Calculating heading angle from 2 points?

Question

I'm trying to get the heading angle from two points as shown below, within -180/180 range. Clockwise measures should be positive while counterclockwise should be negative.

;Assume a view from above:
;
;              C
;              |
;              |
;              |
;            , A------->  (A's heading [A is facing this direction])
;        , '    `
;    , '          `
;  D                `
;                     `
;                       `
;                         B

float fAngle = A.GetHeadingAngle(B) ;fAngle will equal 45.0
float gAngle = A.GetHeadingAngle(C) ;gAngle will equal -90.0
float hAngle = A.GetHeadingAngle(D) ;hAngle will equal 150.0 (approx)

I've tried using atan2 and the difference in coordinates of the two points to get the angle:

headingangle = atan2(dy, dx) * (180.0 / MATH_PI);

but it doesn't give the expected results. for example:

expected HeadingAngle = -10.443440
actual HeadingAngle = 107.463173

Can any math gurus help?

not sure if this is the problem, but in math (hence also for `atan2`) angles are counterclockwise — 463035818_is_not_an_ai, Jan 12 '18 at 14:09
And your `GetHeadingAngle` implementation is where? Do you normalize the vector to length 1 before trying to calculate the angle? — grek40, Jan 12 '18 at 14:10
Possible duplicate of [Using atan2 to find angle between two vectors](https://stackoverflow.com/questions/21483999/using-atan2-to-find-angle-between-two-vectors) — grek40, Jan 12 '18 at 14:14
I'm trying to recreate what the GetHeadingAngle function does (used in the game Skyrim) by writing it in C++. The above is how it should work essentially. I'm not sure whether the vector is normalized in that function — cruelflames, Jan 12 '18 at 14:15
I'm not a math guru, but you might be able to get some help also on math.stackexchange.com — Leonardo Alves Machado, Jan 12 '18 at 14:15

score 1 · Answer 1 · answered Jan 12 '18 at 15:08

You can try something like this:

#include <cmath>
#include <iostream>

struct Obj
{
    double x, y, dir;
    Obj(double x, double y, double dir) :x{ x }, y{ y }, dir{ dir } {}
    double GetHeadingAngle(Obj const &other)
    {
        double heading = std::atan2(other.x - this->x, other.y - this->y) * 180 / 3.1415926535897 - this->dir;
        if (heading < -180) heading += 360;
        if (heading > 180) heading -= 360;
        return heading;
    }
};


int main() 
{
    Obj A(10, 10, 90);

    Obj B(15, 5, 0);
    std::cout << A.GetHeadingAngle(B) << std::endl; // 45

    Obj C(10, 15, 0);
    std::cout << A.GetHeadingAngle(C) << std::endl; // -90

    Obj D(5, 5, 0);
    std::cout << A.GetHeadingAngle(D) << std::endl; // 135

    return 0;
}

https://ideone.com/MyKdOy

@cruelflames it is heading, just like the one you are trying to calculate. the north is 0, current dir of the A is 90 from your diagram, etc. — Killzone Kid, Jan 12 '18 at 15:47

score 0 · Answer 2 · answered Jan 12 '18 at 14:13

atan2 will give you an angle relative to the x axis (one of the two polar coordinates). So, to calculate the angle between B and A, calculate B's azimuth, calculate that of A, and subtract one of another (possibly adding or subtracting 2*pi, to reduce result's absolute value).

Note that your definition of angles is the opposite of standard one (the positive direction is usually counter-clockwise), so you'll have to negate the result of atan2 (or, simply, subtract the second azimuth from the first one).

Calculating heading angle from 2 points?

2 Answers2