What is the best way to find bounding box for binary mask?

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Hi If you are having multiple objects or otherwise multiple contours in your image & you simply want the Bounding box of the overall mask you can easily find the Bounding box as follows!

Mat mSource_Bgr,mSource_Gray,mThreshold;
mSource_Bgr= imread(List_Files[i],1);
cvtColor(mSource_Bgr,mSource_Gray,COLOR_BGR2GRAY);
threshold(mSource_Gray,mThreshold,254,255,THRESH_BINARY_INV);

Mat Points;
findNonZero(mThreshold,Points);
Rect Min_Rect=boundingRect(Points);

//Here you can also use minAreaRect() also to find Rotated Rect.

rectangle(mSource_Bgr,Min_Rect.tl(),Min_Rect.br(),Scalar(0,255,0),2);

imshow("Result",mSource_Bgr);

Source Image

Threshold Mask

Cropped Bounding Rect

Try to use findContours then pass the findContours result into boundingRect