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Line Read And Write in a Hardware Graphics Device

IP.com Disclosure Number: IPCOM000100978D
Original Publication Date: 1990-Jun-01
Included in the Prior Art Database: 2005-Mar-16
Document File: 3 page(s) / 97K

Publishing Venue

IBM

Related People

Butler, ND: AUTHOR

Abstract

Graphics display systems are often required to draw lines temporarily and later remove them to leave the original pixels intact. This is typically done by software and hence is slow. This article describes a technique for reading pixel data from along a line, or curve, storing it in a form such that the data can subsequently be re-drawn along that line, or any other line by means of a hardware line drawing device.

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Line Read And Write in a Hardware Graphics Device

       Graphics display systems are often required to draw lines
temporarily and later remove them to leave the original pixels
intact.  This is typically done by software and hence is slow.  This
article describes a technique for reading pixel data from along a
line, or curve, storing it in a form such that the data can
subsequently be re-drawn along that line, or any other line by means
of a hardware line drawing device.

      A typical graphics system manipulates pixels stored in
rectangular arrays, or "pixel maps".  The pixel maps are defined by
the start address in memory, the map width (in pixels), the map
height (in pixels), and the number of bits-per-pixel (Fig. 1).

      Drawing Patterned Lines (Line Write) A typical graphics
operation involves drawing lines or curves in a pixel map.  In the
simple case this line may be of a constant colour.  A more complex
case allows patterned coloured lines to be drawn.  In this case the
colour for each pel, and the pattern (choice of update function) can
be taken from registers programmed by the user, or indeed from their
own pixel maps.  A previous article (*) describes the way hardware
can perform this operation and is the context of the current
disclosure. The pattern is held in a pixel map, stored horizontally
along a single line of that map.  As the X and Y coordinates in the
destination are stepped according to, e.g., the Bresenham algorithm,
the Y coordinate of the source and pattern is held constant and the X
coordinate is moved in the positive X direction only.  This has the
effect of drawing a pattern defined in one direction only along a
line at an arbitary angle (as illustrated in Fig. 2).  It could also
apply to drawing patterned curves.

      Reading Data Along Lines (Line Read) There are circumstances
where lines need to be drawn in temporarily and then removed later
without destroying the orginal pixels under the...