gnuplot-beta

clipping filled polygons

[Gerard C. <ca...@kn...>]

LS,
I encountered a problem when I tried to fill a closed polygon which 
extended beyond the xrange, yrange. This is conform the documentation, 
which says:
"Zoom of a filled curve drawn from a datafile may produce empty or 
incorrect area because gnuplot is clipping points and lines, and not areas."
I use Version 4.2 patchlevel 2.
I guess that it is desirable to remove this "feature" from gnuplot.

A well-known  algorithm to clip a filled polygon is by Sutherland and 
Hodgman. Some very ingenious implementations exist, e.g.
http://www.codeguru.com/Cpp/misc/misc/graphics/article.php/c8965/, but I 
wrote a simple one, in Perl, with which I was able to clip my polygons 
myself, before sending them to gnuplot. The advantage of my code is that 
it is simple and very fast, but its disadvantage is that it needs to 
store both the input and the output polygons, so it needs more memory 
than more sophisticated solutions. Yet, with current memory prices, this 
should not really be a problem anymore. My implementation is 
specifically to clip to rectangular areas.

I include my Perl program. Perhaps a gnuplot code guru is interested to 
implement Sutherland/Hodgman clipping; if so, (s)he may find it useful 
to have a look at my Perl program.
I myself cannot write C (easily), but I think conversion of Perl to C 
code will not be a great challenge. The real challenge lies in adjusting 
the code to gnuplot data structures.

-- 
<PRE>
R G   s
 e ard
G R

Gerard Cats, KNMI, PO Box 201, 3730 AE De Bilt, The Netherlands.
Tel. +31302206442, Fax +31302211371,  e-mail ca...@kn...
URL: http://www.knmi.nl/~cats
</PRE>

Re: clipping filled polygons

[Ethan A M. <merritt@u.washington.edu>]

On Friday 11 January 2008 05:47, Gerard Cats wrote:
> 
> I encountered a problem when I tried to fill a closed polygon which 
> extended beyond the xrange, yrange. This is conform the documentation, 
> which says:
> "Zoom of a filled curve drawn from a datafile may produce empty or 
> incorrect area because gnuplot is clipping points and lines, and not areas."
> I use Version 4.2 patchlevel 2.
> I guess that it is desirable to remove this "feature" from gnuplot.

I think you are correct that it would be worth re-visiting the generic
filled-area code in gnuplot.  I suspect that the existing code was written
in terms of clipping points and lines just because those clipping routines
were already implemented.  But yes, the code in graphics.c 
 plot_filledcurves()
 finish_filled_curve()
 fill_missing_corners()
and maybe also
 plot_betweencurves()

is quite tangled, and could probably be entirely replaced by a polygon-
clipping algorithm such as you suggest below.  As a side benefit it would
reduce the size of the code by ~500 lines.

That would be a nice project for someone.  


	Ethan

> A well-known  algorithm to clip a filled polygon is by Sutherland and 
> Hodgman. Some very ingenious implementations exist, e.g.
> http://www.codeguru.com/Cpp/misc/misc/graphics/article.php/c8965/, but I 
> wrote a simple one, in Perl, with which I was able to clip my polygons 
> myself, before sending them to gnuplot. The advantage of my code is that 
> it is simple and very fast, but its disadvantage is that it needs to 
> store both the input and the output polygons, so it needs more memory 
> than more sophisticated solutions. Yet, with current memory prices, this 
> should not really be a problem anymore. My implementation is 
> specifically to clip to rectangular areas.
> 
> I include my Perl program. Perhaps a gnuplot code guru is interested to 
> implement Sutherland/Hodgman clipping; if so, (s)he may find it useful 
> to have a look at my Perl program.
> I myself cannot write C (easily), but I think conversion of Perl to C 
> code will not be a great challenge. The real challenge lies in adjusting 
> the code to gnuplot data structures.
> 

-- 
Ethan A Merritt