Copy Mat at Contour/Mask into another Mat.

edit

in short - NO. only rectangular areas can be copied.

but , what you can do is: make a mask from your contour, and mask out (blacken or such) anything inside the boundingRect but outside the contour:

Rect roi = boundingRect(contours[i])
Mat mask = Mat::zeros(roi.size(), img.type());
drawContours(mask,contours,i,Scalar::all(255), -1); // -1 == filled
Mat final;
img(roi).copyTo(final, mask);