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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.