[Help-gsl] Using GSL to draw a 2D B-Spline path using unevenly spaced points

[Michael Petrie]

Hi, my name's Michael, and I'm new to the mailing list, so forgive me if
I'm asking this question in the wrong place.

I'm trying to using the GNU Scientific Library (GSL) to draw a smooth path
from A to B. I'm using an API that returns a small number (8 in this case)
of irregularly spaced points (in red), that you can see picture [1].

The purple points represent the points that I would like to see returned
from GSL.

Firstly, is this kind of 2D B-Spline shape obtainable by using GSL? I don't
know much about B-Splines, let alone 2D B-Splines. I was able to get the
B-Splines example at link [2] running and creating a smooth .ps file
without problem, but that example uses uniform breakpoints with the
following code:

    /* use uniform breakpoints on [0, 15] */
    gsl_bspline_knots_uniform(0.0, 15.0, bw);

In my case, given that the data I'm given is erratic and not evenly spaced,
would I have to use non-uniform knots? I tried using `gsl_bspline_knots()`,
in order to use non uniform breakpoints within the following test code, but
I'm really not sure if this is the right direction or not.

    #define NCOEFFS 8 // not sure what this number should be - number of
data points?
    #define NBREAK   (NCOEFFS - 2)
    const size_t nbreak = NBREAK;

    int main (void) {

        // (example code)...

        gsl_vector *non_uniform = gsl_vector_alloc(nbreak);

        // create some random breakpoint values
        for (i=0; i<nbreak; i++) {
            double val = gsl_ran_gaussian(r, 2.0);
            printf("val: %f\n", val);
            gsl_vector_set(non_uniform, i, val);
        }

        gsl_bspline_knots(non_uniform, bw);

        // (more example code)...
    }

Further more, how would I translate the above example for drawing B-Splines
in a 2D x/y coordinate space? If GNU Scientific Library is not suitable for
this, could someone make a recommendation for a more suitable C/C++ library?

Any help or pointers in the direction would be much appreciated.

  [1]: http://i.stack.imgur.com/mpxbx.png
  [2]:
http://www.gnu.org/software/gsl/manual/html_node/Example-programs-for-B_002dsplines.html#Example-programs-for-B_002dsplines

PS, I have also asked this question at Stack Overflow too:
http://stackoverflow.com/questions/24194909/using-gnu-scientific-library-gsl-to-draw-a-2d-b-spline-path-using-unevenly-spa

-- 
Michael Petrie
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+64 21 022 99121
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