OpenCV: cv::Mat圖像像素值訪問及其座標系問題

1. 創建存儲三通道圖像的矩陣:

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

2. 灰度圖（8UC1）的(x,y)位置的像素值：

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

   intensity.val[0]中保存圖像的灰度值（0-255），另外intensity.val[1]，intensity.val[2]爲0
3. 獲取3通道圖像像素值：

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

4. 獲取cv::Mat的座標系以縱向爲x軸，橫向爲y軸。而cvPoint等則相反:

   Mat img = imread("F:\\lena.jpg");
   	for(int i = 0;i<100;i++)
   		for(int j = 100;j<200;j++)
   			img.at<Vec3b>(i,j) = 0;
   	line(img,cvPoint(0,0),cvPoint(200,400),Scalar(255,0,0),2);
   	imshow("Lena",img);
   	cvWaitKey(0);