How to compare two image to check whether they are almost equal or not. For e.g. I will take a image from camera and then again open the camera. It will instantly check whether it matches with the previously taken image and give a green indicator that it matches 90 % or more. Can anybody know how to handle this?

Use Google to look on GitHub.com Perhaps you might find the code in that repository.

Do you want the camera to indicate whether it matches before the second image is taken or compare when both photographs have been taken. What type of images are used photographs, icons blank and white, color Either way this is going to be extremely complicated just comparing uiimages is difficult due to lighting conditions, image quality, size of image. There are augmented reality that do an excellent job at image recognition and masking. It is a very interesting challenge though

I did some work on a project for someone one time using Wikitude's sdk (http://www.wikitude.com). It wasn't too bad to setup, although I wouldn't recommend it for a novice. It has an option to do this sort of thing. You point the camera on the phone at an object and if it matches (relatively close) to a pre-defined image then it performs whatever action you specify. In their stock example, it shows another image overtop, like an ad overlay. But I was able to manipulate it to switch to a different screen in the app.