diff --git a/src/basics.h b/src/basics.h
index 41223d6..d46afab 100644
--- a/src/basics.h
+++ b/src/basics.h
@@ -32,6 +32,14 @@ struct User {
 	Size size{0,0};
 	Position position{0,0};
 
+	template <typename Ball>
+	bool is_inside(Ball b){
+		float x = b.position.x;
+		float y = b.position.y;
+		return position.x < x && x < position.x + size.width
+			&& position.y < y && y < position.y + size.height;
+	}
+
 	User& operator=(IncomingPacket const & p){
 		name = p.name;
 		size = p.size;
diff --git a/src/json_ext.h b/src/json_ext.h
index 76a2760..2c19f7c 100644
--- a/src/json_ext.h
+++ b/src/json_ext.h
@@ -4,16 +4,24 @@
 
 struct Identity{
 	template <typename T>
-	T operator()(T t){
+	T operator()(T t) const {
 		return t;
 	}
 };
 
-template <typename C, typename F = Identity>
-inline js::Value vector_to_json(C const & container, F f = Identity()){
+struct All{
+	template <typename T>
+	bool operator()(T const & t) const {
+		return true;
+	}
+};
+
+template <typename C, typename F1 = Identity, typename F2 = All>
+inline js::Value vector_to_json(C const & container, F1 const & transform = Identity(), F2 const & filter = All()){
 	js::Array array;
 	for(auto x : container){
-		array.push_back(to_json(f(x)));
+		if(!filter(x)) continue;
+		array.push_back(to_json(transform(x)));
 	}
 	return array;
 }
diff --git a/src/main.cpp b/src/main.cpp
index 964bfc2..742ad61 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -39,9 +39,9 @@ websockets::TestProtocol<User> default_protocol{
 			user.update_lines = false;
 		} else {
 			if(app->uberclient){
-				ret["balls"] = vector_to_json(app->offline_sim.balls);
+				ret["balls"] = vector_to_json(app->offline_sim.balls, Identity(), [&user](cheap_ball_type const & b){ return user.is_inside(b); });
 			} else {
-				ret["balls"] = vector_to_json(app->online_sim.sim.balls, [](ball_type const & b){return cheap_ball_type(b);});
+				ret["balls"] = vector_to_json(app->online_sim.sim.balls, [](ball_type const & b){return cheap_ball_type(b);}, [&user](ball_type const & b){ return user.is_inside(b); });
 			}			
 		}
 		return write_json(ret);