【opencv学习-C++】透视变换

男娘i 2021-12-23 23:55 709阅读 0赞

# 透视变换 #

先看一下图，在牌照的过程中，由于角度问题，难免会出现一些倾斜的问题，如下图，

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3BsU29uZ19DU0RO_size_16_color_FFFFFF_t_70][]

我们要解决的就是通过一系列的操作，将上图变为

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3BsU29uZ19DU0RO_size_16_color_FFFFFF_t_70 1][]

# 解决的思路 #

用opencv中的透视变换的API，输入四个角点的坐标，完成透视变换。

看下一主要的两个opecnv的API

getPerspectiveTransform( InputArray src, InputArray dst );//获取透视变换矩阵
    warpPerspective( InputArray src, OutputArray dst,
                                       InputArray M, Size dsize,
                                       int flags = INTER_LINEAR,
                                       int borderMode = BORDER_CONSTANT,
                                       const Scalar& borderValue = Scalar());//透视变换

从函数的需求上可以看出，透视变换的主要任务就是：找到原图像的角点。主要流程如下

（1）灰度处理、二值化、形态学操作形成连通区域

（2）轮廓发现、将目标的轮廓绘制出来

（3）在绘制的轮廓中进行直线检测

（4）找出四条边，求出四个交点

（5）使用透视变换函数，得到结果

下面是主要的几个中间效果图

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3BsU29uZ19DU0RO_size_16_color_FFFFFF_t_70 2][]

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3BsU29uZ19DU0RO_size_16_color_FFFFFF_t_70 3][]

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3BsU29uZ19DU0RO_size_16_color_FFFFFF_t_70 4][]

# 完全示例代码 #

#include<iostream>
    #include<opencv2\opencv.hpp>
    
    using namespace cv;
    using namespace std;
    
    int main()
    {
    	//input image
    	Mat src = imread("D:/images/shebaoka.png");
    	imshow("input image", src);
    
    	//bgr 2 gray 转为灰度图像
    	Mat src_gray;
    	cvtColor(src, src_gray, COLOR_BGR2GRAY);
    
    	//binary 二值化
    	Mat binary;
    	threshold(src_gray, binary, 0, 255, THRESH_BINARY_INV|THRESH_OTSU); //THRESH_BINARY_INV二值化后取反
    	//imshow("binary", binary);//因为有一些斑点存在
    
    	//形态学 闭操作：可以填充小的区域
    	Mat morhp_img;
    	Mat kernel = getStructuringElement(MORPH_RECT, Size(5, 5), Point(-1, -1));
    	morphologyEx(binary, morhp_img, MORPH_CLOSE, kernel, Point(-1, -1), 3);
    	//imshow("morphology", morhp_img);
    
    	Mat dst;
    	bitwise_not(morhp_img, dst);//在取反
    	imshow("dst", dst);//
    
    	//轮廓发现
    	vector<vector<Point>> contous;
    	vector<Vec4i> hireachy;
    	findContours(dst, contous, hireachy, CV_RETR_TREE, CHAIN_APPROX_SIMPLE, Point());
    	cout << "contous.size:" << contous.size() << endl;
    
    
    	//轮廓绘制
    	int width = src.cols;
    	int height = src.rows;
    	Mat drawImage = Mat::zeros(src.size(), CV_8UC3);
    	cout << contous.size() << endl;
    	for (size_t t = 0; t < contous.size(); t++)
    	{
    		Rect rect = boundingRect(contous[t]);
    		if (rect.width > width / 2 && rect.height > height / 2 && rect.width<width-5 && rect.height<height-5)
    		{
    			drawContours(drawImage, contous, static_cast<int>(t), Scalar(0, 0, 255), 2, 8, hireachy, 0, Point(0, 0));
    		}
    	}
    	imshow("contours", drawImage);//显示找到的轮廓
    
    	//直线检测
    	vector<Vec4i> lines;
    	Mat contoursImg;
    	int accu = min(width*0.5, height*0.5);
    	cvtColor(drawImage, contoursImg, COLOR_BGR2GRAY);
    	imshow("contours", contoursImg);
    
    	Mat linesImage = Mat::zeros(src.size(), CV_8UC3);
    	HoughLinesP(contoursImg, lines, 1, CV_PI / 180.0, accu,accu,0);
    	for (size_t t = 0; t < lines.size(); t++)
    	{
    		Vec4i ln = lines[t];
    		line(linesImage, Point(ln[0], ln[1]), Point(ln[2], ln[3]), Scalar(0, 0, 255), 2, 8, 0);//绘制直线
    	}
    	cout << "number of lines:"<<lines.size() << endl;
    	imshow("linesImages", linesImage);
    
    	//寻找与定位上下 左右 四条直线
    	int deltah = 0; //高度差
    	int deltaw = 0; //宽度差
    	Vec4i topLine, bottomLine; //直线定义
    	Vec4i rightLine, leftLine;
    
    	for (int i = 0; i < lines.size(); i++)
    	{
    		Vec4i ln = lines[i];//?????
    		/*
    		Opencv中的累计霍夫变换HoughLinesP()，输出的是一个Vector of Vec4i，
    		Vector每个元素代表一条直线，是由一个4元浮点数构成，
    		前两个一组x_1,y_1，后两个一组x_2,y_2，代表了图像中直线的起始点和结束点。		
    		*/
    		deltah = abs(ln[3] - ln[1]); //计算高度差(y2-y1)
    		//topLine
    		if (ln[3] < height / 2.0 && ln[1] < height / 2.0 && deltah < accu - 1)
    		{
    			topLine = lines[i];
    		}
            //bottomLine
    		if (ln[3] > height / 2.0 && ln[1] >height / 2.0 && deltah < accu - 1)
    		{
    			bottomLine = lines[i];
    		}
    
    		deltaw = abs(ln[2] - ln[0]); //计算宽度差(x2-x1)	
    		//leftLine
    		if (ln[0] < height / 2.0 && ln[2] < height / 2.0 && deltaw < accu - 1)
    		{
    			leftLine = lines[i];
    		}
    		//rightLine
    		if (ln[0] > width / 2.0 && ln[2] >width / 2.0 && deltaw < accu - 1)
    		{
    			rightLine = lines[i];
    		}
    	}
    
    	// 打印四条线的坐标
    	cout << "topLine : p1(x,y)= " << topLine[0] << "," << topLine[1] << "; p2(x,y)= " << topLine[2] <<  "," << topLine[3] << endl;
    	cout << "bottomLine : p1(x,y)= " << bottomLine[0] << "," << bottomLine[1] << "; p2(x,y)= " << bottomLine[2] <<  "," << bottomLine[3] << endl;
    	cout << "leftLine : p1(x,y)= " << leftLine[0] << "," << leftLine[1] << "; p2(x,y)= " << leftLine[2] <<  "," << leftLine[3] << endl;
    	cout << "rightLine : p1(x,y)= " << rightLine[0] << "," << rightLine[1] << "; p2(x,y)= " << rightLine[2] << "," << rightLine[3] << endl;
    
    	//拟合四条直线
    	float k1, k2, k3, k4, c1, c2, c3, c4;
    	k1 = float(topLine[3] - topLine[1]) / float(topLine[2] - topLine[0]);
    	c1 = topLine[1] - k1*topLine[0];
    
    	k2 = float(bottomLine[3] - bottomLine[1]) / float(bottomLine[2] - bottomLine[0]);
    	c2 = bottomLine[1] - k2*bottomLine[0];
    
    	k3 = float(leftLine[3] - leftLine[1]) / float(leftLine[2] - leftLine[0]);
    	c3 = leftLine[1] - k3*leftLine[0];
    
    	k4 = float(rightLine[3] - rightLine[1]) / float(rightLine[2] - rightLine[0]);
    	c4 = rightLine[1] - k4*rightLine[0];
    
    	//求四个角点,
    	Point p1;//topLine  leftLine 左上角
    	p1.x = static_cast<int>(c1 - c3) / k3 - k1;
    	p1.y = k1*p1.x + c1;
    
    	Point p2;//topLine  rightLine 右上角
    	p2.x = static_cast<int>(c1 - c4) / k4 - k1;
    	p2.y = k1*p2.x + c1;
    
    	Point p3;//bottomLine  leftLine 左下角
    	p3.x = static_cast<int>(c2 - c3) / k3 - k2;
    	p3.y = k2*p3.x + c2;
    
    	Point p4;//bottomLine  rightLine 右下角
    	p4.x = static_cast<int>(c2 - c4) / k4 - k2;
    	p4.y = k2*p4.x + c2;
    
    	cout << "Point p1: (" << p1.x << "," << p1.y << ")" << endl;
    	cout << "Point p2: (" << p2.x << "," << p2.y << ")" << endl;
    	cout << "Point p3: (" << p3.x << "," << p3.y << ")" << endl;
    	cout << "Point p4: (" << p4.x << "," << p4.y << ")" << endl;
    
    	//显示四个点
    	circle(linesImage, p1, 2, Scalar(0,255, 0), 2);
    	circle(linesImage, p2, 2, Scalar(0,255, 0), 2);
    	circle(linesImage, p3, 2, Scalar(0, 255, 0), 2);
    	circle(linesImage, p4, 2, Scalar(0, 255, 0), 2);
    	imshow("find four points", linesImage);
    
    	//透视变换
    	vector<Point2f> src_corners(4);
    	src_corners[0] = p1;
    	src_corners[1] = p2;
    	src_corners[2] = p3;
    	src_corners[3] = p4;
    
    	Mat result_images = Mat::zeros(height*0.7, width*0.9, CV_8UC3);
    	vector<Point2f> dst_corners(4);
    	dst_corners[0] = Point(0, 0);
    	dst_corners[1] = Point(result_images.cols, 0);
    	dst_corners[2] = Point(0, result_images.rows);
    	dst_corners[3] = Point(result_images.cols, result_images.rows);
    	
    	Mat warpmatrix = getPerspectiveTransform(src_corners, dst_corners); //获取透视变换矩阵
    	warpPerspective(src, result_images, warpmatrix, result_images.size()); //透视变换
    	imshow("final result", result_images);
    	imwrite("D:/images/warpPerspective.png", result_images);
    	waitKey(0);
    	return 0;
    }

控制台的输出

> \*\*\*\*\* VIDEOINPUT LIBRARY - 0.1995 - TFW07 \*\*\*\*\*
> 
> contous.size:6  
> 6  
> number of lines:5  
> topLine : p1(x,y)= 20,64; p2(x,y)= 487,56  
> bottomLine : p1(x,y)= 145,329; p2(x,y)= 361,332  
> leftLine : p1(x,y)= 13,75; p2(x,y)= 47,320  
> rightLine : p1(x,y)= 479,328; p2(x,y)= 497,66  
> Point p1: (11,64)  
> Point p2: (497,55)  
> Point p3: (47,327)  
> Point p4: (479,333)

[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3BsU29uZ19DU0RO_size_16_color_FFFFFF_t_70]: /images/20211223/3b7ba4f262b141068328388256b59eaf.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3BsU29uZ19DU0RO_size_16_color_FFFFFF_t_70 1]: /images/20211223/a651721c80c2404392708f068964e141.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3BsU29uZ19DU0RO_size_16_color_FFFFFF_t_70 2]: /images/20211223/b2c3784ad8f14fdb8d48f39a16097d50.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3BsU29uZ19DU0RO_size_16_color_FFFFFF_t_70 3]: /images/20211223/e94aed1f7ec8404c9a20dc7ae56381c9.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3BsU29uZ19DU0RO_size_16_color_FFFFFF_t_70 4]: /images/20211223/3c571c3520694f84997df63ad5fbe9e8.png