Re: [Lib2geom-devel] lib2geom basics: define pathvector, define ellipse, define intersection

[mathog <ma...@ca...>]

On 02-Sep-2012 15:03, Krzysztof Kosiński wrote:
> 2012/8/31 mathog <ma...@ca...>:

>> 1.  generate a second PathVector "pathB".  In one case this is a
>> rectangle (with corners known), and in the the other it is an 
>> elliptical
>> ring (center, inner/outer radii, minor, major unit axis vectors all
>> known).
>
> If your rectangle has corners at points a, b, c, d, do the following:
>
> Geom::Point
> Geom::Path x(a);
> x.appendNew<Geom::LineSegment>(b);
> x.appendNew<Geom::LineSegment>(c);
> x.appendNew<Geom::LineSegment>(d);
> x.close();

This one works.

>
> For an elliptical ring, do the same, but use two
> Geom::SVGEllipticalArc. The constructors are the same as for
> Geom::EllipticalArc and are documented with Doxygen comments.
>

The elliptical path is giving me conniptions.  This is what is seen 
when the "cutter" is viewed
(sorry about any line wraps):

   Geom::Path cutter;
   cutter.start(Geom::Point(250, 1000) );
// just the point
//   cutter.appendNew<Geom::SVGEllipticalArc>( 50.0, 500.0, true, true, 
M_PI,  Geom::Point(250, 1000) );
// long skinny ellipse at 1:30 on clock
//   cutter.appendNew<Geom::SVGEllipticalArc>( 50.0, 500.0, true, true, 
M_PI,  Geom::Point(250.1, 1000) );
// ditto
//   cutter.appendNew<Geom::SVGEllipticalArc>( 50.0, 500.0, true, true, 
M_PI,  Geom::Point(251.0, 1000) );
// ditto
//   cutter.appendNew<Geom::SVGEllipticalArc>( 50.0, 500.0, true, true, 
M_PI,  Geom::Point(270.0, 1000) );
// ditto
//   cutter.appendNew<Geom::SVGEllipticalArc>( 50.0, 500.0, true, true, 
M_PI/4.0,  Geom::Point(270.0, 1000) );
// long skinny elipse at 7:30 on clock
//   cutter.appendNew<Geom::SVGEllipticalArc>( 50.0, 500.0, true, true, 
0.0,  Geom::Point(270.0, 1000) );

I didn't pick these numbers arbitrarily, they were derived using the 
svg-elliptical-arc-test-toy test program. From that
I expected the first one to be a 1000 tall, 100 wide ellipse with its 
lowest point at 250,1000.  Here are
the bezier points (not the control points) for this form

   cutter.appendNew<Geom::SVGEllipticalArc>( 50.0, 500.0, true, true, 
M_PI/4.0,  Geom::Point(270.0, 1000) );

as seen when they hit the "stroke" part of my driver:

244.204    996.223
276.971    953.438
365.389    878.694
496.893    782.607
652.798    678.83
810.968    582.097
948.966    506.129
1047.23    461.692
1091.83    455.077
1076.46    487.222
1003.26    553.574
  882.61    644.735
680.347    782.252
425.566    931.384
313.999    985.863

If you plot them in Excel or gnuplot you can see the sort of ellipse 
that is coming out.  Long and skinny, but
the wrong angle.


Is there maybe a function in 2geom to convert from ellipse(center, long 
radius, short radius, rotation angle) to
this other path specification system?

Thanks,

David Mathog
ma...@ca...
Manager, Sequence Analysis Facility, Biology Division, Caltech