CODEWITHSUNDEEP
javascriptmathangle
kohloth
kohloth

Reputation: 904

Find angle between three points, ([x,y], [x,y], [x,y]) allow for obtuse results. (Math and Javascript)

I have the following function, to find the angle between 3 points.

(Where a point is defined as an array containing the x position as its first element and the y position as its second element, and the angle to measure is the angle created when a line is drawn through all three points.)

findAngle: function(a, b, c) {
    var ab = Math.sqrt(Math.pow(b[0] - a[0], 2) + Math.pow(b[1] - a[1], 2));
    var bc = Math.sqrt(Math.pow(b[0] - c[0], 2) + Math.pow(b[1] - c[1], 2));
    var ac = Math.sqrt(Math.pow(c[0] - a[0], 2) + Math.pow(c[1] - a[1], 2));
    var o1 = (bc * bc + ab * ab - ac * ac) / (2 * bc * ab);
    var o2 = Math.acos(o1);
    var o3 = o2 * (180 / Math.PI);
    return o3;
}

However, this only ever returns angles between 0 and 180 degrees. How can I have it return values between 0 - 180 degrees, and values between 180 and 360 when the angle is in fact obtuse?

Upvotes: 0

Views: 1027

Answers (1)

MBo
MBo

Reputation: 80177

Use atan2 function that has two arguments and returns result in range -Pi..Pi (add 2*Pi to negative angles if needed)

angle = Math.atan2(crossproduct(c-b,b-a), dotproduct(c-b,b-a))
where
crossproduct(c-b,b-a) = (c.x-b.x)*(b.y-a.y) - (c.y-b.y)*(b.x-a.x)
dotproduct(c-b,b-a) = (c.x-b.x)*(b.x-a.x) + (c.y-b.y)*(b.y-a.y)

Upvotes: 1

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