CG Dictionary.

[Hundred]

To Everyone: Please note that much of this data is rather old, and many of these terms may no longer be in use. The primary source from which I obtained the majority of this information, was put out (online) in the year 2000. I believe. A lot of edits and updates must be made, as new terms have, no doubt, been introduced into the 3D graphics world since that time. However from the amount of Googling I did yesterday afternoon, Edinburgh's Online Graphics Dictionary, appeared to be the broadest and most comprehensive source availible. Which is why I posted it. In any case, I will endeavor to update and amend the information, as well as the PDF, as quickly as possible.

And as always, to the Moderators and Administrators. If the thread is in any wise offensive and or inappropriate. Or if such a thread already exists. Please delete it.

Thank you, .


#

2D
Two-dimensional

3D
Three-dimensional


A.

A-buffering
An antialiased extension to Z-buffering. An A-buffer identifies visible segments within a sub-pixel area which are represented with bit masks and area sampled for pixel intensity. The technique employs logical operations on the bit masks and thus avoids floating point geometry calculations.

Achromatic
Light without color. The quantity of light is the only attribute associated with achromatic light. In physical terms this is the intensity or luminance or in the psychological sense it is the perceived intensity in which case the term brightness is used. In the YIQ or YUV representations, this is the Y component. In the HSV representations, it is the V (value) component. In the HSL representations, it is the L (lightness or intensity) component.

Adaptive forward differencing
An efficient way to evaluate parametric functions describing curves or surfaces. Each value of the function is determined as the sum of the previous value and a difference term. The distance between points at which the function is evaluated is adapted to the flatness of the function. The value can be a vector as well as a scalar, and this is useful for calculating B-splines.

Adaptive sampling
Adaptive sampling is a method of reducing aliasing artifacts when rendering by adapting the sampling rate in response to the local characteristic of the object being rendered. This technique is often useful to reduce the jagged edges at the edges of objects (or jaggies).

Adaptive subdivision
A paradigm for representing data in a hierarchical manner by repeatedly dividing and classifying it until no further definition is necessary, given an error tolerance. E.g. see octree and quadtree.

Additive color model
In an additive color model, colors are defined as a sum of contributions from primary colors. The most commonly used additive color model is the Red-Green-Blue model.

Affine map/transform
A geometrical transformation from an affine space to another. In computer graphics it is used for 2D spaces (image) or 3D spaces (3D image). It is defined by : p' = M p, where M is a real matrix and p is a real vector. Affine transformations preserve parallelism.

Algebraic surface
A surface defined by the set of points for which an algebraic function is equal to a constant value.

Alpha blending/alpha-channel compositing
A technique for computing the color of a pixel when multiple structures contribute to the pixel ( e.g. at a region boundary where we want to avoid aliasing problems arising from partial pixel coverage or transparency). Alpha is the percent of a pixel covered by a given structure, and can be used as part of the color description of every pixel associated with a structure. Computing the resulting color of a combined pixel uses the alpha values of the source pixels, plus other information, such as any relation between the surfaces.

Alpha channel
The collection of alpha values associated with an image where each alpha value represents the coverage of each pixel in the image. The alpha values are used in the process of alpha blending.

Ambient lighting
A global (artificial) illumination level representing infinite diffuse reflections from all surfaces within a scene ensuring that all surfaces are visible (lit) particularly those without direct illumination. Ambient lighting is usually treated as a constant in local shading functions but is simulated directly in radiosity calculations.

Anaglyph
A stereoscopic picture consisting of two images of the same object, taken from slightly different angles, in two complementary colors. When viewed through colored spectacles, the images merge to produce a stereoscopic sensation.

Animation
(1) A medium that provides the illusion of a moving scene using a sequence of still images. (2) Techniques used in the production of animated films. In computer graphics this primarily concerns controlling the motion of computer models and the camera.

Anisotropic filtering
Image filtering that produces different amounts filtering (e.g. smoothing filtering in different directions at each pixel in an image. Two uses of anisotropic filtering in graphics are to: 1) produce textures with different spatial frequency distributions in different directions and 2) to reduce aliasing effects along edges without blurring the edges as much. Anisotropic filtering can be done in either the image or the frequency domains.

Antialiasing
Antialiasing is a method of reducing or preventing aliasing artifacts when rendering by using color information to simulate higher screen resolutions. In one technique, blurred pixels are introduced by filtering the image, or individual elements, to remove spatial frequencies that are greater than the pixel sample rate by convolution. If high frequencies remain they may cause other visual artifacts such as Moiré patterns. An alternative and often preferable technique is supersampling, where many samples per pixel are estimated and combined.

Artifacts/Artefact
A classifiable visual error. E.g., a loss of resolution when zooming into an image or incorrect depth sorting due to the painter's algorithm.

Atmosphere effects
Atmospheric effects arise because light is affected by the properties of the medium through which it passes. The main effects are attenuation, where distant objects get lower contrast (see depth cueing) and blurring, such as might occur with dust, fog or haze, which scatters the light.

Attenuation
1. Atmospheric attenuation: the simulation of the atmospheric attenuation from the object to the viewer which affects both the illumination strength and color.

Augmented reality
The idea that an observer's experience of an environment can be augmented with computer generated information. Usually this refers to a system in which computer graphics are overlaid onto a live video picture or projected onto a transparent screen as in a head-up display.

The attachment below is a printable PDF of the online dictionary, from where I obtained much of this information. However please note that much of this data is rather old, and many of these terms may no longer be in use. I will endeavor to update and amend the information, as well as the PDF, as quickly as possible. Enjoy, .

[Samurai]

I'm sure Nightshadow did something like this pre-crash a couple of years back....

[Hundred]

B.

B-spline
A multi-segment spline curve representation based on local polynomials having continuity of curve orientation and curvature at the points (knots) where different segments join. Cubic b-splines are popular, but linear, quadratic, quartic, etc. splines are also used. The B in B-spline stands for basis, because the b-spline segments are formed from the weighted sum of four local basis functions. The local shape of the spline segment is controlled by four control points; in the case of b-splines, these control points do not lie on the curve itself ( i.e. b-splines are not interpolating). One important advantage of b-splines is that the movement of a control point affects only four segments of the curve. B-spline surfaces can be defined from b-spline curves lying in both directions on the surface. Here, 16 non-interpolated control points are needed per patch, but each patch then has tangent and curvature continuity where it joins a neighboring patch. The b-spline is defined over a uniform parameter domain, and is evaluated as a simple polynomial function. More complex forms, such as NURBS, relax these assumptions.

Backfacing polygons
Polygons whose surface normals point away from the camera position, which can be easily tested by the dot product of the polygon surface normal n and the ray v from the viewer to the polygon.

Background color
The intensity level of pixels which are not intersected by any of the displayed surfaces.

Backwards ray tracing
Backwards ray tracing is used to render a scene on a view plane by tracing imaginary "eye rays" from the viewer's eye to the surface of the objects in a scene, to determine the objects' visibility. A grid on the view plane is used to cast eye rays from the center of projection (the viewer's eye). It is convenient for the grid to correspond to the pixels of the display screen. For every pixel on the view plane, an eye ray is cast from the center of projection, through the center of the pixel and into the scene. The pixel's color is determined by the eye ray's point of first intersection with an object in the scene.

The basic backwards ray tracing algorithm can be extended to render shadows in a scene. This extension involves firing an additional ray from the first point of intersection to each of the scene's light sources. If the ray intersects with an object on its path to the light source, then the point of first intersection is in shadow for that light source. The combination of the effect of each ray to the light source determines the first intersection point's color.

Basis spline
A spline curve or surface that can be formulated as a weighted sum of polynomial basis functions. Commonly known as a B-Spline.

Beam tracing
Beam tracing is a method of rendering similar to ray-tracing but using an arbitrarily shaped projection, commonly a polygonal cone, rather than a single ray. It is an improvement on ray-tracing since it reduces the CPU overhead and reduces aliasing artifacts by taking advantage of known spatial coherence in the beam.

Bézier curve
A spline curve that (in the usual case of a cubic Bézier curve) is represented by four control points defining a cubic polynomial.

Bicubic surface
A type of parametric two variable polynomial surface patch where the polynomials are cubic in both parameters.

Bilinear filtering
An averaging technique applied to the color values of adjacent pixels so that textures look smooth rather than blocky. It aims to make the texture looks more realistic

Bilinear interpolation
An algorithm for interpolating image data in order to estimate the intensity or color of the image in between pixel centers. The interpolated value is calculated as a weighted sum of the neighboring pixel values.

Binary space partition tree
A method for representing a polyhedron that explicitly uses the planes that bound the polyhedron. The technique represents the object as a binary tree, with planes at each non-leaf node. The planes bound a face of the polyhedron, and divide space into two subregions, which in turn can be further bounded by the two children at a node. Leaf nodes (at the edge of the tree) are either completely object or completely free space. A similar idea can be used in 2D for representing polygons. This representation is useful for hidden surface removal and point classification (determining whether a point is inside or outside the object).

BitBlt/RasterOp
An abbreviation of bit block transfer. This is an efficient technique for copying rectangular arrays of pixels that exploits the fact that computer memory is organized into multi-bit words.

Bitmap (Amended. Source: Wikipedia)
In computer graphics, a bitmap or pixmap is a type of memory organization or image file format used to store digital images. The term bitmap comes from the computer programming terminology, meaning just a map of bits, a spatially mapped array of bits. Now, along with pixmap, it commonly refers to the similar concept of a spatially mapped array of pixels. Raster images in general may be referred to as bitmaps or pixmaps, whether synthetic or photographic, in files or in memory.

In some contexts, the term bitmap implies one bit per pixel, while pixmap is used for images with multiple bits per pixel.

Bits per pixel
The number of bits used to describe the color or intensity of a pixel. For example, using 8 bits for to store a value from the RGB color model would permit 3 bits to be used for both red and green values and 2 bits for the blue value. Blue gets a smaller range because the human eye contains less blue cones and is thus is less sensitive to blue variations.

Blend surface
A surface added to two or more others surfaces to provide a continuous join between them.

Blitter
A blitter is a special-purpose chip or hardware system used for fast implementations of bitmapped graphics. Blitters are used to copy sections of video memory from one place to another. During the copy operation several source areas may be used and logical operations may be performed on them. One application of blitters is the provision of fast animated graphics, known as sprites.

Boundary representation/B-rep
A paradigm for representing graphical data in terms of the boundaries of the objects involved. E.g., representing a cube as a collection of bounding faces, or a polygon by its edges.

Bounding box/volume
The smallest regular shaped box that encloses an object, usually rectangular in shape.

Bounding boxes are used to accelerate tests such as visibility or ray-object intersection by providing a pre-test which can eliminate many cases.

Bresenham's algorithm
A technique developed in the framework of raster graphics for generating lines and circles. These algorithms use only integer arithmetic, avoid rounding and perform an iterative computation of the primitive points by approximating the distance to the nearest pixel center along either the x or y axis. These characteristics make for efficient algorithms.

Brightness
The perceived intensity of a radiating object.

Bump mapping
A technique used to increase the realism of a surface by changing how light reflects from that surface. Usually, the surface normal at a given point on a surface is used in the calculation of the brightness of the surface at that point.

[Hundred]

C.

CAD
Abbreviation of Computer Aided Design. In the context of graphics, CAD refers to the use of computer based models of objects for visualization or testing as an aid in the design process.

Camera
A virtual viewpoint in world space with position and view direction to provide a view of a scene in the same way as a photographer would position a camera.

Candela
Derived from candle and denoted by the symbol "cd", it is the basic SI unit of luminous intensity. It is defined as the radiation intensity, in a perpendicular direction, of a surface of 1/600000 square meter of a black body at the temperature of freezing platinum under a pressure of 101325 newtons per square meter.

Canvas
A two-dimensional region of graphics information. The canvas may be displayed on screen or be recorded in off-screen display memory.

Cartesian coordinates
A common system of representing a point in two or more dimensions using an ordered set corresponding to its projection on a spanning orthogonal base set. Commonly encountered Cartesian coordinate systems are the XY plane 2D coordinate system, (row,column) 2D image coordinate system and XYZ 3D scene coordinate system. 3D coordinates in graphics are usually specified with x and y being aligned with x and y on the screen, +x is to the right, +y is upwards, and +z goes into the space 'behind' the screen. This is a left-handed coordinate system with the property that most z-values are thereby positive. It's why z-buffers are called z-buffers when they are actually depth-value buffers.

CAVE
An immersive virtual environment where the viewer stands inside a room upon whose walls are projected images. The images may be in stereo requiring stereo shutter glasses to be worn. The name CAVE comes from Computer Augmented Virtual Environment.

Caustic
The effect given when light is transmitted through a specular surface and then strikes a diffuse surface. If the specular surface is of high curvature the light will be tend to be focused. When this effect is taken into account, rendered scenes involving liquids or glass are much more photorealistic. Caustics can also arise when light is reflected from a specular surface. The classic example is the caustic on the surface of a liquid. Refraction may make it happen too. The caustic shape is the envelope of the reflected rays.

Center of projection/viewpoint
Part of the model representing the the projection from a 3D space (the world) to 2D planar space (the image). It is the point of intersection of all the straight projection rays emanating from the object points in the 3D space and intersecting the projection plane to form the projection.

Chroma
1) A characterization of how much a color differs from both the pure color and the grey of the same intensity. Also called saturation. 2) The color component of a composite video signal.

Chromaticity coordinates/tristimulus coordinates
Chromaticity coordinates are based on the Commission Internationale de l'Éclairage (CIE) color scheme, which uses three standard (but physically unrealizable) primary colors called X, Y and Z. (These are different from red, green and blue, and are chosen to represent human color matching performance.)

Chrominance
Information describing hue, or the color components orthogonal to the brightness. YUV and YIQ are chrominance/luminance color models.

Clipping
The selective removal of an object disjoint with the display area or the non-visible parts of an object that does intersect the display area. Parts of an object intersecting the display area may lie outside of the display area or be partially or fully obscured by another intersecting object.

Collision detection
Collision detection is used in a virtual environment to monitor the relative locations of solid objects. If the virtual environment manager detects that the proximity of two or more objects is sufficiently close, a collision event occurs. As a result of this event the object's movement can be controlled so their surfaces do not intersect. In an environment which models a natural system, the kinetic energy of a moving object is (partially) transferred to the object it collides with, making the second object move.

Color keying/chroma keying
Using the pixel color of one image to designate that pixel data from another image should replace the first pixel's color. The first image might be a binary image, which would select regions of interest from the second image. Another use is in blue-screening, where an actor works against a blue background. In the output image, the blue pixels get replaced by another image. For example, a weather map can be placed behind the weather presenter who is actually standing in front of a blue screen.

Color models
A color model is a method of specifying a color (position) in color space, often using a co-ordinate system. Examples include RGB and the Munsell Color System.

Color space
A mathematical space defining a range and encoding of colors. E.g. see RGB, LUV, HSV, HSL, YIQ, YUV and XYZ.

Compositing
The process of combining multiple images into a single image. Usually this is performed in films to make a computer graphics generated character appear on a previously filmed background. The term is also used in traditional photographic manipulation to refer to the process by which cel animation is recorded onto film under a rostrum camera. In film the 'mechanical' process is usually called matte photography (see color keying), and the process, when used in film sequences is ambiguously called traveling matte.

Concave/convex polygon
A concave polygon has the property that some points within its area can be joined by a line segment that passes outside the polygon. A convex polygon has the property that any line segment joining two points belonging to the polygon area is completely inside the polygon.

Cone tracing
An alternative to ray tracing in which cones are projected from the camera center through each pixel, where the intersection of the cone and the scene model is used to determine the pixel's color.

Contour
This is an image curve, often used to represent the set of points where a given function has a given constant value. A familiar example is a contour line on a topographic map. Here the contour denotes where the land has a given elevation. Another type of map contour might denote the boundary between increasing and decreasing population density. The equivalent concept in 3D is the level surface or isosurface.

Contrast
The range of colors in an image. Increasing the contrast of a color palette makes different colors easier to distinguish, while reducing the contrast makes them appear washed out.

Control point
One of a set of points which control the shape of a curve by their intuitively by position. The curves may go through some (see Bézier curve, an interpolating spline) or all (e.g. the Catmull-Rom interpolating splines) of the control points. Positioning is often interactive and the points are combined by blending functions to generate the shape desired. See also B-spline and Bézier curve. Note the distinction between knots and control points: in an interpolating spline, knots and control points are at the same positions in space. In a quadratic or higher order approximating spline they are in different places: the knots lie on the curve and control points lie near the knots, but not on the curve.

Convex hull
The convex hull of a given set of points is the smallest convex set that contains all the points.

Coons patch
A Coons patch is a form of parametric bicubic spline representation for surface patches. It allows explicit control of patch boundary position and tangent plane continuity. It is an example of a lofted surface.

Coordinate system
A coordinate system is a minimal set of mutually orthogonal vectors which span a given space. All points or vertices in the space may then be represented using a linear combination of these spanning vectors.

CSG/Constructive solid geometry
A paradigm for representing 3D shapes in terms of mathematically based compositions of geometric primitives. Any volumetric primitives can be used provided the primitive can satisfy an 'inside-outside' test which uniquely partitions points in the space near it. Typically, boolean set theoretic composition operators (e.g. intersection, union, difference) are used. Affine transformations may be applied to alter the shape of the primitives. For example, the exterior of an igloo may be represented as the union of a sphere and a cylinder, intersected with a cube.

Cuberille
A representation of 3D space consisting of a regular array of cubes, often referred to as voxels.

[Hundred]

D.

Data visualization
The set of techniques used to turn a set of data into visual insight. It aims to give the data a meaningful representation by exploiting the powerful discerning capabilities of the human eye. The data is displayed as 2D or 3D images using techniques such as colorization, 3D imaging, animation and spatial annotation to create an instant understanding from multi-variable data.

Delta frame
The difference between two consecutive images. Often used in video compression algorithms that exploit the temporal coherence of image sequences.

Depth buffer/Z-buffer
A method for solving the visible (or hidden) surface problem using two aligned pixel buffers or images. The first buffer stores the current color of the pixel and the second buffer stores the distance from the viewer to the surface. When rendering a point a on a scene surface, if the distance from the observer to a is greater than that of a previous point b that projects to the same image pixel, then point a can be ignored (as it cannot be seen). If the distance to a is less than the stored distance to b, then distance and color of a replace the color and distance buffer entry of b. A z-buffer is often efficiently implemented as a hardware buffer with entries aligned with pixels. Unfortunately, these z-buffers suffer a lot from aliasing effects and A-buffers are much better at dealing with visibility problems at sub-pixel accuracy.

Depth complexity
A measure of the complexity of an algorithm. It is equivalent to the number of pieces of data written to a framebuffer divided by the total number of pixels in the framebuffer, when a whole frame is rendered.

Depth cueing
Objects closer to the viewer appear brighter and more distinct than distant objects. Thus more distant objects or distant parts of objects are displayed with less intensity to simulate this phenomenon and enhance perception of depth.

Diffuse reflection
The portion of light that falls on a facet (small piece of the surface) which is radiated diffusely in all directions.

Direction cube
A technique used for representing spatial directions, often used by recursive direction decomposition algorithms. The cube is placed at the origin and aligned so that the coordinate axes are orthogonal to the faces. Each face of a cube is subdivided into a number of squares. Each square represents a collection of similar directions. Subdividing the squares on a face increases the resolution of the directions.

Directional lighting
A light source that radiates in such a way that rays from it are non-parallel.

Dissolve
An animation effect that is a transition between two sequences involving a fade from one directly to the other.

Dithering
One of many processes for reducing the total number of colors present in an image while retaining visual fidelity. Dithering can be done by interleaving pixels of selected colors to locally approximate the desired color. Dithering can be applied to either a color or a greyscale color space and may be necessary due to a limited number of colors available on the display device.

Double-buffering
A mechanism for duplicating the frame-buffer memory by using a two buffer system in which the image in one buffer is displayed while the image in the other buffer is computed. The newly created image is then displayed by swapping buffer pointers rather than having to copy memory. Double buffering allows the CPU to have uninterrupted access to one of the buffers while the video controller has access to the other.

[Northern_Mind]

I'm guessing you have a lot of time on your hands...

[Hundred]

E.

Edge merging
The process of replacing the edge of a polygon with the adjacent edges of neighboring polygons to prevent cracks appearing during rendering.

Emittance
The light emitted by a surface. This may have different intensities and spectral characteristics in different directions.

Explicit surface
(To be added.)

Extended light source
A light source with surface area which will cast shadows with both umbra and penumbra and thus is more difficult to model than a point source.

[Hundred]

F.

Face/facet/patch normal
A solid object can be constructed from many surface pieces which fit together. Each piece is called a face/facet/patch. Its normal is the direction from the surface of the object that is perpendicular to the piece's surface.

Facet/faceting
A facet is a small piece (usually a planar polygon) of a larger surface. Faceting is the technique used to construct a surface from multiple facets; triangulation is an example of faceting.

Fading
Fading is a method of switching between video sources, or images, using a black image as an intermediate. Fading without this intermediate is called a dissolve.

False coloring
See pseudo-color.

Field rendering
In interlaced video, a single image frame is sent as two fields - composed of even scanlines and odd scanlines. Field rendering refers to a method of rendering where fields are rendered separately in order to reduce motion artifacts.

Fill/flood fill
These are techniques for coloring areas bounded by line edges. The algorithms that fill interior-defined regions (the largest connected region of pixels whose values are the same as a given starting pixel) are called flood fill algorithms.

Filter
1) An optical device that selectively attenuates the intensity of light passing through it according to the light's properties. Common filters attenuate light according to either wavelength or polarization state. 2) An algorithm that selectively modifies the intensity or color of image data according to the image's properties. 3) An element (software or hardware) which takes in a stream of data and produces a stream of results, on average one output for each input.

Flat shading
Shading a polygonal patch with a single color and intensity. The shade chosen is a function of a variety of factors, such as light source position, viewer position and surface normal, according to the shading model used. A single shade is how the patch would appear if the surface is genuinely planar, rather than just being approximated by polygons, and if several viewing environment conditions hold (distant viewer and light source).

Fogging
The blending of a color, often light grey, with parts of an image such that the farther objects become increasingly obscured. (See Atmosphere effects.) In other words, the contrast between the fog color and objects in the image gets lower the deeper an object appears in the scene. Fogging may be used to provide a back-clipping plane where objects too distant to be seen clearly are removed to speed up the rendering of a scene.

FPS/Frames-per-second/Feet-per-second
1) A measure of the speed of an animation in terms of the number of complete, fully rendered images or frames which can be displayed in one second. 2) The same, except FPS refers to the number of feet (30.48 cm) of cinema film displayed in one second.

Fractal
A fractal has statistical self-similarity at all resolutions and is generated by an infinitely recursive process. In reality, those fractals generated by finite processes may exhibit no visible change in detail after some stage so are adequate approximations. So, for computer graphics we can extend the definition to include anything that has a substantial measure of exact or statistical self-similarity.

Frame
A still two-dimensional image. Often a frame is a raster image as used in the frame buffer of a graphics display system. In computer animation frames per second is a measurement of the number of still frames displayed in one second to give the impression of a moving image.

Frame rate
The frame rate of a video source is determined by the speed at which it completes the rendering of a new image. This is limited by both the speed at which image data can be created and the rate at which video images can be presented on a display. For example the NTSC system redraws at 30Hz, PAL is 25Hz and computer displays are now usually 72-75Hz.

Frame size
A term used to refer to the dimensions of the array of pixels forming a frame of an animation, or alternatively the memory requirement and hence indirectly the resolution and dimensions.

Free-form/free-form surface
A surface that does not have a simple geometric description (e.g. not a plane or quadric surface). It is usually represented using a spline surface or a triangulated surface.

Frequency
The number of time that a periodic function or vibration repeats itself in a specified time or space. It is often measured in cycles per second, or cycles per centimeter, or cycles per degree of visual arc.

Fresnel equation
An equation used to determine the attenuation of unpolarized light reflected from a surface, given the refractive index of the surface material and the angle of incidence of the light relative to the surface normal.

Frustrum of vision
The visible region of 3D space. Projecting rays from the viewer through all pixels in the image plane defines an infinite pyramid-like solid shape within which all visible objects appear. The pyramid is then truncated by a distant plane to eliminate the space which is too far away to render, and by a closer plane which eliminates object too close to render. The space in between is the frustrum of vision.

[Hundred]

G.

Gamut
Normally refers to the full range of colors available in a color space. The gamut varies with resource: photographic film, printing inks, color displays, etc.

Gaze direction
View direction is specified as a target object rather than the more usual vector form from a camera or eye position and direction.

Generalized polygon
A generalized polygon is a planar shape that is constructed using an ordered number of vertices that are connected to form an enclosed polygonal area. It is a graphics engine's most abstract internal representation of a shape. Specific shapes such as square and triangle have a fixed number of vertices (3 and 4 in this example) and can be generalized using a generalized polygon. A generalized polygon may have holes or be concave.
Other shapes such as circle and ellipse have an infinite number of vertices. A generalized polygon can provide the graphics engine with an approximate representation by using a large number of vertices.

Gloss
An object is said to have a gloss surface when specular reflection is observed. This causes a highlight on the surface when a bright light is directed at the object.

Gouraud shading
Gouraud shading computes an intensity for each vertex of a polygon using Lambert-law shading and then interpolates the computed intensities across the polygon by performing a bilinear interpolation of the intensities down and then across scan lines. It thus eliminates the sharp changes at polygon boundaries.

Graphics application programmers interface (API)
A software library enabling a programmer to produce a graphical application, typically incorporating input handling (mouse, keyboard etc.). E.g. OpenGL, Java3D, Allegro.

Gray scale
A color space where colors are represented by their luminance values only, i.e. saturation and hue are zero.

Gupta-Sproull algorithm
A technique developed in the framework of Raster Graphics. It aims to reduce aliasing when doing line drawing.

[Hundred]

H.

Hedgehog
A visual representation of a surface in which the surface normals are rendered like pins sticking out of the surface.

Hidden line removal
A technique used in wireframe rendering (which is when one draws the straight line boundaries of the polygonal patches, or polyhedral solids that define the scene). If all boundaries are drawn, this is as if all surfaces and objects are transparent. If all surfaces and objects are opaque, then some boundaries would not be visible because they are hidden by closer surfaces. Removing the obscured or occluded portions of the boundaries is hidden line removal.

Hidden surface problem
Sometimes called visible-surface determination or hidden-surface removal. It is the problem of only displaying the parts of a surface in a scene which are visible to the user. For a scene to make sense to a user, any surface that is obscured by an opaque surface must not be rendered. For raster graphics, an example of a rendering algorithm which solves the hidden-surface problem is the z-buffer algorithm.

Highlight
The area of a glossy object over which specular reflection can be viewed. It is normally the color of the light source, not of the object.

Hollow fill
A three-dimensional object whose internal volume (defined as the space enclosed by the object's skin) is not rendered. Such three-dimensional solid objects are frequently used in virtual environments and are constructed using infinitely thin polygons to form the skin of the object. If the user's viewpoint is positioned within the skin of the object, the reverse of the surface will be rendered if there is sufficient illumination.

Homogeneous coordinates
Normally, the transformations for scaling, rotation and translation are treated differently. Scaling and rotation use matrix multiplication whereas translation uses vector addition. When the homogeneous coordinate system is used, all three transformations can be performed using matrix multiplication. This representation is commonly used in graphics systems because of its simplicity of representation and use.

HSL/Hue-Saturation-Lightness
HSL, also known as HSI (Hue-Saturation-Intensity) is a color space used to represent images. HSL is based on polar coordinates, while the RGB color space is based on a three-dimensional Cartesian coordinate system. Intensity is the vertical axis of the polar system, hue is the relative angle and saturation is the planar distance from the axis. HSL is thought to be more intuitive to manipulate than RGB space. For example, in the HSI space, to change red to pink requires only changing the saturation parameter.

HSV/Hue-Saturation-Value
A color space that describes color using three basis components: hue, saturation and brightness. (See also HSL and Munsell color system.)

Hue
A perceptual term referring to the colorimetry quantity 'dominant wavelength' of a color. Hue can be used together with saturation and luminance to define the HSL color space.