I want to implement the bicubic interpolation of a hermite spline (specifically the akima spline). That means I have the points, and the derivatives in x & y direction for each point. But I do not know how to calculate the cross derivative from that.
I know there is this in the Wikipedia article:
To find the cross derivative fxy, take the derivative in both axes, one at a time. For example, one can first use the fx procedure to find the x derivatives of the points above and below the target point, then use the fy procedure on those values (rather than, as usual, the values of f for those points) to obtain the value of fxy(x, y) for the target point.
But I do not understand how those fx and fy are put together to form fxy.
