Intel® oneAPI Math Kernel Library Developer Reference - C

Configuring and Computing an FFT in C/C++

The table below summarizes information on configuring and computing an FFT in C/C++ for all kinds of transforms and possible combinations of input and output domains.

FFT to Compute

Input Data

Output Data

Required FFT Function Calls

Complex-to-complex,

in-place,

forward or backward

Interleaved complex numbers

Interleaved complex numbers 

/* Configure a Descriptor */
status = DftiCreateDescriptor(&hand, <precision>, DFTI_COMPLEX, <dimension>, <sizes>);
status = DftiCommitDescriptor(hand);

/* Compute an FFT */
/* forward FFT */
status = DftiComputeForward(hand, X_inout);
/* or backward FFT */
status = DftiComputeBackward(hand, X_inout);

Complex-to-complex,

out-of-place,

forward or backward

Interleaved complex numbers

Interleaved complex numbers 

/* Configure a Descriptor */
status = DftiCreateDescriptor(&hand, <precision>, DFTI_COMPLEX, <dimension>, <sizes>);
status = DftiSetValue(hand, DFTI_PLACEMENT, DFTI_NOT_INPLACE);
status = DftiCommitDescriptor(hand);

/* Compute an FFT */
/* forward FFT */
status = DftiComputeForward(hand, X_in, Y_out);
/* or backward FFT */
status = DftiComputeBackward(hand, X_in, Y_out);

Complex-to-complex,

in-place,

forward or backward

Split-complex numbers

Split-complex numbers 

/* Configure a Descriptor */
status = DftiCreateDescriptor(&hand, <precision>, DFTI_COMPLEX, <dimension>, <sizes>);
status = DftiSetValue(hand, DFTI_COMPLEX_STORAGE, DFTI_REAL_REAL);
status = DftiCommitDescriptor(hand);

/* Compute an FFT */
/* forward FFT */
status = DftiComputeForward(hand, Xre_inout, Xim_inout);
/* or backward FFT */
status = DftiComputeBackward(hand, Xre_inout, Xim_inout);

Complex-to-complex,

out-of-place,

forward or backward

Split-complex numbers

Split-complex numbers 

/* Configure a Descriptor */
status = DftiCreateDescriptor(&hand, 
<precision>, DFTI_COMPLEX, <dimension>, <sizes>);
status = DftiSetValue(hand, DFTI_COMPLEX_STORAGE, DFTI_REAL_REAL);
status = DftiSetValue(hand, DFTI_PLACEMENT, DFTI_NOT_INPLACE);
status = DftiCommitDescriptor(hand);

/* Compute an FFT */
/* forward FFT  */
status = DftiComputeForward(hand, Xre_in, Xim_in, Yre_out, Yim_out);
/* or backward FFT  */
status = DftiComputeBackward(hand, Xre_in, Xim_in, Yre_out, Yim_out);

Real-to-complex,

in-place,

forward

Real numbers

Numbers in the CCE format 

/* Configure a Descriptor */
status = DftiCreateDescriptor(&hand, <precision>, DFTI_REAL, <dimension>, <sizes>);
status = DftiSetValue(hand, DFTI_CONJUGATE_EVEN_STORAGE,  DFTI_COMPLEX_COMPLEX);
status = DftiSetValue(hand, DFTI_PACKED_FORMAT, DFTI_CCE_FORMAT);
status = DftiSetValue(hand, DFTI_INPUT_STRIDES, <real_strides>);
status = DftiSetValue(hand, DFTI_OUTPUT_STRIDES, <complex_strides>);
status = DftiCommitDescriptor(hand);

/* Compute an FFT */
status = DftiComputeForward(hand, X_inout);

Real-to-complex,

out-of-place,

forward

Real numbers

Numbers in the CCE format 

/* Configure a Descriptor */
status = DftiCreateDescriptor(&hand, <precision>, DFTI_REAL, <dimension>, <sizes>);
status = DftiSetValue(hand, DFTI_CONJUGATE_EVEN_STORAGE, DFTI_COMPLEX_COMPLEX);
status = DftiSetValue(hand, DFTI_PACKED_FORMAT, DFTI_CCE_FORMAT);
status = DftiSetValue(hand, DFTI_PLACEMENT, DFTI_NOT_INPLACE);
status = DftiSetValue(hand, DFTI_INPUT_STRIDES, <real_strides>);
status = DftiSetValue(hand, DFTI_OUTPUT_STRIDES, 
<complex_strides>);
status = DftiCommitDescriptor(hand);

/* Compute an FFT */
status = DftiComputeForward(hand, X_in, Y_out);

Complex-to-real,

in-place,

backward

Numbers in the CCE format

Real numbers 

/* Configure a Descriptor */
status = DftiCreateDescriptor(&hand, <precision>, DFTI_REAL, <dimension>, <sizes>);
status = DftiSetValue(hand, DFTI_CONJUGATE_EVEN_STORAGE,  DFTI_COMPLEX_COMPLEX);
status = DftiSetValue(hand, DFTI_PACKED_FORMAT, DFTI_CCE_FORMAT);
status = DftiSetValue(hand, DFTI_INPUT_STRIDES, <complex_strides>);
status = DftiSetValue(hand, DFTI_OUTPUT_STRIDES, <real_strides>);
status = DftiCommitDescriptor(hand);

/* Compute  an FFT */
status = DftiComputeBackward(hand, X_inout);

Complex-to-real,

out-of-place,

backward

Numbers in the CCE format

Real numbers 

/* Configure a Descriptor */
status = DftiCreateDescriptor(&hand, <precision>, DFTI_REAL, <dimension>, <sizes>);
status = DftiSetValue(hand, DFTI_CONJUGATE_EVEN_STORAGE,  DFTI_COMPLEX_COMPLEX);
status = DftiSetValue(hand, DFTI_PLACEMENT, DFTI_NOT_INPLACE);
status = DftiSetValue(hand, DFTI_PACKED_FORMAT, DFTI_CCE_FORMAT);
status = DftiSetValue(hand, DFTI_INPUT_STRIDES, <complex_strides>);
status = DftiSetValue(hand, DFTI_OUTPUT_STRIDES, <real_strides>);
status = DftiCommitDescriptor(hand);

/* Compute an FFT */
status = DftiComputeBackward(hand, X_in, Y_out);

You can find C programs that illustrate configuring and computing FFTs in the examples/dftc/subdirectory of your Intel® oneAPI Math Kernel Library directory.

Parent topic: FFT Computation Functions