Best way to access a mobile phone's camera from a webpage.

Hi I'm looking to make a webpage for mobile users that can scan a qrcode through the user's mobile phone camera and display the resulting url on the page. This functionality needs to be cross platform(android,ios,windows,bb,etc...) I want to know if this is even doable. If so then, let me know how you would approach this problem, or even better show me a demo that I can test and I will award you the project.