Calculating degrees between 2 points with inverse Y axis

javascript math trigonometry degrees
skerit picture skerit · Jul 22, 2010 · Viewed 17.7k times · Source

I'm creating a simple 2D game in javascript/canvas. I need to figure out the angle of a certain object relative to my position.

So: say I'm at (10,10) and the object is at (10,5) - that would result in 90 degrees (as positive Y is down, negative Y is up) (10,10) vs (10,15) would be 270 degrees.

How would I go about this?

Answer

kennytm picture kennytm · Jul 22, 2010

Suppose you're at (a, b) and the object is at (c, d). Then the relative position of the object to you is (x, y) = (c - a, d - b).

Then you could use the Math.atan2() function to get the angle in radians.

var theta = Math.atan2(-y, x);

note that the result is in the range [-π, π]. If you need nonnegative numbers, you have to add

if (theta < 0)
   theta += 2 * Math.PI;

and convert radians to degrees, multiply by 180 / Math.PI.

Related questions

How can I generate random points on a circles circumference in javascript

I am trying to write a function that will randomly return an (x,y) co-ordinates around a given circumference so if I have a point that's at (0,0) (being the center of the div) how can I write a function that …

Read More
Why does Math.cos(90 * Math.PI/180) yield 6.123031769111... and not zero?

I convert degrees to radians (degrees * Math.PI/180) but why does the following: Math.cos(90 * Math.PI/180) yield 6.123031769111... and not zero? I'm trying to perform 2D rotations uses matrixes and the results are completely out of whack.

Read More
HTML5 Canvas: Calculating a x,y point when rotated

I developing a HTML5 Canvas App and it involves reading a xml file that describes the position of arrows, rectanges and other shapes I need to to draw on the canvas. Example of the XML layout: <arrow left="10" top="20" …

Read More