hi
The solution is VERY easy for a circle...
of course it is easy!. jesus, i must go back to my primary school books ... thanks
Point P, with coord Px, Py
Circle C, with Center Cx, Cy, and Radius R.
If sqrt((Cx-Px)^2 + (Cy-Py)^2) <R, then the point is within the circle. The part in front of the '<' is simply the formula for finding the distance between two points.
One can drop the expensive SQRT op, and say (Cx-Px)^2 + (Cy-Py)^2 < R^2
IE, if the distance between P and C is < R, than the point is inside the circle.
If it = R, the point is on the edge of circle, and well, it's a design choice as to whether that counts as inside/outside the circle.
-- enrike