Use X,Y coordinates to plot points inside a circle

Question

Is there a way in javascript to plot x,y coordinates so they fall into a circle rather than a square?

For example if I have the following code:

  circleRadius = 100;
  context.drawImage(img_elem, dx, dy, dw, dh);

I need to figure out a combination of x,y values that would fall inside a 100 pixel circle.

Thanks!

Answer 1

1. choose an x at random between -100 and 100
2. a circle is defined by x^2 + y^2 = r^2, which in your case equals 100^2 = 10000
3. From this equation you can get that y^2 = 10000 - x^2 , therefore the points with a chosen x and y = +/-sqrt(10000 - x^2) will lye on the circle.
4. choose an y at random between the two coordinates found at point 3
5. You're set!

EDIT: In JS:

var radius = 100;
x = Math.random() * 2 * radius - radius;
ylim = Math.sqrt(radius * radius - x * x);
y = Math.random() * 2 * ylim - ylim;

Another edit: a jsFiddle Example