To learn about the 'clever list' that chromatic refers to, check 'binary space partitioning' on Wikipedia.

If you intend to draw circles and you get ovals, check to make sure that your pixels are square, and that the X and Y axes of your graphics system have the same scale factors. To check this, draw a square first. That way you won't have trigonometric problems or rounding problems while you verify your graphical coordinate system.