Finding damages in the image using Feature Matching

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Given an image and a template image, I would like to match the images and find possible damages, if any.

Undamaged Image

Damaged Image

Template Image

Note: Above image shows the example of a damage, which can be of any size and shape. Assume that proper preprocessing has been done and both the template and the image are converted to binary with a white background.

I used the following approach to detect the key points and match it:

1. Find all the keypoints and the descriptors from the template as well as the image using ORB. For that, I used the inbuilt function of OpenCV named detectAndCompute().
2. After this, I used the Brute Force Matcher and matched it using the knnMatch().
3. Then I used the Lowe's Ratio Test to find good matches.

Results: If I match the template with itself template-template, I get 1751 matches which should be an ideal value for a perfect match.

In the undamaged image, I got 847 good matches.

In the damaged image, I got 346 good matches.

We can perceive the differences from the number of matches, but I have a few questions:

1. How to pin point the exact location of the damages?
2. How can I conclude that the image contains damages by looking at the number of good matches in the image-template and template-template?

Here is the code for your reference.

    #include <iostream>

    #include <opencv2/features2d/features2d.hpp>
    #include <opencv2/calib3d/calib3d.hpp>
    #include <opencv2/imgproc/imgproc.hpp>
    #include <opencv2/highgui/highgui.hpp>

    using namespace std;
    using namespace cv;

    int main() {

            Mat image = imread("./Images/PigeonsDamaged.jpg");
            Mat temp = imread("./Templates/Pigeons.bmp");

            Mat img_gray, temp_gray;

            cvtColor(image, img_gray, CV_RGB2GRAY);
            cvtColor(temp, temp_gray, CV_RGB2GRAY);

            /**** Pre-processing *****/

            threshold(temp_gray, temp_gray, 200, 255, THRESH_BINARY);
            adaptiveThreshold(img_gray, img_gray, 255, ADAPTIVE_THRESH_GAUSSIAN_C, THRESH_BINARY_INV, 221, 0);

            /*****/

            /***** ORB keypoint detector *****/

            Mat img_descriptors, temp_descriptors;
            vector<KeyPoint> img_keypoints, temp_keypoints;

            vector<KeyPoint> &img_kp = img_keypoints;
            vector<KeyPoint> &temp_kp = temp_keypoints;

            Ptr<ORB> orb = ORB::create(100000, 1.2f, 4, 40, 0, 4, ORB::HARRIS_SCORE, 40, 20);

            orb -> detectAndCompute(img_gray, noArray(), img_kp, img_descriptors, false);
            orb -> detectAndCompute(temp_gray, noArray(), temp_kp, temp_descriptors, false);

            cout << "Temp Keypoints " << temp_kp.size() << endl;

            /*****/

            vector<vector<DMatch> > featureMatches;
            vector<vector<DMatch> > &matches = featureMatches;

            Mat & img_desc_ref = img_descriptors;
            Mat & temp_desc_ref = temp_descriptors;

            BFMatcher bf(NORM_HAMMING2, false);    /** Never keep crossCheck true when using knnMatch. Imp: Use NORM_HAMMING2 for WTA_K = 3 or 4 **/
            bf.knnMatch(img_descriptors, temp_descriptors, matches, 3);

            /*****/

            /***** Ratio Test *****/

            vector<DMatch> selected;
            vector<Point2f> src_pts, temp_pts;

            float testRatio = 0.75;

            for (int i = 0; i < featureMatches.size(); ++i) {

                    if (featureMatches[i][0].distance < testRatio * featureMatches[i][1].distance) {
                            selected.push_back(featureMatches[i][0]);
                    }

            }


            cout << "Selected Size: " << selected.size() << endl;

            /*****/

            /*** Draw the Feature Matches ***/

            Mat output;
            vector <DMatch> &priorityMatches = selected;

            drawMatches(image, img_kp, temp, temp_kp, priorityMatches, output, Scalar(0, 255, 0), Scalar::all(-1));

            namedWindow("Output", CV_WINDOW_FREERATIO);
            imshow("Output", output);
            waitKey();

            /******/

            return 0;
    }

P.S.: I am expecting an elaborate answer as I am new to OpenCV.