Finding Angles 0-360

Need help with a math issue: i need to get the true angle from 0 degrees using x and y cordinates im using this at the moment: Math.atan((x2-x1)/(y1-y2))/(Math.PI/180) but /(Math.

Solution 1:

The atan function only gives half the unit circle between -pi/2 and +pi/2 (0 on x axis), there is another library function that can give the whole unit circle between -pi and + pi, atan2

I would think you are better of using atan2 to get the right quadrant rather than branching yourself, then just scale as you have been, something like

Math.atan2(y2 - y1, x2 - x1) * 180 / Math.PI + 180

The multiply by 180 over pi is just the scale from radians to degrees as in the question (but with the division by a division simplified), the +180 makes sure its always positive i.e. 0-360 deg rather than -180 to 180 deg

Solution 2:

Math.atan limits you to the two rightmost quadrants on the unit circle. To get the full 0-360 degrees:

if x < 0add180 to the angle
elseif y < 0add360 to the angle. 

Your coordinate system is rotated and inverted compared to mine (and compared to convention). Positive x is to the right, positive y is up. 0 degrees is to the right (x>0, y=0, 90 degrees is up (x=0,y>0) 135 degrees is up and to the left (y>0, x=-y), etc. Where are your x- and y-axes pointing?

Solution 3:

The answers of @jilles de wit and @jk. led me on the right path but for some reason did not provide the right solution for my problem that i think is very similar to the original question.

I wanted to get up = 0°, right = 90°, down = 180°, left = 270° as in aeronautical navigation systems.

Presuming the question was referring to canvas drawing i reached this solution:

I first translated the canvas origin using ctx.translate(ctx.canvas.width / 2, ctx.canvas.height / 2). I also halved e.offsetX and e.offsedY i got from a mouse event on the canvas to get x and y with the same coordinate system as the canvas.

let radianAngle = Math.atan2(y, x);    // x has the range [-canvas.width/2 ... +canvas.width/2], y is similarlet northUpAngle = radianAngle * 180 / PI + 90;    // convert to degrees and add 90 to shift the angle counterclockwise from it's default "left" = 0°if (x < 0 && y < 0) {    // check for the top left quadrant
    northUpAngle += 360;    // add 360 to convert the range of the quadrant from [-90...0] to [270...360] (actual ranges may vary due to the way atan2 handles quadrant boundaries)
}
northUpAngle.toFixed(2)    // to avoid getting 360° near the "up" position

There might be a more concise solution using the modulo operation but i could't find it.

Solution 4:

Also note:

if (y1==y2) {
    if (x1>x2)
        angle = 90;
    elseif (x1<x2)
        angle = 270;
    else
        angle = 0;
}

Solution 5:

This should do the trick:

1. If y2
2. If < 0, add 360.

Examples:

(x1,y1) = 0

(x2,y2) = (-1,1), atan() =-45, [add360], 270
(x2,y2) = (1,1), atan() =45
(x2,y2) = (1,-1), atan() =-45, [add180], 135
(x2 ,y2) = (-1,-1), atan() =45, [add180], 225