Opencv中Homography

转自 https://blog.csdn.net/datase/article/details/78420251

1. 什么是Homography
   在图1中有两张书的平面图，两张图分别有四个相对位置相同的点，Homography就是一个变换（3*3矩阵），将一张图中的点映射到另一张图中对应的点
   

因为Homography是一个3*3矩阵，所以可以写成

两张图间的H映射关系就可以表示成

Homography应用：图像对齐

上面公式得出的H ，对于图一中的所有点都是正确的，换句话说，可以用H将第一个图中的点映射到第二张图。

如何得到一个Homography

要得到两张图片的H,就必须至少知道4个相同对应位置的点，opencv中可以利用findHomography正确得到

  
  
// pts_src and pts_dst are vectors of points in source
// and destination images. They are of type vector<Point2f>.
// We need at least 4 corresponding points.

Mat h = findHomography(pts_src, pts_dst);

// The calculated homography can be used to warp
// the source image to destination. im_src and im_dst are
// of type Mat. Size is the size (width,height) of im_dst.
warpPerspective(im_src, im_dst, h, size);
  
  

• OpenCV C++ Homography的一个简单例子：

  
      
      
  #include "opencv2/opencv.hpp"
  
  using namespace cv;
  using namespace std;
  
  int main( int argc, char ** argv)
  {
  // Read source image.
  Mat im_src = imread( "book2.jpg");
  // Four corners of the book in source image
  vector<Point2f> pts_src;
  pts_src.push_back(Point2f( 141, 131));
  pts_src.push_back(Point2f( 480, 159));
  pts_src.push_back(Point2f( 493, 630));
  pts_src.push_back(Point2f( 64, 601));
  
  
  // Read destination image.
  Mat im_dst = imread( "book1.jpg");
  // Four corners of the book in destination image.
  vector<Point2f> pts_dst;
  pts_dst.push_back(Point2f( 318, 256));
  pts_dst.push_back(Point2f( 534, 372));
  pts_dst.push_back(Point2f( 316, 670));
  pts_dst.push_back(Point2f( 73, 473));
  
  // Calculate Homography
  Mat h = findHomography(pts_src, pts_dst);
  
  // Output image
  Mat im_out;
  // Warp source image to destination based on homography
  warpPerspective(im_src, im_out, h, im_dst.size());
  
  // Display images
  imshow( "Source Image", im_src);
  imshow( "Destination Image", im_dst);
  imshow( "Warped Source Image", im_out);
  
  waitKey( 0);
  }
      
      
  

  • 41

  Homography应用：图像矫正

  假设你有一张如下所示的图片
  

  你想点击图中书的四个顶点，然后得到正放的书：
  

  该如何做？
  利用Homography可以做到这点。
  1.首先获取书本四个顶点的坐标 pts_src
  2.然后我们需要知道书本的宽高比，此书的宽高比是3/4,所以可使输出图像的size 为300*400，就可设其四个点的坐标为（0,0），（299,0），（299,399），（0,399）保存在pts_dst中
  3.通过pts_src和pts_dst 获取homography
  4.对原图应用homography 得到输出

  
      
      
  #include <opencv2/opencv.hpp>
  
  using namespace cv;
  using namespace std;
  
  struct userdata{
  Mat im;
  vector<Point2f> points;
  };
  
  
  void mouseHandler( int event, int x, int y, int flags, void * data_ptr)
  {
  if ( event == EVENT_LBUTTONDOWN )
  {
  userdata *data = ((userdata *) data_ptr);
  circle(data->im, Point(x,y), 3,Scalar( 0, 0, 255), 5, CV_AA);
  imshow( "Image", data->im);
  if (data->points.size() < 4)
  {
  data->points.push_back(Point2f(x,y));
  }
  }
  
  }
  
  
  
  void main()
  {
  
  // Read source image.
  Mat im_src = imread( "book1.jpg");
  
  // Destination image. The aspect ratio of the book is 3/4
  Size size( 300 , 400 );
  Mat im_dst = Mat::zeros(size,CV_8UC3);
  
  
  // Create a vector of destination points.
  vector<Point2f> pts_dst;
  
  pts_dst.push_back(Point2f( 0, 0));
  pts_dst.push_back(Point2f(size.width - 1, 0));
  pts_dst.push_back(Point2f(size.width - 1, size.height - 1));
  pts_dst.push_back(Point2f( 0, size.height - 1 ));
  
  // Set data for mouse event
  Mat im_temp = im_src.clone();
  userdata data;
  data.im = im_temp;
  
  cout << "Click on the four corners of the book -- top left first and" << endl
  << "bottom left last -- and then hit ENTER" << endl;
  
  // Show image and wait for 4 clicks.
  imshow( "Image", im_temp);
  // Set the callback function for any mouse event
  setMouseCallback( "Image", mouseHandler, &data);
  waitKey( 0);
  
  // Calculate the homography
  Mat h = findHomography(data.points, pts_dst);
  
  // Warp source image to destination
  warpPerspective(im_src, im_dst, h, size);
  
  // Show image
  imshow( "Image", im_dst);
  waitKey( 0);
  
  
  }
      
      
  

  Homography应用：虚拟广告牌

  在足球或者棒球体育直播中，经常可以看到球场旁边有虚拟广告，并且还会根据地区，国家的不同播放不同的广告，这是如何做到的？
  看完此篇博客，你应该就能知道如何实现了。原理跟前一个差不多，这里直接上代码

  
      
      
  #include <opencv2/opencv.hpp>
  
  using namespace cv;
  using namespace std;
  
  struct userdata{
  Mat im;
  vector<Point2f> points;
  };
  
  
  void mouseHandler( int event, int x, int y, int flags, void * data_ptr)
  {
  if ( event == EVENT_LBUTTONDOWN )
  {
  userdata *data = ((userdata *) data_ptr);
  circle(data->im, Point(x,y), 3,Scalar( 0, 255, 255), 5, CV_AA);
  imshow( "Image", data->im);
  if (data->points.size() < 4)
  {
  data->points.push_back(Point2f(x,y));
  }
  }
  
  }
  
  
  
  int main( int argc, char ** argv)
  {
  
  // Read in the image.
  Mat im_src = imread( "first-image.jpg");
  Size size = im_src.size();
  
  // Create a vector of points.
  vector<Point2f> pts_src;
  pts_src.push_back(Point2f( 0, 0));
  pts_src.push_back(Point2f(size.width - 1, 0));
  pts_src.push_back(Point2f(size.width - 1, size.height - 1));
  pts_src.push_back(Point2f( 0, size.height - 1 ));
  
  
  
  // Destination image
  Mat im_dst = imread( "times-square.jpg");
  
  
  // Set data for mouse handler
  Mat im_temp = im_dst.clone();
  userdata data;
  data.im = im_temp;
  
  
  //show the image
  imshow( "Image", im_temp);
  
  cout << "Click on four corners of a billboard and then press ENTER" << endl;
  //set the callback function for any mouse event
  setMouseCallback( "Image", mouseHandler, &data);
  waitKey( 0);
  
  // Calculate Homography between source and destination points
  Mat h = findHomography(pts_src, data.points);
  
  // Warp source image
  warpPerspective(im_src, im_temp, h, im_temp.size());
  
  // Extract four points from mouse data
  Point pts_dst[ 4];
  for( int i = 0; i < 4; i++)
  {
  pts_dst[i] = data.points[i];
  }
  
  // Black out polygonal area in destination image.
  fillConvexPoly(im_dst, pts_dst, 4, Scalar( 0), CV_AA);
  
  // Add warped source image to destination image.
  im_dst = im_dst + im_temp;
  
  // Display image.
  imshow( "Image", im_dst);
  waitKey( 0);
  
  return 0;
  }
      
      
  

  
  

  结果：