Scalar mean(InputArray src, InputArray mask = noArray());

#include<iostream>
#include<opencv2/opencv.hpp>
#include<ctime>
using namespace std;
using namespace cv;
int main(void)
{
	cv::Mat src = cv::Mat::zeros(5, 5, CV_32FC1);
	cv::Mat mask = cv::Mat::zeros(5, 5, CV_8UC1);
	int rows = src.rows;
	int cols = src.cols;
	cv::circle(mask, cv::Point(2, 2), 2, 255, -1);
	int number = 0;
	float sum = 0.0f;
	for (int i = 0; i < rows; ++i)
	{
		for (int j = 0; j < cols; ++j)
		{
			if (mask.at<uchar>(i, j) == 255)
			{
				src.at<float>(i, j) = rand() % 255+rand()%100/100.0f;
				sum += src.at<float>(i, j);
				number++;
			}
		}
	}
	float result=cv::mean(src, mask==255)[0];
	float mean_ = sum / number;
	cout << "result:" << result << endl;
	cout << "mean_" << mean_ << endl;
	system("pause");
	return 0;
}