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this is probably an 'expectation mismatch'.

the outcome of findContours is not your img_threshold, but the contours vector.

if you care to lok at the docs again you will see: "The function modifies the image while extracting the contours." again, this is just a by-product, not the desired result.

if you need your img_threshold for further operations, supply a clone() for findContours:

vector<vector<Point>>contours;
cv::findContours(img_threshold.clone(), contours, CV_RETR_EXTERNAL, CV_CHAIN_APPROX_SIMPLE);

// let's draw some things, assuming, Mat ocv is the original rgb image:
drawContours(ocv, contours, -1, Scalar(200), 2);
rectangle(ocv, boundingRect(contours[0]), Scalar(0,0,200), 2);

this is probably an 'expectation mismatch'.

the outcome of findContours is not your img_threshold, but the contours vector.

if you care to lok at the docs again you will see: "The function modifies the image while extracting the contours." again, this is just a by-product, not the desired result.

if you need your img_threshold for further operations, supply a clone() for findContours:

vector<vector<Point>>contours;
cv::findContours(img_threshold.clone(), contours, CV_RETR_EXTERNAL, CV_CHAIN_APPROX_SIMPLE);

// let's draw some things, assuming, Mat ocv is the original rgb image:
drawContours(ocv, contours, -1, Scalar(200), 0, Scalar(200,0,0), 2);
rectangle(ocv, boundingRect(contours[0]), Scalar(0,0,200), 2);

this is probably an 'expectation mismatch'.

the outcome of findContours is not your img_threshold, but the contours vector.

if you care to lok look at the docs again you will see: "The function modifies the image while extracting the contours." again, this is just a by-product, not the desired result.

if you need your img_threshold for further operations, supply a clone() for findContours:

vector<vector<Point>>contours;
cv::findContours(img_threshold.clone(), contours, CV_RETR_EXTERNAL, CV_CHAIN_APPROX_SIMPLE);

// let's draw some things, assuming, Mat ocv is the original rgb image:
drawContours(ocv, contours, 0, Scalar(200,0,0), 2);
rectangle(ocv, boundingRect(contours[0]), Scalar(0,0,200), 2);