This is a demo OpMode that shows how to use the PTC Vuforia library (which is built into the FTC SDK) to locate and track target images.
This tutorial has a companion GitHub repository.
https://github.com/gearsincorg/FTCVuforiaDemo
There is an animation video that follows to explain the different coordinate systems:
https://www.youtube.com/watch?v=T3-F2xpaesg