How to calculate the angle between a line and the

Question

In a programming language (Python, C#, etc) I need to determine how to calculate the angle between a line and the horizontal axis?

I think an image describes best what I want:

Given (P1_x,P1_y) and (P2_x,P2_y) what is the best way to calculate this angle? The origin is in the topleft and only the positive quadrant is used.

Answer 1

deltaY = Math.Abs(P2.y - P1.y);
deltaX = Math.Abs(P2.x - P1.x);

angleInDegrees = Math.atan2(deltaY, deltaX) * 180 / PI

if(p2.y > p1.y) // Second point is lower than first, angle goes down (180-360)
{
  if(p2.x < p1.x)//Second point is to the left of first (180-270)
    angleInDegrees += 180;
  else (270-360)
    angleInDegrees += 270;
}
else if (p2.x < p1.x) //Second point is top left of first (90-180)
  angleInDegrees += 90;

Answer 2

I have found a solution in Python that is working well !

from math import atan2,degrees

def GetAngleOfLineBetweenTwoPoints(p1, p2):
    return degrees(atan2(p2 - p1, 1))

print GetAngleOfLineBetweenTwoPoints(1,3)