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  • OpenCV】透视变换 Perspective Transformation（续）

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    【OpenCV】透视变换 Perspective Transformation（续）

    分类： 【图像处理】 【编程语言】 2014-05-27 09:39 2776人阅读 评论(13) 收藏 举报

    透视变换的原理和矩阵求解请参见前一篇《透视变换 Perspective Transformation》。在OpenCV中也实现了透视变换的公式求解和变换函数。

    求解变换公式的函数：

    [cpp] view plaincopyprint?在CODE上查看代码片派生到我的代码片
    1. Mat getPerspectiveTransform(const Point2f src[], const Point2f dst[])  
    Mat getPerspectiveTransform(const Point2f src[], const Point2f dst[])
    输入原始图像和变换之后的图像的对应4个点，便可以得到变换矩阵。之后用求解得到的矩阵输入perspectiveTransform便可以对一组点进行变换：
    [cpp] view plaincopyprint?在CODE上查看代码片派生到我的代码片
    1. void perspectiveTransform(InputArray src, OutputArray dst, InputArray m)  
    void perspectiveTransform(InputArray src, OutputArray dst, InputArray m)
    注意这里src和dst的输入并不是图像，而是图像对应的坐标。应用前一篇的例子，做个相反的变换：
    [cpp] view plaincopyprint?在CODE上查看代码片派生到我的代码片
    1. int main( )  
    2. {  
    3.     Mat img=imread("boy.png");  
    4.     int img_height = img.rows;  
    5.     int img_width = img.cols;  
    6.     vector<Point2f> corners(4);  
    7.     corners[0] = Point2f(0,0);  
    8.     corners[1] = Point2f(img_width-1,0);  
    9.     corners[2] = Point2f(0,img_height-1);  
    10.     corners[3] = Point2f(img_width-1,img_height-1);  
    11.     vector<Point2f> corners_trans(4);  
    12.     corners_trans[0] = Point2f(150,250);  
    13.     corners_trans[1] = Point2f(771,0);  
    14.     corners_trans[2] = Point2f(0,img_height-1);  
    15.     corners_trans[3] = Point2f(650,img_height-1);  
    16.   
    17.     Mat transform = getPerspectiveTransform(corners,corners_trans);  
    18.     cout<<transform<<endl;  
    19.     vector<Point2f> ponits, points_trans;  
    20.     for(int i=0;i<img_height;i++){  
    21.         for(int j=0;j<img_width;j++){  
    22.             ponits.push_back(Point2f(j,i));  
    23.         }  
    24.     }  
    25.   
    26.     perspectiveTransform( ponits, points_trans, transform);  
    27.     Mat img_trans = Mat::zeros(img_height,img_width,CV_8UC3);  
    28.     int count = 0;  
    29.     for(int i=0;i<img_height;i++){  
    30.         uchar* p = img.ptr<uchar>(i);  
    31.         for(int j=0;j<img_width;j++){  
    32.             int y = points_trans[count].y;  
    33.             int x = points_trans[count].x;  
    34.             uchar* t = img_trans.ptr<uchar>(y);  
    35.             t[x*3]  = p[j*3];  
    36.             t[x*3+1]  = p[j*3+1];  
    37.             t[x*3+2]  = p[j*3+2];  
    38.             count++;  
    39.         }  
    40.     }  
    41.     imwrite("boy_trans.png",img_trans);  
    42.   
    43.     return 0;  
    44. }  
    int main( )
{
	Mat img=imread("boy.png");
	int img_height = img.rows;
	int img_width = img.cols;
	vector<Point2f> corners(4);
	corners[0] = Point2f(0,0);
	corners[1] = Point2f(img_width-1,0);
	corners[2] = Point2f(0,img_height-1);
	corners[3] = Point2f(img_width-1,img_height-1);
	vector<Point2f> corners_trans(4);
	corners_trans[0] = Point2f(150,250);
	corners_trans[1] = Point2f(771,0);
	corners_trans[2] = Point2f(0,img_height-1);
	corners_trans[3] = Point2f(650,img_height-1);

	Mat transform = getPerspectiveTransform(corners,corners_trans);
	cout<<transform<<endl;
	vector<Point2f> ponits, points_trans;
	for(int i=0;i<img_height;i++){
		for(int j=0;j<img_width;j++){
			ponits.push_back(Point2f(j,i));
		}
	}

	perspectiveTransform( ponits, points_trans, transform);
	Mat img_trans = Mat::zeros(img_height,img_width,CV_8UC3);
	int count = 0;
	for(int i=0;i<img_height;i++){
		uchar* p = img.ptr<uchar>(i);
		for(int j=0;j<img_width;j++){
			int y = points_trans[count].y;
			int x = points_trans[count].x;
			uchar* t = img_trans.ptr<uchar>(y);
			t[x*3]  = p[j*3];
			t[x*3+1]  = p[j*3+1];
			t[x*3+2]  = p[j*3+2];
			count++;
		}
	}
	imwrite("boy_trans.png",img_trans);

	return 0;
}

    得到变换之后的图片：

    注意这种将原图变换到对应图像上的方式会有一些没有被填充的点，也就是右图中黑色的小点。解决这种问题一是用差值的方式，再一种比较简单就是不用原图的点变换后对应找新图的坐标，而是直接在新图上找反向变换原图的点。说起来有点绕口，具体见前一篇《透视变换 Perspective Transformation》的代码应该就能懂啦。

    除了getPerspectiveTransform()函数，OpenCV还提供了findHomography()的函数，不是用点来找，而是直接用透视平面来找变换公式。这个函数在特征匹配的经典例子中有用到，也非常直观：

    [cpp] view plaincopyprint?在CODE上查看代码片派生到我的代码片
    1. int main( int argc, char** argv )  
    2. {  
    3.     Mat img_object = imread( argv[1], IMREAD_GRAYSCALE );  
    4.     Mat img_scene = imread( argv[2], IMREAD_GRAYSCALE );  
    5.     if( !img_object.data || !img_scene.data )  
    6.     { std::cout<< " --(!) Error reading images " << std::endl; return -1; }  
    7.   
    8.     //-- Step 1: Detect the keypoints using SURF Detector   
    9.     int minHessian = 400;  
    10.     SurfFeatureDetector detector( minHessian );  
    11.     std::vector<KeyPoint> keypoints_object, keypoints_scene;  
    12.     detector.detect( img_object, keypoints_object );  
    13.     detector.detect( img_scene, keypoints_scene );  
    14.   
    15.     //-- Step 2: Calculate descriptors (feature vectors)   
    16.     SurfDescriptorExtractor extractor;  
    17.     Mat descriptors_object, descriptors_scene;  
    18.     extractor.compute( img_object, keypoints_object, descriptors_object );  
    19.     extractor.compute( img_scene, keypoints_scene, descriptors_scene );  
    20.   
    21.     //-- Step 3: Matching descriptor vectors using FLANN matcher   
    22.     FlannBasedMatcher matcher;  
    23.     std::vector< DMatch > matches;  
    24.     matcher.match( descriptors_object, descriptors_scene, matches );  
    25.     double max_dist = 0; double min_dist = 100;  
    26.   
    27.     //-- Quick calculation of max and min distances between keypoints   
    28.     for( int i = 0; i < descriptors_object.rows; i++ )  
    29.     { double dist = matches[i].distance;  
    30.     if( dist < min_dist ) min_dist = dist;  
    31.     if( dist > max_dist ) max_dist = dist;  
    32.     }  
    33.   
    34.     printf("-- Max dist : %f  ", max_dist );  
    35.     printf("-- Min dist : %f  ", min_dist );  
    36.   
    37.     //-- Draw only "good" matches (i.e. whose distance is less than 3*min_dist )   
    38.     std::vector< DMatch > good_matches;  
    39.   
    40.     for( int i = 0; i < descriptors_object.rows; i++ )  
    41.     { if( matches[i].distance < 3*min_dist )  
    42.     { good_matches.push_back( matches[i]); }  
    43.     }  
    44.   
    45.     Mat img_matches;  
    46.     drawMatches( img_object, keypoints_object, img_scene, keypoints_scene,  
    47.         good_matches, img_matches, Scalar::all(-1), Scalar::all(-1),  
    48.         vector<char>(), DrawMatchesFlags::NOT_DRAW_SINGLE_POINTS );  
    49.   
    50.     //-- Localize the object from img_1 in img_2   
    51.     std::vector<Point2f> obj;  
    52.     std::vector<Point2f> scene;  
    53.   
    54.     for( size_t i = 0; i < good_matches.size(); i++ )  
    55.     {  
    56.         //-- Get the keypoints from the good matches   
    57.         obj.push_back( keypoints_object[ good_matches[i].queryIdx ].pt );  
    58.         scene.push_back( keypoints_scene[ good_matches[i].trainIdx ].pt );  
    59.     }  
    60.   
    61.     Mat H = findHomography( obj, scene, RANSAC );  
    62.   
    63.     //-- Get the corners from the image_1 ( the object to be "detected" )   
    64.     std::vector<Point2f> obj_corners(4);  
    65.     obj_corners[0] = Point(0,0); obj_corners[1] = Point( img_object.cols, 0 );  
    66.     obj_corners[2] = Point( img_object.cols, img_object.rows ); obj_corners[3] = Point( 0, img_object.rows );  
    67.     std::vector<Point2f> scene_corners(4);  
    68.     perspectiveTransform( obj_corners, scene_corners, H);  
    69.     //-- Draw lines between the corners (the mapped object in the scene - image_2 )   
    70.     Point2f offset( (float)img_object.cols, 0);  
    71.     line( img_matches, scene_corners[0] + offset, scene_corners[1] + offset, Scalar(0, 255, 0), 4 );  
    72.     line( img_matches, scene_corners[1] + offset, scene_corners[2] + offset, Scalar( 0, 255, 0), 4 );  
    73.     line( img_matches, scene_corners[2] + offset, scene_corners[3] + offset, Scalar( 0, 255, 0), 4 );  
    74.     line( img_matches, scene_corners[3] + offset, scene_corners[0] + offset, Scalar( 0, 255, 0), 4 );  
    75.   
    76.     //-- Show detected matches   
    77.     imshow( "Good Matches & Object detection", img_matches );  
    78.     waitKey(0);  
    79.     return 0;  
    80. }  
    int main( int argc, char** argv )
{
	Mat img_object = imread( argv[1], IMREAD_GRAYSCALE );
	Mat img_scene = imread( argv[2], IMREAD_GRAYSCALE );
	if( !img_object.data || !img_scene.data )
	{ std::cout<< " --(!) Error reading images " << std::endl; return -1; }

	//-- Step 1: Detect the keypoints using SURF Detector
	int minHessian = 400;
	SurfFeatureDetector detector( minHessian );
	std::vector<KeyPoint> keypoints_object, keypoints_scene;
	detector.detect( img_object, keypoints_object );
	detector.detect( img_scene, keypoints_scene );

	//-- Step 2: Calculate descriptors (feature vectors)
	SurfDescriptorExtractor extractor;
	Mat descriptors_object, descriptors_scene;
	extractor.compute( img_object, keypoints_object, descriptors_object );
	extractor.compute( img_scene, keypoints_scene, descriptors_scene );

	//-- Step 3: Matching descriptor vectors using FLANN matcher
	FlannBasedMatcher matcher;
	std::vector< DMatch > matches;
	matcher.match( descriptors_object, descriptors_scene, matches );
	double max_dist = 0; double min_dist = 100;

	//-- Quick calculation of max and min distances between keypoints
	for( int i = 0; i < descriptors_object.rows; i++ )
	{ double dist = matches[i].distance;
	if( dist < min_dist ) min_dist = dist;
	if( dist > max_dist ) max_dist = dist;
	}

	printf("-- Max dist : %f 
", max_dist );
	printf("-- Min dist : %f 
", min_dist );

	//-- Draw only "good" matches (i.e. whose distance is less than 3*min_dist )
	std::vector< DMatch > good_matches;

	for( int i = 0; i < descriptors_object.rows; i++ )
	{ if( matches[i].distance < 3*min_dist )
	{ good_matches.push_back( matches[i]); }
	}

	Mat img_matches;
	drawMatches( img_object, keypoints_object, img_scene, keypoints_scene,
		good_matches, img_matches, Scalar::all(-1), Scalar::all(-1),
		vector<char>(), DrawMatchesFlags::NOT_DRAW_SINGLE_POINTS );

	//-- Localize the object from img_1 in img_2
	std::vector<Point2f> obj;
	std::vector<Point2f> scene;

	for( size_t i = 0; i < good_matches.size(); i++ )
	{
		//-- Get the keypoints from the good matches
		obj.push_back( keypoints_object[ good_matches[i].queryIdx ].pt );
		scene.push_back( keypoints_scene[ good_matches[i].trainIdx ].pt );
	}

	Mat H = findHomography( obj, scene, RANSAC );

	//-- Get the corners from the image_1 ( the object to be "detected" )
	std::vector<Point2f> obj_corners(4);
	obj_corners[0] = Point(0,0); obj_corners[1] = Point( img_object.cols, 0 );
	obj_corners[2] = Point( img_object.cols, img_object.rows ); obj_corners[3] = Point( 0, img_object.rows );
	std::vector<Point2f> scene_corners(4);
	perspectiveTransform( obj_corners, scene_corners, H);
	//-- Draw lines between the corners (the mapped object in the scene - image_2 )
	Point2f offset( (float)img_object.cols, 0);
	line( img_matches, scene_corners[0] + offset, scene_corners[1] + offset, Scalar(0, 255, 0), 4 );
	line( img_matches, scene_corners[1] + offset, scene_corners[2] + offset, Scalar( 0, 255, 0), 4 );
	line( img_matches, scene_corners[2] + offset, scene_corners[3] + offset, Scalar( 0, 255, 0), 4 );
	line( img_matches, scene_corners[3] + offset, scene_corners[0] + offset, Scalar( 0, 255, 0), 4 );

	//-- Show detected matches
	imshow( "Good Matches & Object detection", img_matches );
	waitKey(0);
	return 0;
}

    代码运行效果：

    findHomography()函数直接通过两个平面上相匹配的特征点求出变换公式，之后代码又对原图的四个边缘点进行变换，在右图上画出对应的矩形。这个图也很好地解释了所谓透视变换的“Viewing Plane”。
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  • 原文地址：https://www.cnblogs.com/wqj1212/p/3915887.html
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