get angle of a line from horizon?

Question

I want to know how to get an angle of a line A-B from horizontal axis X. Other questions in SO do that only between two lines. I'm aware I can always draw second line A-C and calculate but I'm wondering if there's a faster method. EDIT: I'm very sure I'm not doing a premature optimization.

Answer 1

Given a line A-B, you should have coordinate pairs Ax,Ay and Bx,By. You should then be able to calculate the angle with respect to the X axis by taking the ArcTan of the slope calculated by By-Ay / Bx-Ax. The ArcTan gives the angle in radians which then need to be converted to degrees by multiplying by 180/pi. Some current C# code would be similar to the following.

---- Program.cs ----

using AngleOfLine;

var radians = AngleCalculator.CalculateAngleOfLine(0,0,3,3);

var degrees = AngleCalculator.RadiansToDegrees(radians);

Console.WriteLine($"Angle in radians:{radians}");

Console.WriteLine($"Angle in degrees:{degrees}");

---- AngleCalculator.cs ----

namespace AngleOfLine{

  public static class AngleCalculator{

    public static double CalculateAngleOfLine(int LineAx, int LineAy, int LineBx, int LineBy){

      var AngleInRadians = Math.Atan(GetSlopeOfLine(LineAx, LineAx, LineBx, LineBy));

      return AngleInRadians;

    }

    public static double GetSlopeOfLine(int LineAx, int LineAy, int LineBx, int LineBy){

      return (LineBy - LineAy)/(LineBx-LineAx);

    }

    public static double RadiansToDegrees(double radians){

      return (180 / Math.PI) * radians;

    }

  }

}