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148 lines
4.9 KiB
148 lines
4.9 KiB
.. _feature_homography: |
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Features2D + Homography to find a known object |
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********************************************** |
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Goal |
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===== |
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In this tutorial you will learn how to: |
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.. container:: enumeratevisibleitemswithsquare |
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* Use the function :find_homography:`findHomography<>` to find the transform between matched keypoints. |
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* Use the function :perspective_transform:`perspectiveTransform<>` to map the points. |
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Theory |
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====== |
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Code |
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==== |
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This tutorial code's is shown lines below. You can also download it from `here <https://code.ros.org/svn/opencv/trunk/opencv/samples/cpp/tutorial_code/features2D/SURF_Homography.cpp>`_ |
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.. code-block:: cpp |
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#include <stdio.h> |
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#include <iostream> |
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#include "opencv2/core/core.hpp" |
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#include "opencv2/features2d/features2d.hpp" |
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#include "opencv2/highgui/highgui.hpp" |
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#include "opencv2/calib3d/calib3d.hpp" |
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using namespace cv; |
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void readme(); |
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/** @function main */ |
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int main( int argc, char** argv ) |
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{ |
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if( argc != 3 ) |
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{ readme(); return -1; } |
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Mat img_object = imread( argv[1], CV_LOAD_IMAGE_GRAYSCALE ); |
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Mat img_scene = imread( argv[2], CV_LOAD_IMAGE_GRAYSCALE ); |
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if( !img_object.data || !img_scene.data ) |
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{ std::cout<< " --(!) Error reading images " << std::endl; return -1; } |
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//-- Step 1: Detect the keypoints using SURF Detector |
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int minHessian = 400; |
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SurfFeatureDetector detector( minHessian ); |
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std::vector<KeyPoint> keypoints_object, keypoints_scene; |
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detector.detect( img_object, keypoints_object ); |
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detector.detect( img_scene, keypoints_scene ); |
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//-- Step 2: Calculate descriptors (feature vectors) |
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SurfDescriptorExtractor extractor; |
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Mat descriptors_object, descriptors_scene; |
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extractor.compute( img_object, keypoints_object, descriptors_object ); |
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extractor.compute( img_scene, keypoints_scene, descriptors_scene ); |
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//-- Step 3: Matching descriptor vectors using FLANN matcher |
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FlannBasedMatcher matcher; |
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std::vector< DMatch > matches; |
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matcher.match( descriptors_object, descriptors_scene, matches ); |
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double max_dist = 0; double min_dist = 100; |
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//-- Quick calculation of max and min distances between keypoints |
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for( int i = 0; i < descriptors_object.rows; i++ ) |
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{ double dist = matches[i].distance; |
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if( dist < min_dist ) min_dist = dist; |
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if( dist > max_dist ) max_dist = dist; |
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} |
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printf("-- Max dist : %f \n", max_dist ); |
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printf("-- Min dist : %f \n", min_dist ); |
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//-- Draw only "good" matches (i.e. whose distance is less than 3*min_dist ) |
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std::vector< DMatch > good_matches; |
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for( int i = 0; i < descriptors_object.rows; i++ ) |
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{ if( matches[i].distance < 3*min_dist ) |
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{ good_matches.push_back( matches[i]); } |
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} |
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Mat img_matches; |
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drawMatches( img_object, keypoints_object, img_scene, keypoints_scene, |
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good_matches, img_matches, Scalar::all(-1), Scalar::all(-1), |
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vector<char>(), DrawMatchesFlags::NOT_DRAW_SINGLE_POINTS ); |
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//-- Localize the object |
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std::vector<Point2f> obj; |
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std::vector<Point2f> scene; |
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for( int i = 0; i < good_matches.size(); i++ ) |
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{ |
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//-- Get the keypoints from the good matches |
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obj.push_back( keypoints_object[ good_matches[i].queryIdx ].pt ); |
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scene.push_back( keypoints_scene[ good_matches[i].trainIdx ].pt ); |
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} |
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Mat H = findHomography( obj, scene, CV_RANSAC ); |
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//-- Get the corners from the image_1 ( the object to be "detected" ) |
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std::vector<Point2f> obj_corners(4); |
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obj_corners[0] = cvPoint(0,0); obj_corners[1] = cvPoint( img_object.cols, 0 ); |
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obj_corners[2] = cvPoint( img_object.cols, img_object.rows ); obj_corners[3] = cvPoint( 0, img_object.rows ); |
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std::vector<Point2f> scene_corners(4); |
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perspectiveTransform( obj_corners, scene_corners, H); |
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//-- Draw lines between the corners (the mapped object in the scene - image_2 ) |
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line( img_matches, scene_corners[0] + Point2f( img_object.cols, 0), scene_corners[1] + Point2f( img_object.cols, 0), Scalar(0, 255, 0), 4 ); |
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line( img_matches, scene_corners[1] + Point2f( img_object.cols, 0), scene_corners[2] + Point2f( img_object.cols, 0), Scalar( 0, 255, 0), 4 ); |
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line( img_matches, scene_corners[2] + Point2f( img_object.cols, 0), scene_corners[3] + Point2f( img_object.cols, 0), Scalar( 0, 255, 0), 4 ); |
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line( img_matches, scene_corners[3] + Point2f( img_object.cols, 0), scene_corners[0] + Point2f( img_object.cols, 0), Scalar( 0, 255, 0), 4 ); |
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//-- Show detected matches |
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imshow( "Good Matches & Object detection", img_matches ); |
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waitKey(0); |
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return 0; |
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} |
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/** @function readme */ |
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void readme() |
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{ std::cout << " Usage: ./SURF_descriptor <img1> <img2>" << std::endl; } |
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Explanation |
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============ |
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Result |
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====== |
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#. And here is the result for the detected object (highlighted in green) |
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.. image:: images/Feature_Homography_Result.jpg |
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:align: center |
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:height: 200pt |
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