Re: postscript dash patterns in parametric vs data plots

[Jonathan T. <jt...@as...>]

Hi,

On Sunday 28 March 2010 10:13:12 I 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.

On Sun, 28 Mar 2010, Ethan Merritt wrote:
> 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?

Ok, I have done this, it's sourceforge bug id #2978743, description
"postscript dash patterns differ between parametric & data pl"
(it looks like they truncated my description string),
https://sourceforge.net/tracker/?func=detail&aid=2978743&group_id=2055&atid=102055
I have attached 3 files to the bug report:
* my gnuplot script 'par-dash.gnuplot'
* the eps output produced by gnuplot 4.2.6 on my computer, 'par-dash.eps'
* an X windows screen dump (from 'xwd', converted to a jpeg image via
  'xwdtopnm | pnmtojpeg -quality 95') showing 'gv' on my computer viewing
  the eps file (with antialiasing turned OFF so as not to confuse things),
  with the wierd dash pattern visible on the left and the normal dash
  pattern visible on the right


> 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.

That does seem plausible.  And maybe there's an uninitialized variable
or suchlike hiding in the code to explain why the output is different
on my computer than on your computer.


> But I could be wrong.  Let's see whether your output file looks the same
> as the one I get here.

Yes, I'm interested in what your postscript viewer shows when pointed
at my eps file -- this should at least resolve the question of whether
the problem I see is in my gnuplot or in my postscript viewer ('gv').

ciao,

-- 
-- "Jonathan Thornburg [remove -animal to reply]" <jt...@as...>
   Dept of Astronomy, Indiana University, Bloomington, Indiana, USA
   "Washing one's hands of the conflict between the powerful and the
    powerless means to side with the powerful, not to be neutral."
                                      -- quote by Freire / poster by Oxfam