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

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

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"/>

Tav