opencv 使用Mat 矩阵进行图像的降采样，像素的访问方式

Mat  halfSizeImage(Mat  im)   {unsigned int i,j;  int w = im.cols/2;  int h = im.rows/2;   Mat imnew = cvCreateMat(h, w, CV_8UC1);imnew.step = im.step/2;for ( j = 0; j < h; j++)   for ( i = 0; i < w; i++) ((float *)(imnew.ptr<float>(0,0) + imnew.step/sizeof(float) *(j)))[(i)]=((float *)(im.ptr<float>(0,0) + im.step/sizeof(float) *(2*j)))[(2*i)] ; /*((float *)((float*)imnew.data + imnew.step/sizeof(float) *(j)))[(i)]=((float *)((float*)im.data + im.step/sizeof(float) *(2*j)))[(2*i)] ;  */return imnew;  }

注释内为第二种方式