Extracting Background Image Using GrabCut - c++

Question

I've an image (.jpg image), and I want to extract the background from the original image. I've googled a lot but have only found tutorials of extracting foreground image.
I've taken the code from another stackoverflow question. The code is working fine for me, and I've successfully extracted the foreground (as per my requirements). Now I want to completely remove this foreground from the original image. I want it to be something like this:-
Background = Original Image - Foreground
The empty space can be filled with black or white color. How can I achieve this?
I've tried using this technique:-
Mat background = image2 - foreground;
but it gives a complete black image.
Code:-
#include <opencv2/opencv.hpp>
#include <iostream>
using namespace cv;
using namespace std;
int main( )
{
// Open another image
Mat image;
image= cv::imread("images/abc.jpg");
Mat image2 = image.clone();
// define bounding rectangle
cv::Rect rectangle(40,90,image.cols-80,image.rows-170);
cv::Mat result; // segmentation result (4 possible values)
cv::Mat bgModel,fgModel; // the models (internally used)
// GrabCut segmentation
cv::grabCut(image, // input image
result, // segmentation result
rectangle,// rectangle containing foreground
bgModel,fgModel, // models
1, // number of iterations
cv::GC_INIT_WITH_RECT); // use rectangle
cout << "oks pa dito" <<endl;
// Get the pixels marked as likely foreground
cv::compare(result,cv::GC_PR_FGD,result,cv::CMP_EQ);
// Generate output image
cv::Mat foreground(image.size(),CV_8UC3,cv::Scalar(255,255,255));
//cv::Mat background(image.size(),CV_8UC3,cv::Scalar(255,255,255));
image.copyTo(foreground,result); // bg pixels not copied
// draw rectangle on original image
cv::rectangle(image, rectangle, cv::Scalar(255,255,255),1);
imwrite("img_1.jpg",image);
imwrite("Foreground.jpg",foreground);
Mat background = image2 - foreground;
imwrite("Background.jpg",background);
return 0;
}
Note: I'm an opencv beginner and don't have much knowledge of it right now. I shall be very thankful to you if you can either post the complete code (as required by me) or just post the lines of code and tell me where these lines of code be placed. Thanks.
P.S. This is my second question at StackOverflow.com. apologies ... if not following any convention.

Answer 1

Instead of copying all the pixels that are foreground, it copies all pixels which are not foreground. You can do this by using ~, which negates the mask:
image.copyTo(background,~result);

Answer 2

What if you //Get the pixels marked as likely background:
// Get the pixels marked as likely background
cv::compare(result,cv::GC_PR_BGD,result,cv::CMP_EQ);
Edit: The above code is missing GC_BGD pixels. Despite a more efficient answer was given, let's finish what we started:
// Get the pixels marked as background
cv::compare(result,cv::GC_BGD,result_a,cv::CMP_EQ);
// Get the pixels marked as likely background
cv::compare(result,cv::GC_PR_BGD,result_b,cv::CMP_EQ);
// Final results
result=result_a+result_b;

Answer 3

Just a small suggestion，#William's
answer can be written more concisely as：
result = result & 1;
in order to get the binary mask.

Answer 4

Maybe another example helps, in which I assumed that the middle portion of the image is definitely foreground.
So try this link.
Example