surface
ST_redcross(float    Kfb = 1, 
                t_cutoff = 0.5,
                t2_cutoff = 0.5,
                s_cutoff = 0.5,
                s2_cutoff = 0.5;
                color block_tint = color(0, 1, 1),
                back_tint =1)
{
color    surfcolor = 1;
  
if( s >= s_cutoff && s <= s2_cutoff || t >= t_cutoff && t <= t2_cutoff )
    surfcolor = block_tint;    
  
else
    surfcolor = back_tint;
  
  
/* STEP 1 - set the apparent surface opacity */
Oi = Os;
  
/* STEP 2 - calculate the apparent surface color */
Ci = Oi * Cs * surfcolor * Kfb;
}