Efficient Bicubic filtering code in GLSL?

Question

I\'m wondering if anyone has complete, working, and efficient code to do bicubic texture filtering in glsl. There is this:

http://www.codeproject.com/Articles/236394/Bi-

Answer 1

I've been using @Maf 's cubic spline recipe for over a year, and I recommend it, if a cubic B-spline meets your needs.

But I recently realized that, for my particular application, it is important for the intensities to match exactly at the sample points. So I switched to using a Catmull-Rom spline, which uses a slightly different recipe like so:

// Catmull-Rom spline actually passes through control points
vec4 cubic(float x) // cubic_catmullrom(float x)
{
    const float s = 0.5; // potentially adjustable parameter
    float x2 = x * x;
    float x3 = x2 * x;
    vec4 w;
    w.x =    -s*x3 +     2*s*x2 - s*x + 0;
    w.y = (2-s)*x3 +   (s-3)*x2       + 1;
    w.z = (s-2)*x3 + (3-2*s)*x2 + s*x + 0;
    w.w =     s*x3 -       s*x2       + 0;
    return w;
}

I found these coefficients, plus those for a number of other flavors of cubic splines, in the lecture notes at: http://www.cs.cmu.edu/afs/cs/academic/class/15462-s10/www/lec-slides/lec06.pdf