code: public void transform(double data[], int rowspan) {
      checkData(data,rowspan);
      for(int i=0; i<nrows; i++){
        rowFFT.transform(data,i*rowspan,2); }
      for(int j=0; j<ncols; j++){
        colFFT.transform(data,2*j,rowspan); }}
docs: Compute the Fast Fourier Transform of data leaving the result in data.
  The array data must be dimensioned (at least) 2*nrows*ncols, consisting of
  alternating real and imaginary parts.