Overlay Canny Edges on Original Image

edit

you can blend the two images with addweighted() function or you can just apply something like src += canny_image; but you will need first convert your canny image to bgr.

Mat dst, detected_edges;
Canny( gray, detected_edges, 30, 100, 3 );

imshow("canny", detected_edges);

dst = Scalar::all(0);

Mat addweight;
src.copyTo( dst, detected_edges); // copy part of src image according the canny output, canny is used as mask
cvtColor(detected_edges, detected_edges, CV_GRAY2BGR); // convert canny image to bgr
addWeighted( src, 0.5, detected_edges, 0.5, 0.0, addweight); // blend src image with canny image
src += detected_edges; // add src image with canny image

imshow("addition", src );
imshow("copyTo", dst );
imshow("addwweighted", addweight );
waitkey();

source:

canny:

addition:

copyTo:

addweighted: