Determine which side of a line a point lies

math geometry line point
Skizit picture Skizit · Aug 11, 2010 · Viewed 35.3k times · Source

I have a set of points. I want to separate them into 2 distinct sets. To do this, I choose two points (a and b) and draw an imaginary line between them. Now I want to have all points that are left from this line in one set and those that are right from this line in the other set.

How can I tell for any given point z whether it is in the left or in the right set? I tried to calculate the angle between a-z-b – angles smaller than 180 are on the right hand side, greater than 180 on the left hand side – but because of the definition of ArcCos, the calculated angles are always smaller than 180°. Is there a formula to calculate angles greater than 180° (or any other formula to chose right or left side)?

Answer

Related questions

Circle line-segment collision detection algorithm?

I have a line from A to B and a circle positioned at C with the radius R. What is a good algorithm to use to check whether the line intersects the circle? And at what coordinate along the circles …

Read More
Calculate distance between 2 GPS coordinates

How do I calculate distance between two GPS coordinates (using latitude and longitude)?

Read More
How to determine if a point is in a 2D triangle?

Is there an easy way to determine if a point is inside a triangle? It's 2D, not 3D.

Read More