[cairo] Fwd: Bezier curve

[Andrea Canciani]

(I forgot to write to the list... sorry for the double mail)


---------- Forwarded message ----------
From: Andrea Canciani <ranma42 at gmail.com>
Date: Tue, Nov 15, 2011 at 10:42 AM
Subject: Re: [cairo] Bezier curve
To: Inderjit Singh <inderjit.singh at spaceapplications.com>


On Mon, Nov 7, 2011 at 3:42 PM, Inderjit Singh
<inderjit.singh at spaceapplications.com> wrote:
> Hi,
>
> I'm new to cairo and I'm trying to draw a smooth curve between using an
> array of points.
>
> Say I have an array with (x,y) coordinates as: POS_T pos = {{0.0, 0.0},
> {1.0,1.0}, {1.5, 2.0}, {0.5, 3.0}, {0.0, 4.0}};
>
> So basically, a half round 'circle'. I've been looking in cairo docs but the
> closest thing I get to is the cairo_curve_to but for that it requires
> control points. How is it determined? I want a simple smooth line but using
> Bezier algorithm requires additional data to make it happen. Is that
> correct?

Yes, the additional data determines how the smoothing between the
points is performed.
You can choose the control points and get different smoothing types
between your points.
You might want to read something about splines (cairo supports cubic
Bezier curves, which in turn can represent any parametric cubic
curve).
You can find an algorithm for cubic spline interpolation here
http://en.wikipedia.org/wiki/Spline_interpolation
(I didn't test it).


>
> Or can I simply use curve_line_to and then interpolate this? currently the
> code looks like this for line:

cairo_line_to() would connect the points with straight line segments.

>
> ...
>     for(uint32_t i = 0; i < boundary; i=i++)
>     {
>         cairo_move_to(cr, station->coverage[i].x, station->coverage[i].y);
>
> //        cairo_line_to(cr,
> //                      station->coverage[(i+1) % boundary].x,
> //                      station->coverage[(i+1) % boundary].y);
>
>         cairo_curve_to(cr,
>                      station->coverage[i].x,
>                      station->coverage[(i].y,
>                      station->coverage[i+1].x,
>                      station->coverage[i+1].y,
>                      station->coverage[i+2].x,
>                      station->coverage[i+2].y);
>     }
>
>     cairo_set_line_width (cr, 1.5);
>     cairo_stroke(cr);
>
>     cairo_destroy(cr);
>
>
> Any help would be appreciated. Thanks.
> Indy

If you need further information, just write ;)

Andrea

>
>
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