FFTFwd

Syntax

Parameters

pSrc	Pointer to the source image ROI.
srcStep	Distance in bytes between starts of consecutive lines in the source image.
pDst	Pointer to the destination image ROI.
dstStep	Distance in bytes between starts of consecutive lines in the destination image.
pSrcDst	Pointer to the source and destination image ROI for the in-place operation.
srcDstStep	Distance in bytes between starts of consecutive lines in the source and destination image for the in-place operation.
scaleFactor	Scale factor (see Integer Result Scaling).
pBuffer	Pointer to the external work buffer, can be NULL.

Description

The function ippiFFTFwd is declared in the ippi.h file. It operates with ROI (see Regions of Interest in Intel IPP).

This function performs a forward FFT on each channel of the source image ROI pSrc (pSrcDst for in-place flavors) and writes the Fourier coefficients into the corresponding channel of the destination buffer pDst (pSrcDst for in-place flavors). The size of ROI is N x M, it is specified by the parameters orderX, orderY calling the function ippiFFTInitAlloc.

The function flavor ippiFFTFwd_RToPack that operates on images with real data takes advantage of the symmetry property and stores the output data in RCPack2D format. It supports processing of the 1-, 3-, and 4-channel images. Note that the functions with AC4 descriptor do not process alpha channel.

The function flavor ippiFFTFwd_CToC that operates on images with complex data does not perform any packing of the transform results as no symmetry with respect to frequency domain data is observed in this case. Memory layout of images with complex data follows the same conventions as for real images provided that each pixel value consists of two numbers: imaginary and real part.

The forward FFT functions use the previously initialized pFFTSpec context structure to set the mode of calculations and retrieve support data.

The function may be used with the external work buffer pBuffer to avoid memory allocation within the functions. Once the work buffer is allocated, it can be used for all following calls to the functions computing FFT. As internal allocation of memory is too expensive operation and depends on operating system and/or runtime libraries used - the use of an external buffer improves performance significantly, especially for the small size transforms.

The size of the external buffer must be previously computed by the function ippiFFTGetBufSize. If the external buffer is not specified (pBuffer is set to NULL), the function itself allocates the memory needed for operation.

The Example “Fast Fourier Transform of a Real Image” illustrates the use of the forward FFT function for processing real data. The Example “Fast Fourier Transform of a Complex Image ” explains FFT processing of complex data. Note that input values are the same as in the previous example, but specified in complex domain.

Return Values

ippStsNoErr	Indicates no error. Any other value indicates an error or a warning.
ippStsNullPtrErr	Indicates an error condition if pSrc, pDst, or pFFTSpec pointer is NULL.
ippStsStepErr	Indicates an error condition if srcStep or dstStep value is zero or negative.
ippStsContextMatchErr	Indicates an error condition if a pointer to an invalid pFFTSpec structure is passed.
ippStsMemAllocErr	Indicates an error condition if memory allocation fails.

Fast Fourier Transform of a Real Image

IppStatus fft( void )
{

   Ipp32f src[64] = {0}, dst[64];

   IppiFFTSpec_R_32f *spec;

   IppStatus status;

   src[0] = -3; src[9] = 1;

   ippiFFTInitAlloc_R_32f(&spec, 3, 3, IPP_FFT_DIV_INV_BY_N, ippAlgHintAccurate);

   status = ippiFFTFwd_RToPack_32f_C1R( src, 8*sizeof(Ipp32f), dst,

                                        8*sizeof(Ipp32f), spec, 0 );

   ippiFFTFree_R_32f( spec );

   return status;

}

On exit from the ippiFFTFwd_RToPack function, the destination buffer contains the following data in RCPack2D format:

-2.00  -2.29  -0.71  -3.00  -1.00  -3.71  -0.71  -4.00  

-2.29  -3.00  -1.00  -3.71  -0.71  -4.00  +0.00  -3.71  

-0.71  -3.71  -0.71  -4.00  +0.00  -3.71  +0.71  +0.71  

-3.00  -4.00  +0.00  -3.71  +0.71  -3.00  +1.00  -3.00  

-1.00  -3.71  +0.71  -3.00  +1.00  -2.29  +0.71  +1.00  

-3.71  -3.00  +1.00  -2.29  +0.71  -2.00  +0.00  -2.29  

-0.71  -2.29  +0.71  -2.00  +0.00  -2.29  -0.71  +0.71  

-4.00  -2.00  +0.00  -2.29  -0.71  -3.00  -1.00  -2.00  

Fast Fourier Transform of a Complex Image

IppStatus fft_cplx( void ) {

   Ipp32fc src[64] = {0}, dst[64], m3 = {-3,0}, one = {1,0};

   IppiFFTSpec_C_32fc *spec;

   IppStatus status;

   src[0] = m3; src[9] = one;

   ippiFFTInitAlloc_C_32fc( &spec, 3, 3, IPP_FFT_DIV_INV_BY_N, ippAlgHintAccurate);

   status = ippiFFTFwd_CToC_32fc_C1R( src, 8*sizeof(Ipp32fc), dst,

                                      8*sizeof(Ipp32fc), spec, 0 );

   ippiFFTFree_C_32fc( spec );

   return status;

}

On exit from the ippiFFTFwd_CToC function, the destination buffer contains the following data (real and imaginary part of complex numbers are given in comma-delimited format):

-2.0, 0.0 -2.3,-0.7 -3.0,-1.0 -3.7,-0.7 -4.0, 0.0 -3.7, 0.7 -3.0, 1.0 -2.3, 0.7

-2.3,-0.7 -3.0,-1.0 -3.7,-0.7 -4.0,-0.0 -3.7, 0.7 -3.0, 1.0 -2.3, 0.7 -2.0, 0.0

-3.0,-1.0 -3.7,-0.7 -4.0, 0.0 -3.7, 0.7 -3.0, 1.0 -2.3, 0.7 -2.0,-0.0 -2.3,-0.7

-3.7,-0.7 -4.0, 0.0 -3.7, 0.7 -3.0, 1.0 -2.3, 0.7 -2.0,-0.0 -2.3,-0.7 -3.0,-1.0

-4.0, 0.0 -3.7, 0.7 -3.0, 1.0 -2.3, 0.7 -2.0, 0.0 -2.3,-0.7 -3.0,-1.0 -3.7,-0.7

-3.7, 0.7 -3.0, 1.0 -2.3, 0.7 -2.0, 0.0 -2.3,-0.7 -3.0,-1.0 -3.7,-0.7 -4.0,-0.0

-3.0, 1.0 -2.3, 0.7 -2.0, 0.0 -2.3,-0.7 -3.0,-1.0 -3.7,-0.7 -4.0,-0.0 -3.7, 0.7

-2.3, 0.7 -2.0,-0.0 -2.3,-0.7 -3.0,-1.0 -3.7,-0.7 -4.0, 0.0 -3.7, 0.7 -3.0, 1.0