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Modified Algorithm for Raster Scan CRT Display

IP.com Disclosure Number: IPCOM000089162D
Original Publication Date: 1977-Sep-01
Included in the Prior Art Database: 2005-Mar-04
Document File: 2 page(s) / 75K

Publishing Venue

IBM

Related People

Brandon, M: AUTHOR

Abstract

Raster scan CRT displays have a staircasing appearance when displaying vector information at nonzero angles to the raster scan lines. Normally, picture elements (pels) on the screen closest to the desired vector line to be represented are those selected for illumination. Thus, in Fig. 1(a) a near horizontal line is represented by the illumination of pels 7 to 18 in scan line 3 and pels 19 to 30 in scan line 4, producing a distinct staircasing effect. If instead of merely illuminating the nearest pels, they are selected so that the line to be represented lies along the visual center of the illuminated pels, then the staircasing effect is not so noticeable.

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Modified Algorithm for Raster Scan CRT Display

Raster scan CRT displays have a staircasing appearance when displaying vector information at nonzero angles to the raster scan lines. Normally, picture elements (pels) on the screen closest to the desired vector line to be represented are those selected for illumination. Thus, in Fig. 1(a) a near horizontal line is represented by the illumination of pels 7 to 18 in scan line 3 and pels 19 to 30 in scan line 4, producing a distinct staircasing effect. If instead of merely illuminating the nearest pels, they are selected so that the line to be represented lies along the visual center of the illuminated pels, then the staircasing effect is not so noticeable.

In order to determine which of the adjacent pels should be illuminated, the normal pel selection algorithm is modified to add the following steps to the normal pel selection procedure: 1. the position error (Yp - Y) between the illuminated pel Yp and the true position of the line Y it represents is accumulated from one pel position X to the next position (X 1) along the line of scan; and 2. a fraction G of the accumulated error A = (Yp - Y) is added to the next desired position of the line and the pel Yp nearest to this modified position, Ym, is selected for illumination instead of the pel nearest to the actual position Y.

A simplified flow diagram illustrating the modified pel selection algorithm is shown in Fig. 2. The two steps required, in addition to the normal...