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First, you're using the old C API. You shouldn't do that if c++ is available.

Secondly, the function should do what you need.

cvMulSpectrums( const CvArr* src1, const CvArr* src2, CvArr* dst, int flags );

The output of DFT is, by default CSS packed in a single channel matrix. If you pass the flag COMPLEX_OUTPUT, whatever the C API equivalent is, then you get a two channel matrix. Look at the documentation for DFT for more on that.