How to trace the corresponding scene texture of given 3d vector?

Hello! I would like to implement the omnidirectional camera (Davide Scaramuzza Model). The main idea of Scaramuzza Model is the backprojection of UV coordinate to the 3d vector. After getting the 3d vector, we can trace the corresponding texture color and map the value into UV coordinate. In post process material editor, how to trace the corresponding scene texture value of calculated 3d points??

218091-2017-10-23-14h42-33.png

the ideal effect