Hi, I'm not sure I've seen that equation before.

Usually for drawing a circle I use this equation:

sx = x + radius * sin(i);
sy = y + radius * cos(i);
where
(x, y) is the center of the circle
radius is the radius of the circle
i is the running angle (0 - 180 degrees for half circle)
(sx, sy) is the running pixel coordinate

and use i as the loop counter.

Note that in JavaScript's Math.sin() and Math.cos(), angle is expressed in radians instead of degrees, so you need to convert degrees to radians with this equation:
radian = degree * PI / 180

Hope this helps.

Peace always,
<jdenny>