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From: isaak@imagen.UUCP (Mark Isaak)
Newsgroups: comp.lang.postscript
Subject: Re: Thin line woes
Message-ID: <15253@imagen.UUCP>
Date: 15 Aug 90 19:39:34 GMT
References: <10244@pt.cs.cmu.edu>
Organization: Imagen Corp., Santa Clara CA
Lines: 39

in article <10244@pt.cs.cmu.edu>, tgl@zog.cs.cmu.edu (Tom Lane) says:
> 
>   1. Line width variation occurs even with "0.12 setlinewidth".  On a
> 300dpi printer this width is exactly half a pixel.  I don't understand
> how this could be translated into a two-pixel-wide line, no matter where
> the endpoint positions fall.  If anyone can explain the exact algorithm
> for deciding which pixels get blackened, I would be grateful.

Adobe's painting algorithm says to blacken every pixel which is touched
by the shape, no matter how little of the pixel falls inside.  This has
the effect of making the line you asked to be n pixels wide appear to be
n+1 pixels wide instead.

The situation is complicated by diagonal lines, which you generally
don't _want_ the same width.  Depending on their width, slope, and
positioning, the lines can appear one pixel wider along some patches
than others.  You can eliminate this artifact by correcting the width,
but the corrections are different for different slopes.  QMS/Imagen's
UltraScript does this correction automatically for thin lines, and
Display PostScript and PostScript Level 2 have a "setstrokeadjust"
operator which will do it.

The simplest solution otherwise is to specify "0 setlinewidth", which
will give a nice-looking one-pixel-wide line at all angles.  For slightly
wider diagonals, some combination of endpoint rounding and/or width
correction may be desirable.

>   2. In the book "Real World PostScript" (Roth, ed.), Michael Fryd
> recommends rounding endpoint coordinates to nearest-integer-plus-0.25
> in device space, whereas the green book just shows rounding to nearest
> integer.  Which of these is better, and why?

For horizontal and vertical lines, it shouldn't matter in theory, although
adding +0.25 might avoid some obscure round-off errors in practice  (That
last part is just a guess).  I'm not sure how the difference would affect
diagonal lines.
-- 
Mark Isaak    {decwrl,sun}!imagen!isaak  or  imagen!isaak@decwrl.dec.com
"Convictions are more dangerous enemies of truth than lies." - Nietzsche