular exponent

This is a scalar value that controls the "tightness" of the specular highlights.  A value of zero causes the specular color to be reflected equally in all direction, just like the diffuse color.  An infinite value causes it to be reflected the same way a mirror would.  Common values are about 5 to 60. 

spline, B

See "B spline."

spline, beta

See "beta spline."

spline patch

A curved surface which takes its shape from the placement of a set of control points. 

spot light source

A light source that does not shine in all directions, usually in a cone.  This is a convenient hack for modeling lamps with shades or reflectors, without actually having to compute the effect of the shade or reflector during rendering. 

subpixel

An input pixel to an operation that uses multiple smaller pixels at higher resolution to compute each resulting pixel at the final resolution.  This is done, for example, in an anti-aliasing filtering operation. 

surface orientation, apparent

See "apparent surface orientation."

surface properties

The collective name for all the object-specific parameters that are used to compute the apparent color of a point on that object.  These include the diffuse color, specular color, etc.  The term "surface properties" is common but not standard.  Other names for the same thing are "visual properties", or "material properties". 

- - - - T - - - -

temporal aliasing

Aliasing in time by animation frame, instead of spatially by pixel.  The visual effect of temporal aliasing has been called "strobing".  Moving objects appear to jump thru a sequence of frozen steps, instead of in smooth paths. 

tessellation level

The relative fineness or granularity of the patches used to model a surface.  A model with smaller, and therefore more patches is said to have a higher tessellation level. 

tessellation

The act of tiling a surface with individual surface patches. 

texil

One pixel of a texture map, where the texture map is an image.  In diffuse color texture mapping, the texture value is a color.  The texture color values are therefore often supplied as an image.  A texil is one pixel of this image, as opposed to one pixel of the final image being rendered. 

texture, 3D

See "3D texture."

texture map

The external function that supplies the texture value in texture mapping.  In diffuse color texture mapping, the texture map is a set of color values in two dimensions.  This is usually specified as an image.  Note, however, that not all texture maps are images, because not all textures are two dimensional color values.  Bump mapping is a form of texture mapping where the texture is not an image, since the texture values are shading normal vector perturbations instead of colors. 

texture mapping

The process where some parameter of the apparent color computation is replaced by an external function, called the texture map.  A common example is texture mapping an object's diffuse color from an image.  This gives the appearance of the image painted onto the object.  Bump mapping is another form of texture mapping discussed in this book. 

texture mapping, diffuse color

A form of texture mapping where the texture defines the object's diffuse color.  This gives the appearance of pasting the texture image onto the object. 

texture mapping, normal vector perturbations

See "bump mapping."

thermal wax printer

See "printer, wax transfer."

TIFF

Tag Image File Format.  A common image file format.  Just about all applications that can import image files support the TIFF format. 

toner

The material that is deposited onto the page to form the image in a photocopier or laser printer. 

transform, 3D

See "3D transform."

transformation matrix, 3D

See "3D transform."

transparency value

See "alpha value."

triad

See "phosphor triad."

triangle strip primitive

A graphic primitive that contains a set of successively abutting triangles.  A triangle strip primitive is more efficient that the equivalent separate triangle primitives.  It is also called a "Tstrip."

true color system

A graphics system that stores true colors, as opposed to pseudo colors, in its bitmap. 

Tstrip primitive

See "triangle strip primitive."

- - - - U - - - -

UNION operator

See "OR operator."

unit vector

A vector of length, or magnitude, one. 

unitize

To make a vector a unit vector.  This means adjusting its length to be one without effecting the direction it's pointing. 

up vector

A vector used to define the orientation of the virtual camera in a scene description. 

- - - - V - - - -

vector

A value that has a direction and a magnitude (length).  Plain numbers can be called "scalars" to explicitly distinguish them from vectors. 

vector addition

The process of adding two vectors, which results in another vector. 

vector, basis

See "basis vector."

vector cross product

See "cross product."

vector dot product

See "dot product."

vector, eye

See "eye vector."

vector, geometric normal

See "geometric normal vector."

vector, left

See "left vector."

vector, light

See "light vector."

vector primitive

A graphics primitive that looks like a line segment.  Unlike mathematical line segments, vector primitives have finite widths. 

vector, reflection

See "reflection vector."

vector, right

See "right vector."

vector scaling

An operation of a vector and a scalar.  The resulting vector is the same as the input vector, except that its magnitude (length) is multiplied by the scalar.  Scaling a vector by the reciprocal of its magnitude results in a unit vector. 

vector, shading normal

See "shading normal vector."

vector thickness

The width of a vector, or line segment, primitive.  Unlike mathematical line segments, computer graphics vector primitives must have a finite width to be visible.  Many graphics systems allow the application to specify a width for such primitives. 

vector, unit

See "unit vector."

vector unitize

See "unitize."

vector, up

See "up vector."

vertical refresh rate

See "refresh rate, vertical."

video back end

The part of a display controller that reads the pixel data in the bitmap and produces the live video signals.  Among other components, the back end contains the color lookup tables (LUTs), the digital to analog converters (DACs), and the logic to generate the video timing signals. 

video card

See "display controller."

view direction

See "gaze direction."

view point

See "eye point."

virtual reality

A name loosely applied to systems that attempt to immerse the user in a virtual world generated by the computer.  This is often done with a stereoscopic display that is updated based on head position, and usually includes some sort of 3D pointing device.  More advanced systems include 3D sound and some form of touch feedback.  This is still a rapidly changing area. 

visual properties

See "surface properties."

voxel

A unit of subdivided space.  Voxel stands for "volume element".  Nodes in an octree, for example, are referred to as voxels. 

VRAM

Video random access memory.  This is really DRAM with additional features specifically for use as bitmap memory in display controllers.  VRAM typically costs twice as much as DRAM, but allows the drawing engine full access to the bitmap independent from the video back end.  This can increase the hardware drawing rate. 

VRML

Virtual Reality Modeling Language.  The standard description language for 3D graphics in world wide web documents. 

- - - - W - - - -

wax transfer printer

See "printer, wax transfer."

web browser

Software that can read and display world wide web documents. 

wire frame

A rendering method where only the outline of objects and primitives are drawn. 

world wide web

A set of interconnected documents on the internet that adhere to the HTML standard.  The world wide web is often abbreviated to WWW. 

WWW

See "world wide web."

- - - - X - - - -

XOR operator

A constructive solid geometry (CSG) modeling operation on two objects.  The resulting object exists where each object existed alone, not coincident with each other.  XOR is an abbreviation for "exclusive or."

X Windows

A window management and 2D graphics environment that is very common on Unix systems. 

- - - - Y - - - -

- - - - Z - - - -

Z buffer

The collective name for all the Z values in a bitmap. 

Z buffer rendering

A rendering method where an additional depth, or Z, value is kept per pixel.  A pixel is only overwritten by a new primitive if the new Z value represents a shorter distance, or depth, from the eye point.  The end result is an image of the front most surface of the front most objects. 

Z value

The name for the value that represents distance from the eye point in a Z buffer rendering system.