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);
}