double treshold = 0.8;
for (double y = 0, n = 2.0 * radius; y < n; y++)
{
for (double x = 0; x < n; x++)
{
double deltaX = radius - x;
double deltaY = radius - y;
double distance = Math.sqrt(deltaX * deltaX + deltaY * deltaY);
double color = Math.max(0, Math.min(1, radius - distance));
if(color>treshold){
//draw pixel (e.g.: openGL: glVertex2i(x, y))
}
}
}