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The elimination of visible stair-step effects on lines drawn on a raster display by distributing the intensity error into neighboring pixels which are not directly intersected by the line.
A plane parallel to the view plane and displaced from it along the view plane normal by the back distance which is measured from the view reference point. When back clipping is enabled, portions of the object which lie behind this plane are not plotted.
A display device configuration in which the intensity of each pixel is described by a single bit (usually) in a computer memory. Typical examples are low-cost raster displays and dot matrix printers.
A world coordinate object is projected onto the view plane with either a perspective or a parallel projection. The projection of an object onto the view plane is found by passing lines through each point of the object and finding their intersection with the view plane. For a perspective projection, all lines emanate from a common point, which is the center of projection. For a parallel projection, the lines are all parallel to a specified vector, in which case the center of projection may be said to be at infinity. In the CORE system, the center of projection for a parallel projection is simply a point such that all projection lines are parallel to the line from it to the view reference point. If the projection lines are parallel to the view plane normal, the parallel projection is called orthographic, or sometimes, axonometric. This includes the isometric projection used in drafting. Skewed parallel projections are obtained when the projection lines are not parallel to the view plane normal. Examples include the Cabinet and cavalier drafting projections.
Removing parts of display elements that lie outside a given boundary, usually a window or viewport.
An integer index into the color lookup table.
A display device dependent table in which the entries specify the values of red, green, and blue intensities defining a particular color. This capability is available on most better quality color raster displays, and serves two purposes. First, only a limited number of memory bits (typically 2 to 16, occasionally up to 24) need be allocated to each pixel on the display, but these form an integer index into the lookup table which can specify red, green, and blue intensities much more precisely, offering a wider range of possible colors than would otherwise be available. Second, the display is dynamically updated from the lookup table which can be changed in less than one refresh cycle to radically alter the appearance of the displayed image; this is of particular importance in image processing and enhancement. If the memory values specified the color mixture directly, this much larger amount of memory could not be updated nearly as quickly.
The description of color in terms of its constituent additive primary intensities of red, green, and blue (RGB) (as used by television monitors), or sometimes in terms of subtractive primaries cyan, magenta, and yellow (CMY) (as used by color printing devices). These two are hardware oriented, and specification of the mix of RGB or CMY to obtain a particular color is rather non-intuitive. Other models which are more natural for a human are the hue, lightness, and saturation (HLS) model and the hue, lightness and value (HLV) models. The CORE system allows color specification in terms of either the RGB or HLS models.
A device independent sequential file which can be read and written by the CORE system. It may be used for long term storage and transmittal and transferral of graphics information. In the PLOT79 implementation of Metafile output, there is a single device driver which can be used in place of any other device driver. The PLOT79 Metafile Translator program can be used with any device driver to graphically display the contents of a Metafile, and the Metafile Dumper program can be used to print summaries of the contents of a Metafile.
A proposal for a 2-D and 3-D graphics standard developed by an international working group (the Graphics Standards Planning Committee) from the early 1970's with the final report published in 1979 (ACM SIGGRAPH Computer Graphics, Volume 13, Number 3, August 1979). The name PLOT79 commemorates this date.
CORE system value that defines the current drawing position in world coordinates. It is set to the origin of the world coordinate system at CORE system initialization time. Its value is affected by calls made to the functions that create output primitives.
Graphics Kernel System - a draft ANSI and ISO 2-D graphics standard originally developed by the West German Standardization Institute, DIN, in 1978. It is published in ACM SIGGRAPH Computer Graphics, Volume 18, Number 2, March 1984.
The device dependent part of a CORE implementation intended to support a graphics device. It generates device specific output in response to device-independent requests from the CORE routines.
A device (for example, refresh display, storage tube display, pen plotter, film recorder, dot matrix printer) on which display images can be represented.
In a display device, that medium on which display images may appear.
A technique for simulation of intensity ranges on display devices which have substantially limited intensity possibilities (usually just black (zero) and white (full)). One such method is the halftone process used for image display in books and newspapers, in which small rectangular areas of paper are printed with areas of ink in proportion to the desired intensity. Techniques for bit-map and dot matrix displays which support only black/white choices at each pixel attempt to redistribute over neighboring pixels the intensity error made at each pixel by having to color it totally black or totally white.
Enhancement of the interior of closed 2-D polygonal regions by filling them with solid color, patterns, or a hatch style.
A text attribute determining the shape of characters and their relative positioning.
A display technology in which the intensities and color of each pixel on the display surface is maintained in computer memory and dynamically redisplayed in raster line order, usually at rates from 30 to 100 times per second, on a television monitor. By comparison, broadcast television refresh rates are 25 to 30 times per second, and movie film rates are 24 frames per second. With an appropriate decay time of the television phosphor and refresh rates somewhere above 40 frames per second, it is possible to obtain motion which is flicker-free to the human eye.
A plane parallel to the view plane and displaced from it along the view plane normal by the front distance which is measured from the view reference point. When front clipping is enabled, portions of the object which lie in front of this plane are not plotted.
An algorithm for the display of mesh or wire frame surfaces in which lines on the mesh which lie behind features between them and the viewpoint are not shown. These algorithms usually work by scanning the mesh line segments in order from front to back comparing each segment against upper and lower visibility horizons which are the silhouette of the object to that point in the scan. The advantages are that the scan time is usually linear in the number of points on the mesh, and the case of elevation data on a grid of points, for which the method is well suited, occurs very frequently, and is easily represented in a computer program by a rectangular array of elevation values. The disadvantages are that the method is only with difficulty extended to multiple surfaces, contours on meshed surfaces, or multiple-valued surfaces, and that it deals with individual line segments, rather than surface patches, making smooth shading impractical.
An algorithm for the display of solid objects in which surface portions are hidden by other parts of the object between them and the viewer. Surface portions which face away from the viewer are similarly hidden. The objects are usually represented by planar polygonal surface patches (or curved patches in newer work) and information about how the patches are connected to form the surface. The advantage of this is that the resulting display can be much more life-like than wire mesh drawings, particularly if the visible parts of the surface are given different intensities according to their shading by a light source. Surface texturing is also possible. The disadvantage is that sophisticated algorithms are needed to avoid the N-squared computation growth from comparing each patch with every other one to find which portion is visible, and sophisticated data structures are necessary to represent the surface patch orientation and connectivity information. Another disadvantage is the general inability to deal with translucent and transparent objects, for which the ray tracing algorithm is required.
A quaint name for the front clipping plane.
The characteristic associated with a color names such as red, yellow, green, blue, etc. Hue is a gradation of color. In the CORE HLS model, blue is at 0 and 1, red at 1/3, green at 2/3, so that the hues form a unit cycle.
A branch of computer graphics involving the enhancement and analysis of images obtained from raster scanning devices such as television cameras, computer-assisted tomography (CAT) scanners, satellites such as LANDSAT, spacecraft such as Mariner and Voyager, and so on.
The characteristic that allows color to be ranked on a scale from light to dark.
A directed line segment. In the CORE system, it is a visible line segment from the current position to the specified point, after which that point becomes the new current point.
An attribute of lines in the CORE system; lines may be solid, or dashed and dotted in a variety of combinations.
A symbol with a specified appearance which is used to identify a particular location.
A coordinate specified in a device independent coordinate system normalized to the range 0 to 1.
The 2-D or 3-D Cartesian space which represents the viewable region of an arbitrary display device. In the case of 3-D NDC space, the coordinate system is left-handed, with the origin at the lower left front face of the unit cube, x positive to the right, y positive upward, and z positive into the cube.
A less-common name for pixel.
The smallest element of a display surface that can be independently assigned a color or intensity. The name is an acronym for "picture element". It is conventionally applied primarily to raster scan displays.
A closed planar figure bounded by straight line segments. Although the CORE system permits this to be specified in 3-D coordinates, it must still lie in a single plane.
A set of connected line segments.
A set of locations, each to be indicated by the same type of marker symbol.
A picture element, such as a line, marker, or text string, having a specific appearance. The CORE system supports five output primitives: line, polyline, marker, polymarker, and text; the raster extensions to the CORE system add a polygon primitive. The GKS system supports polyline, polymarker, text, fill area, cell array, and generalized drawing primitive.
Computer graphics in which a display image is composed of an array of pixels arranged in rows and columns.
A sophisticated (and computationally expensive) algorithm for constructing high-quality hidden surface images including reflection, refraction, and shadowing. For each pixel in the image, the algorithm traces a light ray into the scene from the pixel to the object and after branching due to reflection and refraction, back to the original light source(s).
The graphical display of objects rapidly enough that the scene can be changed to give the appearance of smooth motion to a human viewer in front of the display. Until recently, such displays have been very expensive (in the range of fifty thousand to several million dollars). Television and commercial film computer graphics motion sequences which have become increasingly common from film makers such as Lucasfilm and Disney are actually not done in real time. Each frame may represent many minutes or even hours of computation, and require film exposures of several seconds for each primary color.
Turning all or part of a display image about an axis.
The characteristic which describes the extent to which a color differs from a gray of the same lightness. In the CORE HLS model, zero saturation corresponds to maximum white content at that lightness level, and unity to full saturation.
Enlarging or reducing all or part of a display image by multiplying the coordinates of display elements by a constant value.
A refresh display in which a command list (usually called a display list) of graphics primitives is repeatedly scanned and redrawn directly on the face of the display tube. This is in contrast to a frame buffer display in which the image is displayed by a raster scan. The advantage of a stroke display is that real-time dynamic graphical display is possible, provided that the display list is not too long, and the phosphor decay rate is short enough to avoid ghosting, and long enough to avoid flicker. The main disadvantages are the expense of the hardware required for adequate performance, particularly if color is required, and with few exceptions, the inability to handle shading and polygon filling in real time.
An output primitive consisting of a character string.
The application of a constant displacement to the position of all or part of a display image.
A signed quantity which is the distance from the view reference point to the view plane measured in the direction of the view plane normal.
A 3-D vector specified relative to the view reference point. The view plane containing the window is perpendicular to the view plane normal at a distance from the view reference point specified by the view plane distance. The origin of the window coordinate system is at the point where the line through the view reference point parallel to the view plane normal pierces the view plane. In the CORE default case, the view distance is zero, and the view reference point then lies on the window at the coordinate system origin.
A convenient known point on or near the object being viewed.
A 3-D vector whose projection onto the view plane defines the up direction in the window.
An application program specified part of normalized device coordinate space. This is restricted to a rectangular region in 2-D NDC space, or a truncated rectangular prism with parallel front and back faces in 3-D NDC space.
An application program specified rectangular portion of the view plane in which the view of the object appears according to the viewing parameters and projection type selected. It is mapped onto the viewport in NDC space. If the NDC space is 3-D, the front clipping plane is mapped onto the front plane of the 3-D NDC viewport, and the back clipping plane onto the back plane of the viewport. If the aspect ratio of the window is different than that of the viewport, then the view will be stretched or shrunk accordingly.
A device independent 3-D Cartesian coordinate system used by the application program for specifying graphical input and output. The units are arbitrary, for example, light years, meters, furlongs, time, inches, parsecs, miles, weight, and so on. In the CORE system, the coordinate system may be chosen either right-handed or left-handed.
A quaint name for the back clipping plane.
A refresh buffer in which, for each pixel on the display surface, in addition to color values, is maintained the depth (z coordinate) of the object mapping into that pixel. By retaining only the smallest z value, which corresponds to the object closest to the window, a hidden surface view is obtained.
Enlarging or reducing all or part of a display image by multiplying the coordinates of display elements by a constant value. When this feature is implemented on a raster device, it normally allows only integral enlarging factors of a 2-D display, most frequently in the range 1 to 16. Higher quality raster displays may permit different scale factors for the two coordinates.