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

The missing function cubic() in JAre's answer could look like this:

vec4 cubic(float x)
{
    float x2 = x * x;
    float x3 = x2 * x;
    vec4 w;
    w.x =   -x3 + 3*x2 - 3*x + 1;
    w.y =  3*x3 - 6*x2       + 4;
    w.z = -3*x3 + 3*x2 + 3*x + 1;
    w.w =  x3;
    return w / 6.f;
}

It returns the four weights for cubic B-Spline.

It is all explained in NVidia Gems.