UDT - Camera frame clipping according to a region of the target

Hi,

I have this project that propose realistic AR experience. To achieve this, one of the steps is get the pixel information of determinated region of the user defined target. So what I need to do is "back project" the coordinates of a region of the target to the camera frame and then get the information of the pixels.

How can I do this "back projection" from the "world coordinates" of the target to the camera frame coordinates?

P.S.

I'm already did the part of getting the camera frame and make a texture of it. And I tried to do this back projection by multiplying the model view projection matrix by the points position of my interest according to the target. But when I renderizes the texture of this points of interest I only see noises.