void cvCanny3(  const void* srcarr, void* dstarr,
            void* dxarr, void* dyarr,
            int aperture_size )

CvMat srcstub, *src = cvGetMat( srcarr, &srcstub );
CvMat dststub, *dst = cvGetMat( dstarr, &dststub );

CvMat dxstub, *dx = cvGetMat( dxarr, &dxstub );
CvMat dystub, *dy = cvGetMat( dyarr, &dystub );