废话不多说，直接上代码

鼠标绘制矩形提取ROI

#include <iostream>  
#include <opencv2\opencv.hpp>
#include <opencv2/highgui/highgui.hpp>    
#include <opencv2/core/core.hpp>

using namespace cv;
using namespace std;

// 全局变量图像源
cv::Mat srcImage;
cv::Mat roiImage;
// 起始点
cv::Point startPoint;
// 终止点
cv::Point endPoint;
// 完成所选区域标志位
bool downFlag = false;
bool upFlag = false;

void MouseEvent(int event, int x, int y, int flags, void* data)
{
	// 鼠标左键按下 获取起始点
	if (event == EVENT_LBUTTONDOWN)
	{
		downFlag = true;
		startPoint.x = x;
		startPoint.y = y;
	}
	// 鼠标拖到松开 获取终止点
	if (event == EVENT_LBUTTONUP)
	{
		upFlag = true;
		endPoint.x = x;
		endPoint.y = y;
	}
	// 显示鼠标选择的图像区域
	if (downFlag == true && upFlag == false)
	{
		Point tempPoint;
		tempPoint.x = x;
		tempPoint.y = y;
		// 用于显示图像生成
		cv::Mat tempImage = srcImage.clone();
		cv::rectangle(tempImage, startPoint, tempPoint, Scalar(0, 0, 255), 2, 3, 0);
		cv::imshow("cut", tempImage);
	}   // 选择区域生成
	if (downFlag == true && upFlag == true)
	{
		// 所选矩形区域 
		cv::Rect roiRect;

		// 获取选择区域的ROI
		roiRect.width = abs(startPoint.x - endPoint.x);
		roiRect.height = abs(startPoint.y - endPoint.y);
		roiRect.x = min(startPoint.x, endPoint.x);
		roiRect.y = min(startPoint.y, endPoint.y);
		
		cout << "rect: " << roiRect << endl;
		if (roiRect.width > 0 && roiRect.height > 0)
		{
			cv::Mat roiMat(srcImage, roiRect);
			imshow("ROI", roiMat);
			imwrite("roi.tif", roiMat);
		}
		downFlag = false;
		upFlag = false;
	}
}


int main()
{
	Mat src = cv::imread("tt.jpg");
	if (src.empty())
		return -1;

	namedWindow("cut", WINDOW_AUTOSIZE);

	// 回调事件响应
	::srcImage = src.clone();
	cv::namedWindow("cut");
	cv::imshow("cut", srcImage);
	cv::setMouseCallback("cut", MouseEvent, 0);
	cv::waitKey(0);
	cv::destroyAllWindows();
	return 0;
}

任意形状绘制提前ROI

#include <iostream>  
#include <vector>
#include <opencv2\opencv.hpp>
#include <opencv2/highgui/highgui.hpp>    
#include <opencv2/core/core.hpp>
#define CV_AA 16

using namespace cv;

cv::Mat src, tempMat;
cv::Mat dst, maskImage;
std::vector<cv::Point> vctPoint;
cv::Point ptStart = (-1, -1); //初始化起点
cv::Point cur_pt = (-1, -1);  //初始化临时节点

void on_mouse(int event, int x, int y, int flags, void *ustc)//event鼠标事件代号，x,y鼠标坐标，flags拖拽和键盘操作的代号    
{
	if (event == EVENT_LBUTTONDOWN)
	{
		std::cout << "x:" << x << " y:" << y << std::endl;
		ptStart = cv::Point(x, y);
		vctPoint.push_back(ptStart);
		cv::circle(src, ptStart, 1, cv::Scalar(255, 0, 255), FILLED, CV_AA, 0);
		cv::imshow("图片", src);
	
	}
	else if (event == EVENT_MOUSEMOVE && (flags & EVENT_FLAG_LBUTTON))
	{
		std::cout << "x:" << x << " y:" << y << std::endl;
		cur_pt = cv::Point(x, y);
		cv::line(src, vctPoint.back(), cur_pt, cv::Scalar(0, 255, 0, 0), 1, 8, 0);
		
		cv::imshow("图片", src);
		vctPoint.push_back(cur_pt);
	}
	else if (event == EVENT_LBUTTONUP)
	{
		std::cout << "x:" << x << " y:" << y << std::endl;
		cur_pt = cv::Point(x, y);
		cv::line(src, ptStart, cur_pt, cv::Scalar(0, 255, 0, 0), 1, 8, 0);
		
		cv::imshow("图片", src);
		vctPoint.push_back(cur_pt);

		//把点构成任意多边形进行填充
		const cv::Point * ppt[1] = { &vctPoint[0] };//取数组的首地址
		int len = vctPoint.size();
		int npt[] = { len };
		
		std::cout << len << std::endl;
		src.copyTo(maskImage);
		maskImage.setTo(cv::Scalar(0, 0, 0, 0));
		cv::fillPoly(maskImage, ppt, npt, 1, cv::Scalar(0, 255, 255, 255));
		src.copyTo(dst, maskImage);
		cv::imshow("抠图", dst);
		
		// 清空上次绘制的点
		vctPoint.clear();
		vctPoint.resize(0);
		vctPoint.shrink_to_fit();

		std::cout << "Vector is " << (vctPoint.empty() ? "empty." : "not empty.") << std::endl;
		std::cout << "Capacity is " << vctPoint.capacity() << std::endl;

		dst.setTo(0);
		src = tempMat.clone();
		cv::waitKey(0);
	}
}

int main()
{
	src = cv::imread("src.jpg");
	tempMat = src.clone();
	//鼠标点击
	cv::namedWindow("图片");//定义一个img窗口    
	cv::setMouseCallback("图片", on_mouse, 0);//调用回调函数    
	cv::imshow("图片", src);
	cv::waitKey(0);
	return 0;
}

如果觉得以上代码对你有帮助，那就一键三连吧。