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Antialiased Graphics Display WITH Limited NUMBER of INTENSITY LEVELS

IP.com Disclosure Number: IPCOM000063113D
Original Publication Date: 1986-Dec-01
Included in the Prior Art Database: 2005-Mar-09
Document File: 3 page(s) / 17K

Publishing Venue

IBM

Related People

Chase, BD: AUTHOR

Abstract

In a graphics display with a limited set of grey levels the gamma is optimized for color discrimination yet also provides good antialiasing and grey level facsimile. Eight levels of grey are used, increasing in intensity to a power law. A sub-set of grey levels is selected so that when taken in pairs (on either side of a line to be displayed), the combined intensity is the same for each and the relative intensities appear to be at an intermediate position close to the line. The gamma of a display is defined as the exponent in the power law whereby an electrical input signal is transformed to a light output: light output = constant x (volts in)(gamma) Typically, a CRT may have a gamma of 2.5, so in combination with a linear video amplifier the gamma characteristic of a monitor can be close to 2.5.

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Antialiased Graphics Display WITH Limited NUMBER of INTENSITY LEVELS

In a graphics display with a limited set of grey levels the gamma is optimized for color discrimination yet also provides good antialiasing and grey level facsimile. Eight levels of grey are used, increasing in intensity to a power law. A sub-set of grey levels is selected so that when taken in pairs (on either side of a line to be displayed), the combined intensity is the same for each and the relative intensities appear to be at an intermediate position close to the line. The gamma of a display is defined as the exponent in the power law whereby an electrical input signal is transformed to a light output: light output = constant x (volts in)(gamma) Typically, a CRT may have a gamma of 2.5, so in combination with a linear video amplifier the gamma characteristic of a monitor can be close to 2.5. The eye responds to light in a power law fashion (Stefans Law). For a set of grey scale steps to appear equally distinguishable from one to another, the steps should increase in light intensity according to a power law of 2, i.e.: Brightness of step = constant x (step number)(2) If, for example, we have 7 steps with levels ranging from 0 to 7, then for the steps to appear equal the levels must increase in proportion to 0, 1, 4, 9, 16, 25, 36, 49. Clearly, for a display that has only 8 displayable levels and on which we wish to display a set of well distinguished colors and grey levels, it will be preferable to have a set of levels available that match this power law. This is a fundamental requirement of a display and imposes restraints on the levels generated by the display hardware. To display antialiased graphics, a set of levels is required that exhibits a totally different and largely linear characteristic.

Antialiased line drawing is a method of removing the jaggies from a line on a raster scan display by displaying the line using a combination of grey levels. If the line is required to pass between two pixels (picture elements) on the display screen, then for a conventional non- antialiased line the nearest of the two pels is turned fully on. For near vertical or horizontal lines this method results in a sudden step in the drawn line when the drawing flips to the next pel as the true position of the line moves across the center point. This sudden step gives the line a jagged or stepped appearance. This is referred to as aliasing. When a line is drawn using an antialiasing technique, the point on a line that must be shown between two pixels is represented by turning on two pels on either side of the point. The intensities of the two pels must be such that the light seen by the eye is the same as when the line is shown by a single pel and also in a ratio to one another such that the apparent center point of the light seen by the eye is as close as possible (given the levels available) to the point on the line that is being drawn. Mathematically the intensities are su...