Member Avatar for wwsoft

hello , I need to know how to get the direction from point A to point B

eg. point_direction(0 , 0 , 100,100); would equal 315
x1 y1 x2 y2

how do I compute this ?

Thanks !

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  • Latest Post Latest Post by daviddoria

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There are two separate things to get right: the trig routine and the coordinate/angle convention. was pointing you toward trig, computed the angle in standard mathematical coordinates (which gives 45° for (0,0)->(100,100)), and correctly noted that many graphics APIs use a top-left origin (Y increases downward), which flips the sign and can turn 45° into 315°. Use a robust routine (atan2) and explicitly pick the convention you want.

#include <cmath>
#include <iostream>

const double PI = 3.14159265358979323846;

double direction_deg(double x1, double y1, double x2, double y2)
{
    double dx = x2 - x1;
    double dy = y1 - y2;   // note: invert Y for typical screen coords where +Y is down
    double rad = std::atan2(dy, dx);    // handles quadrants and dx==0 safely
    double deg = rad * 180.0 / PI;
    if (deg < 0.0) deg += 360.0;        // normalize to [0,360)
    return deg;
}

int main()
{
    std::cout << direction_deg(0,0,100,100) << '\n'; // prints 315 with the Y-inversion above
}

Notes and quick conversions:

  • If you want standard math angles (0° = +X, counter-clockwise), compute atan2(y2 - y1, x2 - x1) and normalize.
  • To convert a math angle to a clockwise-from-right value: angle_cw = fmod(360.0 - angle_math, 360.0).
  • To make 0° point up, rotate by 90° (add/subtract 90 and re-normalize).

Troubleshooting tips: cast to double to avoid integer division, prefer atan2 over atan to get correct quadrants and avoid divide-by-zero, and be aware that some platforms don’t define M_PI (use your own PI constant). This approach keeps results predictable across different engines and matches the 315° expectation once the screen Y direction is accounted for.

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Member Avatar for Fbody

Are you looking for the bearing? You would need to use the inverse trig functions found in the <cmath> header then convert the answers from radians to degrees.

Member Avatar for wwsoft

Are you looking for the bearing? You would need to use the inverse trig functions found in the <cmath> header then convert the answers from radians to degrees.

what would the code be to this ?

Member Avatar for daviddoria 
#include <cmath>
#include <iostream>

int main()
{
  double x1 = 0;
  double y1 = 0;
  double x2 = 100;
  double y2 = 100;
  
  std::cout << 180./3.14159 * atan((x2-x1)/(y2-y1)) << std::endl;
  
  return 0;
}

The answer is 45 not 315 going from (0,0), to (100,100), right?

Dave

Edited by WaltP because: Fixed CODE tags -- careful, David. Remember PREVIEW...
Member Avatar for rectifryer

Just look up the distance formula.

Member Avatar for wwsoft 
#include <cmath>
#include <iostream>

int main()
{
  double x1 = 0;
  double y1 = 0;
  double x2 = 100;
  double y2 = 100;
  
  std::cout << 180./3.14159 * atan((x2-x1)/(y2-y1)) << std::endl;
  
  return 0;
}

The answer is 45 not 315 going from (0,0), to (100,100), right?

Dave

well in graphics everything starts at the top left hand side :icon_smile:

Member Avatar for daviddoria

Ah, yes. In mathematics the standard is +x is 0 degrees and the angle increases counter-clockwise. You'll have to adjust accordingly.

Dave

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