/* Shader description goes here */
surface
ST_circle(float    Kfb = 1,
                radius = 1,
                t_centre = 0,
                s_centre = 0; 
                color block_tint = color(0, 1, 1),
                back_tint =1)
{
color    surfcolor = 1;
  
if(pow(t - t_centre, 2) + pow(s - s_centre, 2) <= radius)
    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;
}