/* diagnal*/
surface
constant_test(float    Kfb = 1    /* fake brightness */)
{
color    surfcolor = 1;
  
  
  
if(s >= 1-t)
    surfcolor = color(0.4,0.4,0.0);
else
    surfcolor = color(.5,.4,.1);
  
    
/* STEP 1 - set the apparent surface opacity */
Oi = Os;
  
/* STEP 2 - calculate the apparent surface color */
Ci = Oi * Cs * surfcolor * Kfb;
}