Rotation Matrix again

I'm back to doing som experiment and want to generate some random 3D points and then some random cameras that point to those points. So, I have to set the random focal length (within some range), the position of the center of image (principle point) and the rotation matrix for the camera to face the point cloud.

I forgot how to do them already, so some online readings directed me to these good sources:

Concepture overview, fully illustrated presentation:eScience Lectures Notes : Transformation in 3D

Plain text explanation with code: Matrix and Quaternion FAQ