使用javascript（或html5！）使html中的正文变暗

Question

I would like to know how to be able to "darken" the image of a html page without darkening the area I selected over that same image, just like this example http://odyniec.net/projects/imgareaselect/ . I studied the code but remained the same. Can someone help me? Thank you!

EDIT:

I was able to darken the body but unfortunately, even playing with opacity, the content inside the div referring to the selection remained faded to.

Answer 1

They are achieving this affect using four different div's to do the darkening. One for everything to the top, right, bottom and left. Don't have time to explain further right now, but that is what is happening on the surface level.