Appendix B. Glossary

Aliasing   Technically, the loss of signal information in an image reproduced at some finite resolution. It is most often characterized by the appearance of sharp jagged edges along points, lines, or polygons due to the nature of having a limited number of fixed-sized pixels.

Alpha   A fourth color value added to provide a degree of transparency to the color of an object. An alpha value of 0.0 means complete transparency; 1.0 denotes no transparency (opaque).

Ambient light   Light in a scene that doesn’t come from any specific point source or direction. Ambient light illuminates all surfaces evenly and on all sides. In the OpenGL lighting model, ambient light approximates how light is collectively scattered off all the surfaces in a scene.

Antialiasing   A rendering method used to smooth lines and curves and polygon edges. This technique averages the color of pixels adjacent to the line. It has the visual effect of softening the transition from the pixels on the line and those adjacent to the line, thus providing a smoother appearance. Full-scene Antialiasing is supported by OpenGL via the multisampling feature.

ARB   The Architecture Review Board. The committee body consisting of 3D graphics hardware vendors, previously charged with maintaining the OpenGL specification. The OpenGL ARB is now the name of the Khronos working group that is responsible for maintenance of the OpenGL specification.

Aspect ratio   The ratio of the width of a window to the height of the window. Specifically, the width of the window in pixels divided by the height of the window in pixels.

AUX library   A window-system-independent utility library. Limited but useful for quick and portable OpenGL demonstration programs. Now largely replaced by the GLUT library.

Bézier curve   A curve whose shape is defined by control points near the curve rather than by the precise set of points that define the curve itself.

Bitplane   An array of bits mapped directly to screen pixels.

Buffer   An area of memory used to store image information. This can be color, depth, or blending information. The red, green, blue, and alpha buffers are often collectively referred to as the color buffer.

Cartesian   A coordinate system based on three directional axes placed at a 90° orientation to one another. These coordinates are labeled x, y, and z.

Clip coordinates   The 4D geometric coordinates that result from the modelview and projection transformation.

Clipping   The elimination of a portion of a single primitive or group of primitives. The points that would be rendered outside the clipping region or volume are not drawn. The clipping volume is generally specified by the projection matrix. Clipped primitives are reconstructed such that the edges of the primitive do not lie outside the clipping region.

Color index mode   A color mode in which colors in a scene are selected from a fixed number of colors available in a palette. These entries are referenced by an index into the palette. This mode is rarely used and even more rarely hardware accelerated.

Convex   A reference to the shape of a polygon. A convex polygon has no indentations, and no straight line can be drawn through the polygon that intersects it more than twice (once entering, once leaving).

Culling   The elimination of graphics primitives that would not be seen if rendered. Backface culling eliminates the front or back face of a primitive so that the face isn’t drawn. Frustum culling eliminates whole objects that would fall outside the viewing frustum.

Destination color   The stored color at a particular location in the color buffer. This terminology is usually used when describing blending operations to distinguish between the color already present in the color buffer and the color coming into the color buffer (source color).

Display list   A compiled list of OpenGL functions and commands. When called, a display list executes faster than a manually called list of single commands.

Dithering   A method used to simulate a wider range of color depth by placing different-colored pixels together in patterns that give the illusion of shading between the two colors.

Double buffered   A drawing technique used by OpenGL. The image to be displayed is assembled in memory and then placed on the screen in a single update operation, rather than built primitive by primitive on the screen. Double buffering is a much faster and smoother update operation and can produce animations.

Extruded   The process of taking a 2D image or shape and adding a third dimension uniformly across the surface. This process can transform 2D fonts into 3D lettering.

Eye coordinates   The coordinate system based on the position of the eye. The eye’s position is placed at (0, 0, 0) and looks down the negative z-axis.

Frustum   A truncated pyramid-shaped viewing volume that creates a perspective view. (Near objects are large; far objects are small.)

GLSL   Acronym for the OpenGL Shading Language, a high-level C-like shading language.

GLUT library   The OpenGL Utility Toolkit. A window-system-independent utility library useful for creating sample programs and simple 3D rendering programs that are independent of the operating system and windowing system. Typically used to provide portability between Windows, X-Window, Linux, and so on.

Immediate mode   A graphics rendering mode in which commands and functions are sent individually and have an immediate effect on the state of the rendering engine.

Implementation   A software- or hardware-based device that performs OpenGL rendering operations.

Khronos Group   An industry consortium that now manages the maintenance and promotion of the OpenGL specification in addition to several other industry standards

Literal   A value, not a variable name. A specific string or numeric constant embedded directly in source code.

Matrix   A 2D array of numbers. Matrices can be operated on mathematically and are used to perform coordinate transformations.

Mipmapping   A technique that uses multiple levels of detail for a texture. This technique selects from among the different sizes of an image available, or possibly combines the two nearest sized matches to produce the final fragments used for texturing.

Modelview matrix   The OpenGL matrix that transforms primitives to eye coordinates from object coordinates.

Normal   A directional vector that points perpendicularly to a plane or surface. When used, normals are applied to each vertex in a primitive.

Normalize   The reduction of a normal to a unit normal. A unit normal is a vector that has a length of exactly 1.0.

NURBS   An acronym for non-uniform rational B-spline. This is a method of specifying parametric curves and surfaces.

Orthographic   A drawing mode in which no perspective or foreshortening takes place. Also called parallel projection. The lengths and dimensions of all primitives are undistorted regardless of orientation or distance from the viewer.

Palette   A set of colors available for drawing operations. For 8-bit Windows color modes, the palette contains 256 color entries, and all pixels in the scene can be colored from only this set.

Parametric curve   A curve whose shape is determined by one (for a curve) or two (for a surface) parameters. These parameters are used in separate equations that yield the individual x, y, and z values of the points along the curve.

Perspective   A drawing mode in which objects farther from the viewer appear smaller than nearby objects.

Pixel   Condensed from the words picture element. This is the smallest visual division available on the computer screen. Pixels are arranged in rows and columns and are individually set to the appropriate color to render any given image.

Pixmap   A two-dimensional array of color values that compose a color image. Pixmaps are so called because each picture element corresponds to a pixel on the screen.

Polygon   A 2D shape drawn with three or more sides.

Primitive   A 2D polygonal shape defined by OpenGL. All objects and scenes are composed of various combinations of primitives.

Projection   The transformation of lines, points, and polygons from eye coordinates to clipping coordinates on the screen.

Quadrilateral   A polygon with exactly four sides.

Rasterize   The process of converting projected primitives and bitmaps into pixel fragments in the frame buffer.

Render   The conversion of primitives in object coordinates to an image in the frame buffer. The rendering pipeline is the process by which OpenGL commands and statements become pixels on the screen.

Retained mode   A style of 3D programming in which an object’s representation is held in memory by the programming library.

Scintillation   A sparkling or flashing effect produced on objects when a nonmipmapped texture map is applied to a polygon that is significantly smaller than the size of the texture being applied. This term is also applied to aliasing artifacts.

Shader   A small program that is executed by the graphics hardware, often in parallel, to operate on individual vertices or pixels. See also GLSL.

Source color   The color of the incoming fragment, as opposed to the color already present in the color buffer (destination color). This terminology is usually used when describing how the source and destination colors are combined during a blending operation.

Specification   The design document that specifies OpenGL operation and fully describes how an implementation must work.

Spline   A general term used to describe any curve created by placing control points near the curve, which have a pulling effect on the curve’s shape. This is similar to the reaction of a piece of flexible material when pressure is applied at various points along its length.

Stipple   A binary bit pattern used to mask out pixel generation in the frame buffer. This is similar to a monochrome bitmap, but one-dimensional patterns are used for lines and two-dimensional patterns are used for polygons.

Tessellation   The process of breaking down a complex 2D polygon into a planar mesh of convex polygons.

Texel   Similar to pixel (picture element), a texel is a texture element. A texel represents a color from a texture that is applied to a pixel fragment in the frame buffer.

Texture   An image pattern of colors applied to the surface of a primitive.

Texture mapping   The process of applying a texture image to a surface. The surface does not have to be planar (flat). Texture mapping is often used to wrap an image around a curved object or to produce patterned surfaces such as wood or marble.

Transformation   The manipulation of a coordinate system. This can include rotation, translation, scaling (both uniform and nonuniform), and perspective division.

Translucence   A degree of transparency of an object. In OpenGL, this is represented by an alpha value ranging from 1.0 (opaque) to 0.0 (transparent).

Vertex   A single point in space. Except when used for point and line primitives, it also defines the point at which two edges of a polygon meet.

Viewing volume   The area in 3D space that can be viewed in the window. Objects and points outside the viewing volume are clipped (cannot be seen).

Viewport   The area within a window that is used to display an OpenGL image. Usually, this encompasses the entire client area. Stretched viewports can produce enlarged or shrunken output within the physical window.

Wireframe   The representation of a solid object by a mesh of lines rather than solid shaded polygons. Wireframe models are usually rendered faster and can be used to view both the front and the back of an object at the same time.