opencv使用频域相乘加速卷积速度

void speedy_convolution(
						const CvMat *A,//size:M1 * N1
						const CvMat *B, //size : M2 * N2
						CvMat *C	//size:(M1+M2-1) * (N1+N2-1)
						)
{
	int dft_M = cvGetOptimalDFTSize(A->rows + B->rows -1);
	int dft_N = cvGetOptimalDFTSize(A->cols + B->cols -1);
	
	CvMat *dft_A = cvCreateMat(dft_M , dft_N , A->type);
	CvMat *dft_B = cvCreateMat(dft_M, dft_N, B->type);
	CvMat tmp;

	//copy A TO dft_A and pad dft_A with zeros
	cvGetSubRect(dft_A,&tmp , cvRect(0,0,A->cols,A->rows));
	cvCopy(A,&tmp);
	cvGetSubRect(dft_A,&tmp,cvRect(A->cols,0,dft_A->cols - A->cols , A->rows));
	cvZero(&tmp);

	//no need to pad bottom part of dft_A with zeros because of 
	//use nonzero_rows parameter in cvDFT() call below
	cvDFT(dft_A,dft_A,CV_DXT_FORWARD,A->rows);

	//repeat the same with the second array
	cvGetSubRect(dft_B,&tmp,cvRect(0,0,B->cols,B->rows));
	cvCopy(B,&tmp);
	cvGetSubRect(dft_B,&tmp,cvRect(B->cols,0,dft_B->cols - B->cols , B->rows));
	cvZero(&tmp);

	//no need to pad bottom part of dft_B with zeros because of 
	//use nonzero-rows parameter in cvDFT() call below
	cvDFT(dft_B,dft_B,CV_DXT_FORWARD,B->rows);

	//or CV_DXT_MUL_CONJ to get correlation rather than convolution 
	cvMulSpectrums(dft_A,dft_B,dft_A,0);

	//calculate only the top part
	cvDFT(dft_A,dft_A,CV_DXT_INV_SCALE,C->rows);
	cvGetSubRect(dft_A,&tmp,cvRect(0,0,C->cols,C->rows));
	cvCopy(&tmp,C);
	cvReleaseMat(dft_A);
	cvReleaseMat(dft_B);
}