OpencvMaskTest.java

import org.opencv.calib3d.Calib3d;
import org.opencv.core.*;
import org.opencv.features2d.*;
import org.opencv.highgui.Highgui;
import utils.MatConverter;

import java.util.LinkedList;
import java.util.List;
import java.util.stream.Stream;

/**
 * Created by Marcin on 2014-11-04.
 */
public class OpencvMaskTest {

    static {
        System.loadLibrary(Core.NATIVE_LIBRARY_NAME);
    }

    //setup SIFT detector and SIFT extractor
    static DescriptorExtractor siftDescriptor = DescriptorExtractor.create(DescriptorExtractor.SIFT);
    static FeatureDetector siftDetector = FeatureDetector.create(FeatureDetector.PYRAMID_SIFT);

    public static void main(String[] args) {

        // read images as Mat
        Mat first = Highgui.imread("Cabildo_Catedralicio_(1913),_de_Refugio_Reyes_Rivas.jpg");
        Mat second = Highgui.imread("Cabildo_Catedralicio_(1913),_de_Refugio_Reyes_Rivas_1.jpg");

        //detect keyPoints
        MatOfKeyPoint keyPointsFirst = getKeyPoints(first);
        MatOfKeyPoint keyPointsSecond = getKeyPoints(second);

        // calculate descriptors
        Mat descriptorsFirst = getDescriptor(first, keyPointsFirst);
        Mat descriptorsSecond = getDescriptor(second, keyPointsSecond);

        //match points
        MatOfDMatch matOfDMatch = getMatOfMatchedPoints(descriptorsFirst, descriptorsSecond);

        // find Homography
        MatOfPoint2f firstMatOfPoint2f = converMatchedToMatOfPoint2f(keyPointsFirst, matOfDMatch, true);
        MatOfPoint2f secondMatOfPoint2f = converMatchedToMatOfPoint2f(keyPointsSecond, matOfDMatch, false);
        MatOfByte mask = new MatOfByte();
        Calib3d.findHomography(firstMatOfPoint2f, secondMatOfPoint2f, Calib3d.RANSAC, 1, mask);

        // sum numbers of matched, just for curious
        double sum = Stream.of(MatConverter.matToDouble(mask)).mapToDouble(array -> array[0]).sum();
        System.out.println(sum);

        Mat output = new Mat();
        Features2d.drawMatches(
                first, keyPointsFirst,
                second, keyPointsSecond,
                matOfDMatch,
                output,
                Scalar.all(-1), Scalar.all(-1),
                mask, // MASK!!!!!!!!!!!!!!!!
                Features2d.NOT_DRAW_SINGLE_POINTS
        );
        Highgui.imwrite("matching.jpg", output);


    }
    public static MatOfKeyPoint getKeyPoints(Mat image) {
        MatOfKeyPoint keyPoints = new MatOfKeyPoint();
        siftDetector.detect(image, keyPoints);
        return keyPoints;
    }

    public static Mat getDescriptor(Mat image, MatOfKeyPoint keyPoints) {
        Mat descriptor = new Mat();
        siftDescriptor.compute(image, keyPoints, descriptor);
        return descriptor;
    }

    public static MatOfPoint2f converMatchedToMatOfPoint2f(MatOfKeyPoint keyPoints, MatOfDMatch matOfDMatch, boolean queryIdx) {
        MatOfPoint2f convertedMatOfPoint = new MatOfPoint2f();
        LinkedList<Point> pointList = new LinkedList<>();
        List<KeyPoint> keyPointList = keyPoints.toList();

        for (DMatch dMatche : matOfDMatch.toArray()) {
            int idxOfQueryOrTrain = queryIdx ? dMatche.queryIdx : dMatche.trainIdx;
            pointList.addLast(keyPointList.get(idxOfQueryOrTrain).pt);
        }
        convertedMatOfPoint.fromList(pointList);
        return convertedMatOfPoint;
    }

    public static MatOfDMatch getMatOfMatchedPoints(Mat descriptorsFirst, Mat descriptorsSecond) {
        MatOfDMatch matOfDMatch = new MatOfDMatch();
        DescriptorMatcher descriptorMatcher = DescriptorMatcher.create(DescriptorMatcher.BRUTEFORCE_SL2);
        descriptorMatcher.match(descriptorsFirst, descriptorsSecond, matOfDMatch);
        return matOfDMatch;
    }
}

Images are directly from: http://upload.wikimedia.org/wikipedia/commons/thumb/5/5b/Cabildo_Catedralicio_%281913%29%2C_de_Refugio_Reyes_Rivas.jpg/450px-Cabildo_Catedralicio_%281913%29%2C_de_Refugio_Reyes_Rivas.jpg

and

http://upload.wikimedia.org/wikipedia/commons/thumb/e/ee/Cabildo_Catedralicio_%281913%29%2C_de_Refugio_Reyes_Rivas_1.jpg/450px-Cabildo_Catedralicio_%281913%29%2C_de_Refugio_Reyes_Rivas_1.jpg