How to get point position in left camera reference with point position in right camera reference

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Hello everybody,

This might be a simple questions but I am stuck on it, so any help would be really appreciated.

Reading the book "Learning OpenCV" written by Gary Bradski and Adrian Kaehler, I came upon the following relation relating the position of a point in the right camera reference to the left camera reference :

Where R and T are the rotation matrix and translation vector between the 2 cameras.

As far as I know, a point in a reference Ox'y'z' can be expressed in the reference Oxyz by computing :

It is said just afterwards in the book that using (which comes from what I just said above), we can finally get

So could anyone explain these relations found in the book?

Thank you.