//smoothing smooths a corner, spanned by v1, v2 and v3 with radius r
module smoothing(v1, v2, v3, r, th = 0){
  n1 = (v1-v2)/norm(v1-v2);
  n2 = (v3-v2)/norm(v3-v2);
  alpha = acos(n1*n2)/2;
  n3 = (n1+n2)/norm(n1+n2);  
    
  centre = v2+n3*(r/sin(alpha));
  l = r*sqrt((1/sin(alpha)^2)-1);

  translate(th*n3){
    difference(){
      polygon([v2+l*n1,v2,v2+l*n2]);
      translate(centre)
        circle(r = r, $fn = 64);  
    }
  }  
}