bsplin3val 3d spline arbitrary derivative evaluation function

## Syntax

    [dfp]=bsplin3val(xp,yp,zp,tl,der)

## Arguments

xp, yp, zp

  real vectors or matrices of same size

tl

  tlist of type "splin3d", defining a 3d tensor spline (called `s` in the following)

der

  vector with 3 components `[ox,oy,oz]` defining which derivative of `s` to compute.

dfp

  vector or matrix of same format than `xp`, `yp` and `zp`, elementwise evaluation of the specified derivative of `s` on these points.

## Description

While the function interp3d may compute only the spline `s` and its first derivatives, `bsplin3val` may compute any derivative of `s`. The derivative to compute is specified by the argument `der=[ox,oy,oz]` :

So `der=[0 0 0]` corresponds to s, `der=[1 0 0]` to ds/dx, `der=[0 1 0]` to ds/dy, `der=[1 1 0]` to d2s/dxdy, etc...

For a point with coordinates (xp(i),yp(i),zp(i)) outside the grid, the function returns 0.

## Examples

    deff("v=f(x,y,z)","v=cos(x).*sin(y).*cos(z)");
    deff("v=fx(x,y,z)","v=-sin(x).*sin(y).*cos(z)");
    deff("v=fxy(x,y,z)","v=-sin(x).*cos(y).*cos(z)");
    deff("v=fxyz(x,y,z)","v=sin(x).*cos(y).*sin(z)");
    deff("v=fxxyz(x,y,z)","v=cos(x).*cos(y).*sin(z)");
    n = 20;  // n x n x n  interpolation points
    x = linspace(0,2*%pi,n); y=x; z=x; // interpolation grid
    [X,Y,Z] = ndgrid(x,y,z); V = f(X,Y,Z);
    tl = splin3d(x,y,z,V,[5 5 5]);

    // compute f and some derivates on a point
    // and compare with the spline interpolant
    xp = grand(1,1,"unf",0,2*%pi);
    yp = grand(1,1,"unf",0,2*%pi);
    zp = grand(1,1,"unf",0,2*%pi);

    f_e = f(xp,yp,zp)
    f_i = bsplin3val(xp,yp,zp,tl,[0 0 0])

    fx_e = fx(xp,yp,zp)
    fx_i = bsplin3val(xp,yp,zp,tl,[1 0 0])

    fxy_e = fxy(xp,yp,zp)
    fxy_i = bsplin3val(xp,yp,zp,tl,[1 1 0])

    fxyz_e = fxyz(xp,yp,zp)
    fxyz_i = bsplin3val(xp,yp,zp,tl,[1 1 1])

    fxxyz_e = fxxyz(xp,yp,zp)
    fxxyz_i = bsplin3val(xp,yp,zp,tl,[2 1 1])

## See also

- splin3d
- interp3d

## History

5.4.0 previously, imaginary part of input arguments were implicitly ignored.