【OpenCV】透视变换 Perspective Transformation（续）(2)

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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