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

From: Michael Petrie
Subject: [Help-gsl] Using GSL to draw a 2D B-Spline path using unevenly spaced points
Date: Fri, 13 Jun 2014 10:56:36 +1200

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.


PS, I have also asked this question at Stack Overflow too:

Michael Petrie
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