I have implemented the rlft3 routine into my C code sucessfully as described in the example 1 to perform image filtering. The fft and also the inverse fft is running perfectly.

But I dont get the step of multiplying the arrays "data" and "speq" with a filter function - lets say a simple gaussian.
I think I have a problem with sorting of the output of the first fft, but I have no success by following the explanations of chapter 12.5.

Can anybody help me? It would be great if anybody would have some line of C code how to implement e.g. a gaussian filter for the example 1 code.

Thank you very much!

PS: Using a 2D image I have set input data to: data[1][matrix][matrix]