"Difficult" edge detection

edit

I want to draw rectangles around the finger print like that:

I tried using simple rectangle detection but It didn't work and then I tried activating morphology operations and add threshold

and I got something like that

one of the finger print disappeared as you can see but I hoped I can at least detect the edges of the other one but I didn't detect anything.

I'm using emguCV (C#) but maybe you can help me anyways. Thanks for the help

if you dont see the pictures : wanted result : http://prntscr.com/pgbxrx after morphology operations : http://prntscr.com/pgc0sl