Camera paramerter matrix and calliberation

I have a Webcam which I have placed on top of a assembly line and my goal is to measure the plates which come at slow speeds.
For this, I have used open cv library to compute the intrinsic parameter, distortion coefficient rotation and translation vectors. I have used chessboard pattern to calliberate my Webcam.
Now the problem is to reach from pixel coordinates to real world coordinates.
I have been to many websites where they do some matrix operation …

assembly compute computervision intrinsic library line matrix rotation translation vectors webcam