Calculation of the angle between the horizontal and the line through two points

Question

JavaScript question.

Below is a routine that seems to have some problems. What is the problem? It is assumed that the function defined by two points returns the angle (in radians) formed between the horizontal axis and the line containing two points (X1, Y1) and (X2, Y2).

function GetAngle(X1, Y1, X2, Y2) { if (Y2 == Y1) { return (X1 > X2) ? Math.PI : 0; } if (X2 == X1) { return (Y2 > Y1) ? Math.PI/2 : 1.5*Math.PI; } var tangent = (X2 - X1) / (Y2 - Y1); var ang = Math.atan(tangent); if (Y2-Y1 < 0) ang -= Math.PI; return ang; } 

Answer 1

Why don't you use Math.atan2 , which is more convenient. It automatically does this correctly when both numbers are negative (what information is lost during division) and returns the correct values ​​for edge cases.

 var angle = Math.atan2(Y2 - Y1, X2 - X1); // these return differently, even though 0 / -1 === 0 / 1 Math.atan2( 0, -1); // Math.PI Math.atan2( 0, 1); // 0 // same thing: 1 / 1 === -1 / -1 Math.atan2( 1, 1); // Math.PI / 4 Math.atan2(-1, -1); // -Math.PI * 3 / 4 // other values Math.atan2( 1, 1); // Math.PI / 4 Math.atan2( 1, 0); // Math.PI / 2 Math.atan2(-1, 0); // -Math.PI / 2 

Answer 2

The function calculates the inverse tangent.

 var tangent = (Y2 - Y1) / (X2 - X1); 

But it is preferable to use Math.atan2() , as pimvdb mentioned.

Answer 3

Thanks for the help! I tried atan2, but in some situations this did not work. I wrote my own function to solve this problem. Here. It works great.

 function calcAngle(p1, p2) { // Returns the angle points p1 and p2 form with the horizontal. if (p2.x > p1.x) { // quad 1 or 2 if (p2.y > p1.y) { // quad 2 return arctan(p1, p2)} // should be 1-90 else { if (p2.y==p1.y) { return 0} else { // quad 1 return 2*Math.PI+arctan(p1, p2) // 270-360 } } } else { if (p2.x==p1.x) { // atan undefined if (p2.y == p1.y) { return 0} else { if (p2.y > p1.y) { return Math.PI/2} else { return 1.5*Math.PI } } } else { // else { p2.x < p1.x // quad 3 or 4 if (p2.y == p1.y) { return Math.PI} else { if (p2.y > p1.y) { // quad 3 return Math.PI + arctan(p1, p2)} // 90-180 else { // quad 4 return Math.PI+ arctan(p1, p2) // 180-270 } } } } } function arctan(p1, p2) { // Returns the arcTan of points p1 and p2. rat= (p2.y-p1.y)/(p2.x-p1.x) inradians=Math.atan(rat) //indegrees=180*inradians/Math.PI return inradians }