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Two-Pass Clipping of Thick Lines

IP.com Disclosure Number: IPCOM000035178D
Original Publication Date: 1989-Jun-01
Included in the Prior Art Database: 2005-Jan-28
Document File: 3 page(s) / 53K

Publishing Venue

IBM

Related People

Clegg, JS: AUTHOR [+4]

Abstract

In a graphics display with a Draw Thick Line Command, errors occur at the boundaries of a display area when only simple software clipping is used. Disclosed is an arrangement in which, firstly, an expanded window is clipped by software and, secondly, a smaller display area is defined by hardware scissoring to display thick lines correctly.

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Two-Pass Clipping of Thick Lines

In a graphics display with a Draw Thick Line Command, errors occur at the boundaries of a display area when only simple software clipping is used. Disclosed is an arrangement in which, firstly, an expanded window is clipped by software and, secondly, a smaller display area is defined by hardware scissoring to display thick lines correctly.

Clipping a graphics primitive to a rectangular display area is a common requirement in a computer graphics system. In this article a problem which may occur when clipping a particular graphics primitive, namely thick lines, is addressed.

Many computer graphics workstations now supply thick line drawing as a primitive operation. We assume that our system allows us to draw

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a thick line by the following command: DRAW_TL(XO, YO, X1, Y1, THICKNESS)

This call will cause a line to be drawn from the point (XO, YO) to the point (X1, Y1). The line will be thickened by THICKNESS lines on either side of this line, hence the line will be 1 + (2 * THICKNESS) wide. Although, for clarity, balanced thickened lines are discussed, the result given here is easily generalized to other cases.

The ends of thick line primitives are often, and will be assumed here, drawn using the following algorithm: if the line is X - major then line ends are parallel to the Y axis else line ends are parallel to the X axis

Many computers graphics workstations also supply hardware clipping as a primitive operation in which the beam outside the display area is turned off to display a scissored rectangle. We assume that our system allows us to set the scissor rectangle by the following command: SCISSOR (XO, YO, WIDTH, HEIGHT)

This call causes the scissor rectangle to be set such that only pels (X, Y) where: XO <= X <= (XO+WIDTH) YO <= Y <= (YO+HEIGHT) will be drawn. It should be noted that the points (XO, YO) and (XO+HEIGHT) must lie on the screen. This scissor rectangle stays in effect until the next SCISSOR command is issued.

Most graphics systems work with a coordinate system which i...