Okay dokay. I’m still stuck in the old days (don’t ask 
) when we worried about that stuff. FWIW here’s my version of the interpolate function. It depends on equally spaced knots, but has no divisions. Doesn’t mean it would be faster. Just a FYI
float raw_tca_distort_amt_sq_basis(float x_sq, float *knots, int Nminus1)
{
// x is (squared radius from image center) / (maximum squared radius)
if(x_sq > .9999) return knots[Nminus1] ; // in case of rounding error computing x_sq
float fi = x_sq*(float)(Nminus1) ;
int i = floor(fi) ; // lower bounding knot is i
float p1 = (fi - i) ; // proprtion of knot i+1 to use
return (knots[i]*(1.-p1) + knots[i+1]*p1) ;
}
Cheers,
Jeff Welty