/* circle */
surface
constant_test(float    Kfb = 1    /* fake brightness */)
{
color    surfcolor = 1;
  
if ((s-.5)*(s-.5) + (t-.5)*(t-.5) <.17)
    surfcolor = color(1.0,0.6,0.0);
else
    surfcolor = color(0.0,0.0,1.0);
  
   
/* STEP 1 - set the apparent surface opacity */
Oi = Os;
  
/* STEP 2 - calculate the apparent surface color */
Ci = Oi * Cs * surfcolor * Kfb;
}