The FFT adjusts only a half size.

siinii — Fri, 26 Feb 2010 01:42:15 GMT

I received below source code from Dima. I adjusted it to image.

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You probably did the small size on a static array, defined as x[100][40]. The way the dynamic array is arranged in your example is incorrect and it demonstrates typical mistake. For static array '_complex x' expression 'x' has type '_complex *' whereas for dynamic array that one would use with double-subscript notation 'x' the type of 'x' is '_complex**' that is pointer to pointers. In other words, in this example when you call DftiComputeForward(..., x) the input data to the FFT routine is pointers, not the complex numbers!

I'd suggest you to refrain from the practice of using double-subscript notation for dynamic arrays, and adopt the following usage instead:

const int ROWS=3072, COLS=2560;

const int ROWSIZE=COLS, COLSIZE=ROWS;

_complex *x = new _complex[ROWS*COLS];

#define x(r,c) x[*ROWSIZE+(c)] /* explicit row-major indexing */

memset(x,0,sizeof(x[0])*ROWS*COLS); /* zero fill initialization */

for (r=0;r

The rest of this reply is duplicating your example in the above terms.

DftiCreateDescriptor(rowfft,DFTI_SINGLE,DFTI_COMPLEX,1,ROWSIZE); /* stride 1 by default */

DftiSetValue(rowfft,DFTI_NUMBER_OF_TRANSFORMS,(MKL_LONG)ROWS);

DftiSetValue(rowfft,DFTI_INPUT_DISTANCE,(MKL_LONG)ROWSIZE);

DftiSetValue(rowfft,DFTI_OUTPUT_DISTANCE,(MKL_LONG)ROWSIZE);

DftiCreateDescriptor(colfft,DFTI_SINGLE,DFTI_COMPLEX,1,COLSIZE); /* will set strides */

MKL_LONG colfft_strides[] = { 0, ROWSIZE };

DftiSetValue(colfft,DFTI_INPUT_STRIDES,colfft_strides);

DftiSetValue(colfft,DFTI_OUTPUT_STRIDES,colfft_strides);

DftiSetValue(colfft,DFTI_NUMBER_OF_TRANSFORMS,(MKL_LONG)COLS);

DftiSetValue(colfft,DFTI_INPUT_DISTANCE,(MKL_LONG)1);

DftiSetValue(colfft,DFTI_OUTPUT_DISTANCE,(MKL_LONG)1);

...

DftiComputeForward(rowfft,x);

DftiComputeForward(colfft,x);

Thanks

Dima

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///FFT

DftiComputeForward(rowfft,x);

DftiComputeForward(colfft,x);

//Inverse FFT

DftiComputeBackward(rowfft,x);

DftiComputeBackward(colfft,x);

like this...

but The FFT adjusted only 2560*1536(original image size is 2560*3072)

Which code should I change?

The FFT adjusts only a half size.

Dmitry_B_Intel — Fri, 26 Feb 2010 07:23:51 GMT

The excerpt looks good, and it should process whole array.

Could you provide a self-contained example that shows your problem?

Thanks
Dima

topic The FFT adjusts only a half size. in Intel® oneAPI Math Kernel Library

The FFT adjusts only a half size.

The FFT adjusts only a half size.