Measuring object width

Question

I'd like to develop an iPhone app that does the following: 1. Starts the device camera. 2. Places a layer on the screen containing a stretchable frame for the user to fit to a desired object. 3. Measures the object's width & height.

You may look at this app which does practically what I need and more: http://itunes.apple.com/us/app/easymeasure-measure-your-camera!/id349530105?mt=8

Notice that it doesn't need to be super accurate and can definitely bear some aberration.

Any clue how to do it?

10x

Answer 1

The clue: Geometry and Trigonometry.
By knowing the camera Field-of-View angles, entering the height of the camera above ground and assuming a planar, ie flat, ground, you can use basic geometry and trigonometry to work out everything.