iOS and Android Phone App for identifying distances in Pixels of Digital Photos

My project is to create an iOS and Android app that can measure distances in pixels of two marked points in digital photos taken from close-up shots and landscapes. The app will be used for photos of specific types, not just any photo. I need to ensure that the measurements of the app are accurate and precise, with a desired level of accuracy of within 1 pixel. The app has to be user-friendly with a well-designed interface, and must be able to distinguish between distances in order to provide accurate measurements.