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From: ritter@versatc.VERSATEC.COM (Jack Ritter)
Newsgroups: comp.graphics
Subject: Re: Variable-width Bresenham
Summary: The eye is TOO good.
Message-ID: <19673@versatc.VERSATEC.COM>
Date: 19 Oct 89 21:17:38 GMT
References: <4162@deimos.cis.ksu.edu>
Organization: Versatec, Santa Clara, Ca. 95051
Lines: 59

In article <4162@deimos.cis.ksu.edu>, neil@calvin.ksu.ksu.edu (Neil Erdwien) writes:
> I'm looking for references/ideas for implementing a variable-width
> version of Bresenham's algorithm for quantising line segments.
> --
A simple solution, as already suggested by
someone, is to do the Brez. walk, and simply
replicate N pixels each step, for width N lines.
The problem is, the eye is VERY GOOD at detecting
different effective widths. Consider the following
two lines of width 3:

***             ***
 ***               ***
  ***      &         ***
   ***                  ***
	***                   ***

They will look very different to the eye.
For small N, I would recommend a table, describing
how to step, for a variety of angles & widths. A step description
should be more than just "do 3 pixels". It should be
a more complex sequence like, "do 3,4,4, [repeat]".
You'll have to experiment.

If you connect lines end to end (as in text), you
must position the lines consistently; a line
that is an EVEN number of pixels wide cannot be
exactly centered. You must chose one of the 2 possibilities
(plus, or minus, the half pixel). This scheme must
be consistent for ALL DIRECTIONS.

Note that you arent really drawing thin rectangles,
but rather thin parellograms. For large N, the eye will notice.
For these cases, you must do a Brezenham
walk of half a width from the end point, in a direction
perpendicular to the line's direction, to find the 2
corners of the rectangle. You then fracture the rectangle
into an upper triangle, a middle parellelogram, and a
bottom triangle. Each of these pieces is then drawn by doing 2
simultaneous Brez. walks down its left & right edges, and filling the
pixels in between. This is the fastest way.
  An alternative to fracturing is to walk along the end segment,
drawing N single width, parallel lines. This may cause holes to
appear. To avoid holes, draw the CORNER PIXEL whenever you
take a Brez. step.

-- 
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