Glossary (OpenGL Programming) Part 3

separable filter

A form of 2D convolution filter that can be represented by two ID vectors. The 2D separable filter is determined by computing the outer product of the two vectors.

server

The computer on which OpenGL commands are executed. This might differ from the computer from which commands are issued. See client.

shader program

A set of instructions written in a graphics shading language (The OpenGL Shading Language, also called GLSL) that control the processing of graphics primitives.

shading

The process of interpolating color within the interior of a polygon, or between the vertices of a line, during rasterization.

shininess

The exponent associated with specular reflection and lighting. Shininess controls the degree with which the specular highlight decays.

single-buffering

OpenGL contexts that don’t have back color buffers are single-buffered. You can use these contexts for animation, but take care to avoid visually disturbing flashes when rendering.

singular matrix

A matrix that has no inverse. Geometrically, such a matrix represents a transformation that collapses points along at least one line to a single point.

specular

Specular lighting and reflection incorporates reflection off shiny objects and the position of the viewer. Maximum specular reflectance occurs when the angle between the viewer and the direction of the reflected light is zero. A specular material scatters light with greatest intensity in the direction of the reflection, and its brightness decays, based upon the exponential value shininess.

spotlight

A special type of local light source that has a direction (where it points to) as well as a position. A spotlight simulates a cone of light, which may have a fall-off in intensity, based upon distance from the center of the cone.

sprite

A screen-aligned graphics primitive. Sprites are ususally represented as either a single vertex that is expanded to cover many pixels around the transformed vertex, or as a quadralaterial its vertices specified so that it is perpendicular to the viewing direction (or put another way, parallel to the image plane).

stencil buffer

Memory (bitplanes) that is used for additional per-fragment testing, along with the depth buffer. The stencil test may be used for masking regions, capping solid geometry, and overlapping translucent polygons.

stereo

Enhanced three-dimensional perception of a rendered image by computing separate images for each eye. Stereo requires special hardware, such as two synchronized monitors or special glasses, to alternate viewed frames for each eye. Some implementations of OpenGL support stereo by including both left and right buffers for color data.

stipple

A one- or two-dimensional binary pattern that defeats the generation of fragments where its value is zero. Line stipples are one-dimensional and are applied relative to the start of a line. Polygon stipples are two-dimensional and are applied with a fixed orientation to the window.

tessellation

Reduction of a portion of an analytic surface to a mesh of polygons, or of a portion of an analytic curve to a sequence of lines.

texel

A texture element. A texel is obtained from texture memory and represents the color of the texture to be applied to a corresponding fragment.

texture mapping

The process of applying an image (the texture) to a primitive. Texture mapping is often used to add realism to a scene. For example, you can apply a picture of a building facade to a polygon representing a wall.

texture matrix

The 4×4 matrix that transforms texture coordinates from the coordinates in which they’re specified to the coordinates that are used for interpolation and texture lookup.

texture object

A named cache that stores texture data, such as the image array, associated mipmaps, and associated texture parameter values: width, height, border width, internal format, resolution of components, minification and magnification filters, wrapping modes, border color, and texture priority.

textures

One- or two-dimensional images that are used to modify the color of fragments produced by rasterization.

texture unit

When multitexturing, as part of an overall multiple pass application of texture images, a texture unit controls one processing step for a single texture image. A texture unit maintains the texturing state for one texturing pass, including the texture image, filter, environment, coordinate generation, and matrix stack. Multitexturing consists of a chain of texture units.

transformations

The warping of spaces. In OpenGL, transformations are limited to projective transformations that include anything that can be represented by a 4×4 matrix. Such transformations include rotations, translations, (nonuniform) scalings along the coordinate axes, perspective transformations, and combinations of these.

triangle

A polygon with three edges. Triangles are always convex.

unpack

The process of converting pixels supplied by an application to OpenGL’s internal format.

vertex

A point in three-dimensional space, vertex array

A block of vertex data (vertex coordinates, texture coordinates, surface normals, RGBA colors, color indices, and edge flags) may be stored in an array and then used to specify multiple geometric primitives through the execution of a single OpenGL command.

vertices

Preferred plural of vertex.

viewpoint

The origin of either the eye- or the clip-coordinate system, depending on context. (For example, when discussing lighting, the viewpoint is the origin of the eye-coordinate system. When discussing projection, the viewpoint is the origin of the clip-coordinate system.) With a typical projection matrix, the eye-coordinate and clip-coordinate origins are at the same location.

view volume

The volume in clip coordinates whose coordinates satisfy the following three conditions:

Geometric primitives that extend outside this volume are clipped.

VRML

VRML stands for Virtual Reality Modeling Language, which is (according to the VRML Mission Statement) "a universal description language for multiparticipant simulations." VRML is specifically designed to allow people to navigate through three-dimensional worlds that are placed on the World Wide Web. The first versions of VRML are subsets of the Open Inventor file format with additions to allow hyperlinking to the Web (to URLs— Universal Resource Locators).

window

A subregion of the framebuffer, usually rectangular, whose pixels all have the same buffer configuration. An OpenGL context renders to a single window at a time.

window-aligned

When referring to line segments or polygon edges, implies that these are parallel to the window boundaries. (In OpenGL, the window is rectangular, with horizontal and vertical edges.) When referring to a polygon pattern, implies that the pattern is fixed relative to the window origin.

window coordinates

The coordinate system of a window. It’s important to distinguish between the names of pixels, which are discrete, and the window-coordinate system, which is continuous. For example, the pixel at the lower left corner of a window is pixel (0, 0); the window coordinates of the center of this pixel are (0.5, 0.5, z). Note that window coordinates include a depth, or z, component, and that this component is continuous as well.

wireframe

A representation of an object that contains line segments only. Typically, the line segments indicate polygon edges.

working set

On machines with special hardware that increases texture performance, this is the group of texture objects that are currently resident. The performance of textures within the working set outperforms the textures outside the working set.

X Window System

A window system used by many of the machines on which OpenGL is implemented. GLX is the name of the OpenGL extension to the X Window System.