How to convert x,y coordinates to an angle?

Question

Microsoft provide an excellent SVG gradient maker so IE9 can also have "CSS3" gradients (click Custom).

I currently utilise their logic for my Fireworks and Dreamweaver extensions to convert gradients to SVG, but I only know how to do it for standard top, bottom, left, right directions. If you enter an angle, I don't do the conversion, because I'm not sure how I would convert x1, x2, y1, y2 to CSS3 angle degrees.

The gradient generator provides values like this: x1="0%" y1="0%" x2="56.262833675564686%" y2="68.29999651227678%"

I'm not great with mathematics or trigonometry, so could somebody help me out? I'd also like to use the same math in a Sass mixin to do a similar thing, if possible.

Answer 1

If you get deltaX and deltaY from your coordinates then Math.atan2 returns the arctangent of the quotient of its arguments. The return value is in radians.

var deltaX = x2 - x1;
var deltaY = y2 - y1;
var rad = Math.atan2(deltaY, deltaX); // In radians

Then you can convert it to degrees as easy as:

var deg = rad * (180 / Math.PI)

Edit

There was some bugs in my initial answer. I believe in the updated answer all bugs are addressed. Please comment here if you think there is a problem here.