Canvas - caman js - image filtration in specific areas not in whole canvas

Question

I am using caman js image filtration with canvas in one of my rails4 project. Caman js color effect the whole canvas but my requirement is to effect the specific multi-areas of canvas. Please follow the below links.

This is what i need, color effect (like - grey scale, blur, hue, noise, etc) on the specific marked areas.

http://www.awesomescreenshot.com/image/1020085/180e6924d5e297ecbbffcefb27f08dad

I get the wrong result which effect the whole canvas:

http://www.awesomescreenshot.com/image/1020089/32d7354ae98bd9bdd96b0649e16fa6db

Please I need your help to solve this issue. Thanks in advance :)