为什么在C ++中使用OpenCV似乎无法与knnMatch匹配？

Question

First off I will say that so far I have based a large chunk of this using this very interesting post on the subject.

In the mentioned post, the example uses a webcam and a UI window for the purposes of seeing the output in real time. I am simply just trying to use similar code to compare two images, (as appose to one image and lots of frames), but have ran into some problems.

So I have two images (cv::Mat objects)

Mat object_1 = imread( "image1.jpg", CV_LOAD_IMAGE_GRAYSCALE );
Mat object_2 = imread( "image2.jpg", CV_LOAD_IMAGE_GRAYSCALE );

The following code isn't great, but this is the general idea:

int minHessian = 500;

SurfFeatureDetector detector( minHessian );
std::vector<KeyPoint> kp_object;

SurfDescriptorExtractor extractor;
Mat des_object;

extractor.compute( object_1, kp_object, des_object );

FlannBasedMatcher matcher;

std::vector<Point2f> obj_corners(4);

//Get the corners from the object
obj_corners[0] = cvPoint(0,0);
obj_corners[1] = cvPoint( object_1.cols, 0 );
obj_corners[2] = cvPoint( object_1.cols, object_1.rows );
obj_corners[3] = cvPoint( 0, object_1.rows );

Mat des_image, img_matches;
std::vector<KeyPoint> kp_image;
std::vector<vector<DMatch > > matches;
std::vector<DMatch > good_matches;
std::vector<Point2f> obj;
std::vector<Point2f> scene;
std::vector<Point2f> scene_corners(4);
Mat H;

detector.detect( object_2, kp_image );
extractor.compute( object_2, kp_image, des_image );

matcher.knnMatch(des_object, des_image, matches, 2);

for(int i = 0; i < min(des_image.rows-1,(int) matches.size()); i++) //THIS LOOP IS SENSITIVE TO SEGFAULTS
{
    if((matches[i][0].distance < 0.6*(matches[i][1].distance)) && ((int) matches[i].size()<=2 && (int) matches[i].size()>0))
    {
        good_matches.push_back(matches[i][0]);
    }
}

The issue here is that because matches.size() is equal to 0 , it is not getting into the loop at all.

My question is, (even if both original images are the same) why are there no matches?

Answer 1

You need to detect keypoints in object_1 with detector.detect(object_1, kp_image );

And after that you can call extractor.compute( object_1, kp_object, des_object ); as seen HERE