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Computer Graphics Anti-Aliasing by Ideal Line Pixel Coverage

IP.com Disclosure Number: IPCOM000104360D
Original Publication Date: 1993-Apr-01
Included in the Prior Art Database: 2005-Mar-19
Document File: 6 page(s) / 249K

Publishing Venue

IBM

Related People

Goldsmith, L: AUTHOR [+3]

Abstract

Existing techniques to reduce the effect of aliasing artifacts in computer-generated lines by modulating pixel intensities do not compensate for all irregularities. This invention does so in a manner that is easily reduced to efficient practice.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 25% of the total text.

Computer Graphics Anti-Aliasing by Ideal Line Pixel Coverage

      Existing techniques to reduce the effect of aliasing artifacts
in computer-generated lines by modulating pixel intensities do not
compensate for all irregularities.  This invention does so in a
manner that is easily reduced to efficient practice.

      Aliasing is a term used to describe the effect of sampling a
signal at too low a frequency.  Because a given sample pattern can
result from more than one signal the samples are said to have
aliases, or to be aliased.  Aliases produce the jagged "staircase"
effect that is characteristic of sloped lines on computer graphics
displays.

      Because most computer graphics devices are controlled by
digital circuitry, picture elements, called pixels, may be displayed
only at digitally-addressable coordinate positions.  A line is
represented as brightening or darkening of a series of adjacent
pixels, each of which is as near as possible to the mathematically
pure line between the endpoints of the line segment.  Of course, the
larger the spacing between adjacent pixels becomes, the more obvious
the difference between the mathematically pure line and the displayed
approximation becomes.  However, improving display resolution by
reducing pixel spacing increases the cost and difficulty of producing
a graphics system, so another method of improving visual quality is
desirable.

      Anti-aliasing is the generic name for techniques which reduce
the visual difference between a mathematically pure line and its
displayed approximation.  All currently-known techniques do so by
modulating the intensity of pixels in the vicinity of the
mathematically pure line in accordance with an easily-calculable
value, such as the approximate distance between the pixel and the
pure line (Fig. 1).

      Other factors must also be considered to achieve the best
possible visual approximation of a pure line.  For example,
differences in the distance between adjacent pixels within a line
tend to cause horizontal and vertical lines to appear brighter than
lines at multiples of 45 degrees (Fig. 2).  Similarly, video bandpass
limitations can cause lines aligned with the primary scan axis
(usually horizontal) to be noticeably brighter than lines that form a
steep angle to that axis (Fig. 3).

      Cathode ray tube (CRT) displays exhibit a nonlinear
relationship between the applied brightness control voltage and the
amount of light emitted by the corresponding point on the display
screen.  This relationship may be calculated using the following
expression:

                         L = kB sup 'gamma'
where L is the emitted light, k is a constant, B is the asserted
brightness value, and gamma is a constant which depends upon CRT
characteristics and other factors, such as brightness and contrast
adjustments.  For most displays, the value of gamma lies between 2
and 3.  Adjusting values of B so that CRT light...