OpenCV: cv::Mat图像像素值访问及其坐标系问题

1. 创建存储三通道图像的矩阵:

   Mat img(Size(70,70),CV_8U3C,Scalar(0,0,0)

2. 灰度图（8UC1）的(x,y)位置的像素值：

   Scalar intensity = img.at(x, y);

   intensity.val[0]中保存图像的灰度值（0-255），另外intensity.val[1]，intensity.val[2]为0
3. 获取3通道图像像素值：

   Vec3b intensity = img.at(x, y);
   uchar blue = intensity.val[0];
   uchar green = intensity.val[1];
   uchar red = intensity.val[2];

4. 获取cv::Mat的坐标系以纵向为x轴，横向为y轴。而cvPoint等则相反:

   Mat img = imread("F:\\lena.jpg");
   	for(int i = 0;i<100;i++)
   		for(int j = 100;j<200;j++)
   			img.at(i,j) = 0;
   	line(img,cvPoint(0,0),cvPoint(200,400),Scalar(255,0,0),2);
   	imshow("Lena",img);
   	cvWaitKey(0);