Re: [Lib2geom-devel] Dropping invalid arc segments in parser

[Tavmjong B. <tav...@fr...>]

On Tue, 2014-08-05 at 21:29 +0200, Johan Engelen wrote:
> On 5-8-2014 21:15, Tavmjong Bah wrote:
> > On Tue, 2014-08-05 at 20:46 +0200, Johan Engelen wrote:
> >> Hi all,
> >>    Liam just committed a bug fix to Inkscape trunk, regarding our SVG
> >> parser.
> >>
> >> SVG spec says:
> >> "If the endpoints (x1, y1) and (x2, y2) are identical, then this is
> >> equivalent to omitting the elliptical arc segment entirely." [1]
> >>
> >> Liam changed the parser so that it will simply drop such segments:
> >>
> >>      void _arcTo(double rx, double ry, double angle,
> >>                  bool large_arc, bool sweep, Point p)
> >>      {
> >> +        if (_current == p) {  // sic, probably should be
> >> are_near(_current, p)
> >>
> >> +            return;
> >> +        }
> >>          _quad_tangent = _cubic_tangent = _current = p;
> >>          _sink.arcTo(rx, ry, angle, large_arc, sweep, p);
> >>      }
> >
> >> I am not sure about this change. We are not dropping any degenerate
> >> bezier segments in our parser.
> > I think this is not the same as a degenerate bezier. This describes an
> > ellipse with given major and minor axis but there is no way to figure
> > out where along the ellipse one should start drawing. The following path
> > consists of four very different (almost complete) ellipses where the
> > final points are just a little bit different from the starting points:
> >
> >      <path d="m 50,50
> > 	     a 20 30 45 1 0 1,1
> > 	       20 30 45 1 0 -1,-1
> > 	       20 30 45 1 0 -1,1
> > 	       20 30 45 1 0 1,-1"
> > 	  style="fill:none;stroke:black"/>
> 
> Ok, I see you're saying that "a 20 30 45 1 0 0,0" is ambiguous, hence 
> invalid and the parser can skip it. (degenerate line segments are not 
> ambiguous)
> This would vote in favor of floating point comparison, but I think in 
> the interest of (human) predictability, the equals-check should be made 
> are_near. Do you agree?

Yes, otherwise small rounding errors would cause large visual
differences.

Tav