I am a beginner in `OpenCV` and am currently working on a project requiring the exact mapping of pixels to centimetres/millimetres/any real world unit.

I have performed Stereo Calibration in `OpenCV` followed by Stereo Rectification. And thus I have obtained both the intrinsic and extrinsic parameters.

While performing Stereo Calibration, I have no where entered the exact square size (25mm by 25mm) of the chessboard patter. I have only entered the number of horizontal and vertical inner corners as well as number of boards. So will this have an effect when I reproject to 3D? If so, how do I include the square size in the StereoCalibrate function.

Secondly, the values `fx` and `fy` obtained from the camera matrix are both 513.86 while from the EXIF data it comes out to be 3.7mm. So, what is the exact relation between both?

Thirdly, I have used `reprojectImageTo3D` and obtained the 3d world coordinates. Now what are the exact units of these coordinates (cm/mm/inches/etc.)?

Basically, I want to obtain the exact mapping from pixel to real world units and after lots of reading and searching, I have not been able to do so. Kindly help me with this issue.