How to find sign of directed distance?

Question

I have 2 points, P and Q, on a directed line AB in 3D space. They can be anywhere on the line, i.e. not necessarily between A and B.

Pythagoras gives you the distance, obviously, but how do I calculate the sign of the directed distance from P to Q?

Answer 1

Take the dot product of AB and PQ. Positive => same direction, negative => opposite direction.