Augmented Reality marker iphone sdk

i want to have开发者_如何学Python a feature in my application where the user points the camera at a marker printer on a page and a specific NSString is returned (something like a barcode scanner). What free api's can i use? (i have seen some posts about artoolkit but i cannot find the download source or any tutorials on how to implement it and i believe i need a licence). Thanks for any info.

This page sums up all SDK for most platforms and most main engines/framework

http://socialcompare.com/fr/comparison/augmented-reality-sdks