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Re: postscript dash patterns in parametric vs data plots
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From: Ethan M. <merritt@u.washington.edu> - 2010-03-28 18:29:28
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On Sunday 28 March 2010 10:13:12 Jonathan Thornburg wrote:
> I have stumbled on what looks to me like a bug in postscript dash
> patterns when using parametric mode. In particular, when using
> 'set term postscript eps', the 'with lines linetype 2' dash pattern
> seems to differ between
> (a) plotting a certain horizontal line (about 3 dashes long) using a
> pair of of parametric functions to specify x and y, versus
> (b) plotting that same horizontal line using '-' inline data to explicitly
> specify the endpoint coordinates of the line.
Thank you for your detailed bug report.
Unfortunately, I cannot reproduce it the problem.
Using either 4.2.6 or current CVS, the two lines produced by your example
script are indistinguishable to my eyes even at 10x magnification. Perhaps
in the CVS output one of the component dashes is ever so slightly longer,
but if so it is at the very edge of perceptible difference.
Could you please file this report on SourceForge, and attach an
output PostScript file demonstrating the problem?
Notwithstanding my failure to reproduce this, I'm going to guess that
the problem lies in the PostScript implementation. From your description,
I'm guessing that the dot-dash pattern associated with the line is re-started
for each line segment, so if you draw a line made up of multiple segments
(as is likely for a parametric curve) the pattern is discontinuous at each
break point.
But I could be wrong. Let's see whether your output file looks the same
as the one I get here.
Ethan
> (b) produces the usual dash pattern, while (in my test case) (a) seems
> to be broken.
>
> In particular, consider the gnuplot script given below. (This is
> abstracted from a larger "real-world" script.) Notice that this draws
> two dashed lines of the same length, both 'with lines linetype 2'.
>
> Using gnuplot 4.2.6 compiled from source ('show version long' output
> also given below), this produces an eps file showing a pair of labelled
> dashed lines. Viewing the eps file with gv (version 3.5.8p5, based on
> ghostscript 8.63p7), the left dashed line (the one specified by a pair
> of parametric functions) has a dash pattern which looks like this:
> ------- ------- -------
> This does not match the dash pattern of other 'with lines linetype 2'
> plots in my real-world script. :( In contrast, the right dashed line
> (the one specified by explicit endpoint coordinates) has a dash pattern
> which looks like this:
> ------- ------- -------
> This *does* match the dash pattern of other 'with lines linetype 2' plots.
>
> To me, this (the left dash line's wierd dash pattern) looks like a bug.
> I have checked that the wierd dash pattern is unchanged when I disable
> antialiasing in gv.
>
> --- begin gnuplot script ---
> set term postscript eps enhanced monochrome 18 size 8cm,8cm
> set output 'par-dash.eps'
>
> unset key
>
> set lmargin 9.0
> set rmargin 2.5
>
> unset xtics
> unset ytics
> unset x2tics
> unset y2tics
>
> unset xlabel
> unset ylabel
>
> set border 0
>
> set xrange [20:40]
> set yrange [0.0:1.0]
>
> set parametric
> set dummy ell
> set trange [0:1]
>
> # map t=[0:1] --> [a:b]
> map(a,b,t) = a + (b-a)*t
>
> f6_x = 20.00
> f8_x = 33.00
> dash_dx = 1.00
> dash_y = 0.1625
> label_dx = 1.50
> label_dy = 0.00
>
> set label "f_6(ell) parametric fn" at f6_x+label_dx, dash_y+label_dy
> set label "f_6(ell) data" at f8_x+label_dx, dash_y+label_dy
>
> plot f6_x+map(0,dash_dx,ell), dash_y \
> notitle \
> with lines linetype 2 linewidth 3.0 linecolor -1, \
> '-' \
> notitle \
> with lines linetype 2 linewidth 3.0 linecolor -1
>
> # data for '-' plot above
> 33.00 0.1625
> 34.00 0.1625
> eof
>
> set output
> --- end gnuplot script ---
>
> --- begin 'show version long' output ---
> gnuplot> show version long
>
> G N U P L O T
> Version 4.2 patchlevel 6
> last modified Sep 2009
> System: OpenBSD 4.6
>
> Copyright (C) 1986 - 1993, 1998, 2004, 2007 - 2009
> Thomas Williams, Colin Kelley and many others
>
> Type `help` to access the on-line reference manual.
> The gnuplot FAQ is available from http://www.gnuplot.info/faq/
>
> Send bug reports and suggestions to <http://sourceforge.net/projects/gnuplot>
>
> Compile options:
> -READLINE +LIBREADLINE -HISTORY +BACKWARDS_COMPATIBILITY +BINARY_DATA
> +GD_PNG +GD_JPEG +GD_TTF +GD_GIF +ANIMATION
> -NOCWDRC +X11 +X11_POLYGON +MULTIBYTE +USE_MOUSE +HIDDEN3D_QUADTREE
> +DATASTRINGS +HISTOGRAMS +OBJECTS +STRINGVARS +MACROS +IMAGE
>
> DRIVER_DIR = "/usr/local/libexec/gnuplot/4.2"
> GNUPLOT_PS_DIR = "/usr/local/share/gnuplot/4.2/PostScript"
> HELPFILE = "/usr/local/share/gnuplot/4.2/gnuplot.gih"
>
> gnuplot>
> --- end 'show version long' output ---
>
> ciao,
>
> --
> -- "Jonathan Thornburg [remove -animal to reply]" <jt...@as...>
> Dept of Astronomy, Indiana University, Bloomington, Indiana, USA
> "C++ is to programming as sex is to reproduction. Better ways might
> technically exist but they're not nearly as much fun." -- Nikolai Irgens
>
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--
Ethan A Merritt
Biomolecular Structure Center
University of Washington, Seattle 98195-7742
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