See Selecting a disk point . You generate random theta (0 to 2 * pi) and random r (0 to 1) that are evenly distributed. Then the points will be as follows:
x = Sx + R*sqrt(r)*cos(theta)
y = Sy + R*sqrt(r)*sin(theta)
Another possibility is to create points in the bounding box and discard points that lie outside the circle.
Edit: this would make the probability density function constant in Cartesian coordinates (which you probably want), but not in polar coordinates, since a large r would have a higher probability. You cannot have both of them permanent.
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