surface orange_circle(float    Kfb = 1, 
                        radius = 0.4,
                        s1 = 0.5,
                        t1 = 0.5)
{
color    surfcolor = 1;
  
if ((s-s1)*(s-s1) + (t-t1)*(t-t1) <= radius*radius)
    surfcolor = color(1, 0.5, 0);
  
Oi = Os;
  
Ci = Oi * Cs * surfcolor * Kfb;
}