[cairo] [Fwd: Problem with stroke vs. fill]

Carl Worth cworth at east.isi.edu
Fri Apr 2 19:56:05 PST 2004

On Apr 2, Duncan Mak wrote:
 > Thanks so much for working on Cairo, I've been happily using it to
 > implement the various 2D geometry drawing/filling functions in the GDI+
 > Graphics class. We're making good progress!

You're quite welcome. I have fun working on it, and I'm glad you're
having some luck using it.

 > I found an algorithm [1] from the comp.text.pdf FAQ for drawing
 > elliptical arcs using bezier curves. Using this algorithm, I was able to
 > implement the functions for drawing and filling arcs and pies.

Cool. I don't know if it was there already when you started this work,
but we now have the following function in cairo that may help you drop
a bunch of code:

	cairo_arc (cairo_t *cr,
	           double xc, double yc,
	           double radius,
	           double angle1, double angle2);

That will let you do any circles you want. For an ellipse you'll just
just need to to precede it with a cairo_scale as appropriate.

 > All being the same, with stroke working and fill not working, I'm
 > wondering, is this a Cairo bug?

I don't think so. Looking through your code I see several calls to
cairo_move_to, so your final path is a complex path consisting of
several independent sub-paths. When you call cairo_fill on a complex
path it implicitly closes each sub-path and fills them. So I think
that's what you're seeing here.

To fix this you'll want to fix make_pie so that it creates a simple
path, (ie. one cairo_move_to, followed by several calls to
cairo_line_to and cairo_curve_to). Does that make sense?

Although, frankly, I think it will be a lot easier to just switch to
using cairo_arc directly.

Let me know if you have any more problems.


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