FFT interface in Intel MKL enables the computation of a multiple number of transforms. To compute multiple transforms, you need to be able to specify the data distribution of the multiple sets of data. The distance between the first data element of the consecutive data sets specifies the distribution. This parameter is required if the number of transforms is greater than one. The parameter is a value of the Integer data type in Fortran and MKL_LONG data type in C. Data sets do not have any common elements. The following example illustrates the specification. Consider computing the forward FFT on three 32-length complex sequences stored in X(0:31, 1), X(0:31, 2), and X(0:31, 3). Suppose the results are to be stored in the locations Y(0:31, k), k = 1, 2, 3, of the array Y(0:63, 3). Thus the input distance is 32 and the output distance is 64.
Below is the code fragment:
Complex :: X_2D(0:31,3), Y_2D(0:63, 3)
Complex :: X(96), Y(192)
Equivalence (X_2D, X)
Equivalence (Y_2D, Y)
...................
Status = DftiCreateDescriptor(Desc_Handle, DFTI_SINGLE, DFTI_COMPLEX, 1, 32)
Status = DftiSetValue(Desc_Handle, DFTI_NUMBER_OF_TRANSFORMS, 3)
Status = DftiSetValue(Desc_Handle, DFTI_INPUT_DISTANCE, 32)
Status = DftiSetValue(Desc_Handle, DFTI_OUTPUT_DISTANCE, 64)
Status = DftiSetValue(Desc_Handle, DFTI_PLACEMENT, DFTI_NOT_INPLACE)
Status = DftiCommitDescriptor(Desc_Handle)
Status = DftiComputeForward(Desc_Handle, X, Y)
Status = DftiFreeDescriptor(Desc_Handle)
Note
The data and result parameters in computation functions are all declared as assumed-size rank-one array DIMENSION(0:*). Therefore, transform a two-dimensional array to a one-dimensional array using the EQUIVALENCE statement or other techniques of Fortran.