how to create a masked image in opencv - image

Question

Here is my code (working after taking inputs from zindarod)
#include <stdio.h>
#include "opencv2/core/core.hpp"
#include "opencv2/features2d/features2d.hpp"
#include "opencv2/highgui/highgui.hpp"
#include "opencv2/nonfree/nonfree.hpp"
using namespace cv;
static void help()
{
printf("\nThis program demonstrates using features2d detector, descriptor extractor and simple matcher\n"
"Using the sift desriptor:\n"
"\n"
"Usage:\n matcher_simple <image1> <image2>\n");
}
int main(int argc, char** argv)
{
if(argc != 3)
{
help();
return -1;
}
Mat img1 = imread(argv[1], CV_LOAD_IMAGE_GRAYSCALE);
Mat img2 = imread(argv[2], CV_LOAD_IMAGE_GRAYSCALE);
Rect regionone(151, 115, 42, 27);
Rect regiontwo(141, 105, 52, 37);
Mat dst,mask;
Rect rect(151, 115, 42, 27);
mask = Mat::zeros(img1.size(),CV_8UC1);
mask(Rect(151,115,42,27)) = 1;
img1.copyTo(dst,mask);
if(img1.empty() || img2.empty())
{
printf("Can't read one of the images\n");
return -1;
}
// detecting keypoints
SiftFeatureDetector detector(400);
vector<KeyPoint> keypoints1, keypoints2;
detector.detect(dst, keypoints1);
detector.detect(img2, keypoints2);
// computing descriptors
SiftDescriptorExtractor extractor;
Mat descriptors1, descriptors2;
extractor.compute(dst, keypoints1, descriptors1);
extractor.compute(img2, keypoints2, descriptors2);
// matching descriptors
BFMatcher matcher(NORM_L2);
vector<DMatch> matches;
matcher.match(descriptors1, descriptors2, matches);
// drawing the results
namedWindow("matches", 1);
Mat img_matches;
drawMatches(dst, keypoints1, img2, keypoints2, matches, img_matches);
imshow("masked image",dst);
//imshow("matches", img_matches);
waitKey(0);
return 0;
}
My aim is to compare two different parts of two different images .
You can run above code after using
g++ above_code.cpp -o bincode -I /usr/include/ `pkg-config --libs --cflags opencv`
./bincode image1.png image2.png
It seems that I am passing a rectangular region to keypoint detector as a result , the keypoints1 are saved with coordinates relative to 151,115 .
So , I should pass a masked image to keypoint detector .
How can I create a matrix filled with zeroes (or 255) but with rectangular region at 151,115 copied from img1 ?
thanks.

Answer 1

The following copies source image to destination image based on mask.
Mat src = imread("source.jpg",-1),dst,mask;
Rect rect(151, 115, 42, 27);
mask = Mat::zeros(src.Size(),CV_8UC1);
rectangle(mask, Point(rect.x,rect.y),Point(rect.x+rect.width,rect.y+rect.height),Scalar(255),-1);
src.copyTo(dst,mask);
Although there's a better way for your problem, you can translate your keypoints to the size of the original image.