Drawing Rectangle around difference area - c++

Question

I have a question which i am unable to resolve. I am taking difference of two images using OpenCV. I am getting output in a seperate Mat. Difference method used is the AbsDiff method. Here is the code.
char imgName[15];
Mat img1 = imread(image_path1, COLOR_BGR2GRAY);
Mat img2 = imread(image_path2, COLOR_BGR2GRAY);
/*cvtColor(img1, img1, CV_BGR2GRAY);
cvtColor(img2, img2, CV_BGR2GRAY);*/
cv::Mat diffImage;
cv::absdiff(img2, img1, diffImage);
cv::Mat foregroundMask = cv::Mat::zeros(diffImage.rows, diffImage.cols, CV_8UC3);
float threshold = 30.0f;
float dist;
for(int j=0; j<diffImage.rows; ++j)
{
for(int i=0; i<diffImage.cols; ++i)
{
cv::Vec3b pix = diffImage.at<cv::Vec3b>(j,i);
dist = (pix[0]*pix[0] + pix[1]*pix[1] + pix[2]*pix[2]);
dist = sqrt(dist);
if(dist>threshold)
{
foregroundMask.at<unsigned char>(j,i) = 255;
}
}
}
sprintf(imgName,"D:/outputer/d.jpg");
imwrite(imgName, diffImage);
I want to bound the difference part in a rectangle. findContours is drawing too many contours. but i only need a particular portion. My diff image is
I want to draw a single rectangle around all the five dials.
Please point me to right direction.
Regards,

Answer 1

I would search for the highest value for i index giving a non black pixel; that's the right border.
The lowest non black i is the left border. Similar for j.

Answer 2

You can:
binarize the image with a threshold. Background will be 0.
Use findNonZero to retrieve all points that are not 0, i.e. all foreground points.
use boundingRect on the retrieved points.
Result:
Code:
#include <opencv2/opencv.hpp>
using namespace cv;
int main()
{
// Load image (grayscale)
Mat1b img = imread("path_to_image", IMREAD_GRAYSCALE);
// Binarize image
Mat1b bin = img > 70;
// Find non-black points
vector<Point> points;
findNonZero(bin, points);
// Get bounding rect
Rect box = boundingRect(points);
// Draw (in color)
Mat3b out;
cvtColor(img, out, COLOR_GRAY2BGR);
rectangle(out, box, Scalar(0,255,0), 3);
// Show
imshow("Result", out);
waitKey();
return 0;
}

Answer 3

Find contours, it will output a set of contours as std::vector<std::vector<cv::Point> let us call it contours:
std::vector<cv::Point> all_points;
size_t points_count{0};
for(const auto& contour:contours){
points_count+=contour.size();
all_points.reserve(all_points);
std::copy(contour.begin(), contour.end(),
std::back_inserter(all_points));
}
auto bounding_rectnagle=cv::boundingRect(all_points);