color face( float s_center, t_center, radius; color incolor, outcolor)
  
{
  
color result = 1;
  
if (sqrt(pow( s - s_center,2) + pow( t - t_center,2)) <= radius)
    result = incolor;
return result;
  
}
  
/*-------------------------------------------*/
  
color isInCircle(float s_center, t_center, rad, blur;
                 color incolor, outcolor)
{
color result = 0;
  
point P1 = point(s,t,0);
point P2 = point(s_center, t_center,0);
  
float d = distance (P1, P2);
float blend = smoothstep (rad - blur, rad + blur, d);
result = mix(incolor, outcolor, blend);
return result;
}