[cairo] Adventures in line widths - ellipse example

[Steven Chaplin]

On Thu, 2006-04-27 at 14:28 -0700, Christopher Barker wrote:
> This is all a little beyond me, but I was about to post a question here 
> that's related.
> 
> I'm exploring what I might want to use as the rendering toolkit for a 
> mapping engine. I'm evaluating Cairo and Agg at the moment. I see from 
> the note you referred to that you've been talking to Maxim, which is 
> great; Agg has some very nice features. It's sub-pixel rendering is superb.
> 
> Anyway, I was trying to figure out how to draw an ellipse with Cairo. I 
> found an example in the docs somewhere that indicated that you need to 
> translate and scale appropriately, then draw a circle.
> 
> I got this to work, but I've found that the pen gets scaled with the 
> circle, so I get very fat lines, and they are fatter in the direction 
> that the ellipse is longer.
> 
> Here's the Python code I'm using:
> 
>      def DrawEllipse(self, x, y, width, height):
>          ctx = self.ctx
> 
>          ctx.save()
> 
>          ctx.translate(x + width / 2., y + height / 2.);
>          ctx.scale(1. * (width / 2.), 1. * (height / 2.));
>          ctx.arc(0., 0., 1., 0., 2 * N.pi);
> 
>          self.SetupFill()
>          ctx.fill_preserve()
>          self.SetupStroke()
>          ctx.stroke()
> 
>          ctx.restore();
> 
> Where self.SetupStroke is:
> 
>      def SetupStroke(self):
>          ctx = self.ctx
>          ctx.set_source_rgba(*self.PenColor)
>          ctx.set_line_width(self.LineWidth)
>          ctx.set_dash(*self.DashPattern)
> 
> when I do this:
> 
>      x, y, w, h = 50, 100, 300, 50
> 
>      dc.LineWidth = 1
>      dc.DrawEllipse(x, y, w, h )
>      dc.SaveFile("junk6.png")
> 
> I get the enclosed PNG.
> 
> Could I change the order I make the calls to get this right? Or is this 
> being fixed by what this discussion is about?
> 
> -Chris

Create the ellipse path and do ctx.restore() before you do the stroke.
Example attached.
In general I thinks its best to have path creation functions and
fill/stroke functions which are separate.

Regards,
Steve


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