R help - Sep 2000 - getting lines with non-rounded caps?

I'm trying to get a plot with wide lines that don't have rounded end
caps.
You can see the effect quite clearly with a plot like this:

plot(0:1)
lines(c(1.5,1.5),c(0,.5),lwd=20,col="red")
lines(c(1.5,1.5),c(.5,1),lwd=20,col="green")
abline(h=0)
abline(h=0.5)
abline(h=1)

The colored lines should be between the horizontal lines, but because of the
rounded end caps, they aren't.

Under X11, if I set lty to something like "F0", the end caps are
turned off.
Unfortunately, the "0" appears to get ignored, and the line style is
the same
as "FF", i.e. 16 units of drawn line, then 16 units of space.

With the postscript device, lines styles like "FF" still get rounded
end caps,
unlike X11.

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Trent Piepho wrote:
> 
> I'm trying to get a plot with wide lines that don't have rounded
end caps.
> You can see the effect quite clearly with a plot like this:
> 
> plot(0:1)
> lines(c(1.5,1.5),c(0,.5),lwd=20,col="red")
> lines(c(1.5,1.5),c(.5,1),lwd=20,col="green")
> abline(h=0)
> abline(h=0.5)
> abline(h=1)
> 
> The colored lines should be between the horizontal lines, but because of
the
> rounded end caps, they aren't.
> 
> Under X11, if I set lty to something like "F0", the end caps are
turned off.
> Unfortunately, the "0" appears to get ignored, and the line style
is the same
> as "FF", i.e. 16 units of drawn line, then 16 units of space.
> 
> With the postscript device, lines styles like "FF" still get
rounded end caps,
> unlike X11.

You could use e.g.

plot(0:1, type = "n")
polygon(x = c(1.48, 1.52, 1.52, 1.48, 1.48), y = c(0, 0, 0.5, 0.5, 0),
col = "red")
polygon(x = c(1.48, 1.52, 1.52, 1.48, 1.48), y = c(0.5, 0.5, 1, 1, 0.5),
col = "green")

to avoid rounded caps. Don't know whether that's what you want, but ...


Martina
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At 11:31 19/09/00 -0700, you wrote:
>I'm trying to get a plot with wide lines that don't have rounded end
caps.
>You can see the effect quite clearly with a plot like this:
>
>plot(0:1)
>lines(c(1.5,1.5),c(0,.5),lwd=20,col="red")
>lines(c(1.5,1.5),c(.5,1),lwd=20,col="green")
>abline(h=0)
>abline(h=0.5)
>abline(h=1)
>
>The colored lines should be between the horizontal lines, but because of the
>rounded end caps, they aren't.
>
>Under X11, if I set lty to something like "F0", the end caps are
turned off.
>Unfortunately, the "0" appears to get ignored, and the line style
is the same
>as "FF", i.e. 16 units of drawn line, then 16 units of space.
>
>With the postscript device, lines styles like "FF" still get
rounded end
caps,
>unlike X11.
Hi,

This is a behaviour I already noticed under Windows NT when using plot(...,
type="h", lwd=10), so that the thickness of the lines is expanded all
around their starting- and end-points, rather than only perpandicularly to
the lines (which was what I expected). A workaround for the example you
give is to use rect()

 thi <- 0.025
 plot(0:1)
 rect(1.5-thi, 0, 1.5+thi, 0.5, col="red", border="red")
 rect(1.5-thi, 0.5, 1.5+thi, 1, col="green", border="green")
 abline(h=0); abline(h=0.5); abline(h=1)


Emmanuel Paradis
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> Date: Wed, 20 Sep 2000 10:50:08 +0200
> To: Trent Piepho <xyzzy at speakeasy.org>
> From: Emmanuel Paradis <paradis at isem.univ-montp2.fr>
> Subject: Re: [R] getting lines with non-rounded caps?
> Cc: r-help at stat.math.ethz.ch
> 
> At 11:31 19/09/00 -0700, you wrote:
> >I'm trying to get a plot with wide lines that don't have
rounded end caps.
> >You can see the effect quite clearly with a plot like this:
> >
> >plot(0:1)
> >lines(c(1.5,1.5),c(0,.5),lwd=20,col="red")
> >lines(c(1.5,1.5),c(.5,1),lwd=20,col="green")
> >abline(h=0)
> >abline(h=0.5)
> >abline(h=1)
> >
> >The colored lines should be between the horizontal lines, but because
of the
> >rounded end caps, they aren't.
> >
> >Under X11, if I set lty to something like "F0", the end caps
are turned off.
> >Unfortunately, the "0" appears to get ignored, and the line
style is the same
> >as "FF", i.e. 16 units of drawn line, then 16 units of space.
> >
> >With the postscript device, lines styles like "FF" still get
rounded end
> caps,
> >unlike X11.
> 
> Hi,
> 
> This is a behaviour I already noticed under Windows NT when using plot(...,
> type="h", lwd=10), so that the thickness of the lines is expanded
all
> around their starting- and end-points, rather than only perpandicularly to
> the lines (which was what I expected). A workaround for the example you
> give is to use rect()
It may be what you expected, but it is not what normally happens in
computer graphics.  The idea is that the line is drawn with a round pen
of radius lwd.  If you use a flat-ended pen, you get problems when a
line changes direction discontinuously (and polylines do), as the
line appears half-broken on the outside.

PostScript has different options for the internal changes of
direction (setlinejoin) and the ends (setlinecap).  So does X11, and I
see that the devX11 device uses JoinRound, and CapRound if lty=0
otherwise CapButt.

Is there a case for adding cap-style and join-style to the line type options?
I am not sure if all the devices could support them.  In particular,
I don't know if it is possible to achieve different internal and
end effects under the Windows graphics model.

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272860 (secr)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

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