/* Shader description goes here */
surface
ST_curve(float    Kfb = 1,
                curve = 0.5; 
                color block_tint = color(0, 1, 1),
                back_tint =1)
{
color    surfcolor = 1;
  
if(pow(t, 2) + pow(s, 2)<= curve)
    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;
}