Face Crop by edges from Given Image

We are developing an android app like Bubble which is available on google play store and app store. we are in trouble one of its module which require to crop face by face's edges from given image. actually we need to replace the face of given image to user's face by cropping user's face by edges. Example - user is clicking a picture by mobile's camera, we need to crop that user's face by edges and then create a new image by merging that user's face and existing image which is a cartoon type of image. you can take an idea from reference app URL [login to view URL]