void testOnes() {
 int x=2; int y=2; //arbitrary

 // 1 channel
 cv::Mat img_C1 = cv::Mat::ones(x,y,CV_8UC1);
 uchar px1 = img_C1.at<uchar>(0,0); //not sure of correct data type for px in 1-channel img
 printf("px of 1-channel img: %d n", (int)px1); //prints 1

 // 3 channels
 cv::Mat img_C3 = cv::Mat::ones(x,y,CV_8UC3); //note 8UC3 instead of 8UC1
 cv::Vec3b px3 = img_C3.at<cv::Vec3b>(0,0);
 printf("px of 3-channel img: %d %d %d n", (int)px3[0], (int)px3[1], (int)px3[2]); //prints 1 0 0
}

cv::Mat img_C3( x, y, CV_8UC3, CV_RGB(1,1,1) );