Fixed my timestep!

http://gafferongames.com/game-physics/fix-your-timestep/

6 files changed, 63 insertions, 56 deletions

@@ -73,10 +73,10 @@ void PhysicsBodyComponent::receive(Game&, Entity&, const Message& msg)
 {
   if (msg.type == Message::Type::walkLeft)
   {
-    velocity.first = -1.5;
+    velocity.first = -90;
   } else if (msg.type == Message::Type::walkRight)
   {
-    velocity.first = 1.5;
+    velocity.first = 90;
   } else if (msg.type == Message::Type::stopWalking)
   {
     velocity.first = 0.0;
@@ -89,13 +89,30 @@ void PhysicsBodyComponent::receive(Game&, Entity&, const Message& msg)
   }
 }
 
-void PhysicsBodyComponent::tick(Game&, Entity& entity)
+void PhysicsBodyComponent::tick(Game&, Entity& entity, double dt)
 {
-  velocity.first += accel.first;
-  velocity.second += accel.second;
+  // RK4 integration
+  auto a = std::make_pair(velocity.first, velocity.second);
+  auto b = std::make_pair(velocity.first + a.first*dt*0.5, velocity.second + a.second*dt*0.5);
+  auto c = std::make_pair(velocity.first + b.first*dt*0.5, velocity.second + b.second*dt*0.5);
+  auto d = std::make_pair(velocity.first + c.first*dt, velocity.second + c.second*dt);
   
-  entity.position.first += velocity.first;
-  entity.position.second += velocity.second;
+  double dxxdt = 1.0 / 6.0 * (a.first + 2.0*(b.first + c.first) + d.first);
+  double dxydt = 1.0 / 6.0 * (a.second + 2.0*(b.second + c.second) + d.second);
+  
+  entity.position.first += dxxdt * dt;
+  entity.position.second += dxydt * dt;
+  
+  velocity.first += accel.first * dt;
+  velocity.second += accel.second * dt;
+  
+  // Terminal velocity
+#define TERMINAL_VELOCITY_X (2 * TILE_WIDTH * FRAMES_PER_SECOND)
+#define TERMINAL_VELOCITY_Y (2 * TILE_HEIGHT * FRAMES_PER_SECOND)
+  if (velocity.first < -TERMINAL_VELOCITY_X) velocity.first = -TERMINAL_VELOCITY_X;
+  if (velocity.first > TERMINAL_VELOCITY_X) velocity.first = TERMINAL_VELOCITY_X;
+  if (velocity.second < -TERMINAL_VELOCITY_Y) velocity.second = -TERMINAL_VELOCITY_Y;
+  if (velocity.second > TERMINAL_VELOCITY_Y) velocity.second = TERMINAL_VELOCITY_Y;
 }
 
 void PhysicsBodyComponent::detectCollision(Game& game, Entity& entity, Entity& collider, std::pair<double, double> old_position)
@@ -192,9 +209,9 @@ void PlayerSpriteComponent::receive(Game&, Entity&, const Message& msg)
 
 PlayerPhysicsComponent::PlayerPhysicsComponent()
 {
-  jump_velocity = JUMP_VELOCITY(TILE_HEIGHT*4.5, 0.3*FRAMES_PER_SECOND);
-  jump_gravity = JUMP_GRAVITY(TILE_HEIGHT*4.5, 0.3*FRAMES_PER_SECOND);
-  jump_gravity_short = JUMP_GRAVITY(TILE_HEIGHT*3.5, 0.233*FRAMES_PER_SECOND);
+  jump_velocity = JUMP_VELOCITY(TILE_HEIGHT*4.5, 0.3);
+  jump_gravity = JUMP_GRAVITY(TILE_HEIGHT*4.5, 0.3);
+  jump_gravity_short = JUMP_GRAVITY(TILE_HEIGHT*3.5, 0.233);
   
   accel.second = jump_gravity_short;
 }
@@ -203,11 +220,11 @@ void PlayerPhysicsComponent::receive(Game&, Entity& entity, const Message& msg)
 {
   if (msg.type == Message::Type::walkLeft)
   {
-    velocity.first = -1.5;
+    velocity.first = -90;
     direction = -1;
   } else if (msg.type == Message::Type::walkRight)
   {
-    velocity.first = 1.5;
+    velocity.first = 90;
     direction = 1;
   } else if (msg.type == Message::Type::stopWalking)
   {
@@ -255,7 +272,7 @@ void PlayerPhysicsComponent::receive(Game&, Entity& entity, const Message& msg)
   }
 }
 
-void PlayerPhysicsComponent::tick(Game& game, Entity& entity)
+void PlayerPhysicsComponent::tick(Game& game, Entity& entity, double dt)
 {
   // If frozen, do nothing
   if (frozen)
@@ -268,10 +285,10 @@ void PlayerPhysicsComponent::tick(Game& game, Entity& entity)
   {
     if (direction < 0)
     {
-      velocity.first = -1.5;
+      velocity.first = -90;
     } else if (direction > 0)
     {
-      velocity.first = 1.5;
+      velocity.first = 90;
     }
   }
   
@@ -281,21 +298,10 @@ void PlayerPhysicsComponent::tick(Game& game, Entity& entity)
     accel.second = jump_gravity_short;
   }
   
-  // Apply acceleration
-  velocity.first += accel.first;
-  velocity.second += accel.second;
-  
-  // Terminal velocity
-  if (velocity.first < -16) velocity.first = -16;
-  if (velocity.first > 16) velocity.first = 16;
-  if (velocity.second < -16) velocity.second = -16;
-  if (velocity.second > 16) velocity.second = 16;
-  
   // Do the movement
   std::pair<double, double> old_position = entity.position;
-  entity.position.first += velocity.first;
-  entity.position.second += velocity.second;
-  
+  PhysicsBodyComponent::tick(game, entity, dt);
+    
   // Check for collisions
   game.detectCollision(entity, old_position);
 }
@@ -19,10 +19,10 @@ class UserMovementComponent : public Component {
 class PhysicsBodyComponent : public Component {
   public:
     void receive(Game& game, Entity& entity, const Message& msg);
-    void tick(Game& game, Entity& entity);
+    void tick(Game& game, Entity& entity, double dt);
     void detectCollision(Game& game, Entity& entity, Entity& collider, std::pair<double, double> old_position);
     
-  private:    
+  protected:    
     std::pair<double, double> velocity;
     std::pair<double, double> accel;
 };
@@ -40,15 +40,13 @@ class PlayerSpriteComponent : public Component {
     bool dying = false;
 };
 
-class PlayerPhysicsComponent : public Component {
+class PlayerPhysicsComponent : public PhysicsBodyComponent {
   public:
     PlayerPhysicsComponent();
-    void tick(Game& game, Entity& entity);
+    void tick(Game& game, Entity& entity, double dt);
     void receive(Game& game, Entity& entity, const Message& msg);
     
   private:
-    std::pair<double, double> velocity;
-    std::pair<double, double> accel;
     double jump_velocity;
     double jump_gravity;
     double jump_gravity_short;
@@ -13,11 +13,11 @@ void Entity::send(Game& game, const Message& msg)
   }
 }
 
-void Entity::tick(Game& game)
+void Entity::tick(Game& game, double dt)
 {
   for (auto component : components)
   {
-    component->tick(game, *this);
+    component->tick(game, *this, dt);
   }
 }
 
@@ -37,7 +37,7 @@ class Entity {
   public:
     void addComponent(std::shared_ptr<Component> c);
     void send(Game& game, const Message& msg);
-    void tick(Game& game);
+    void tick(Game& game, double dt);
     void input(Game& game, int key, int action);
     void render(Game& game, Texture& buffer);
     void detectCollision(Game& game, Entity& collider, std::pair<double, double> old_position);
@@ -53,7 +53,7 @@ class Component {
   public:
     virtual void receive(Game&, Entity&, const Message&) {}
     virtual void render(Game&, Entity&, Texture&) {}
-    virtual void tick(Game&, Entity&) {}
+    virtual void tick(Game&, Entity&, double) {}
     virtual void input(Game&, Entity&, int, int) {}
     virtual void detectCollision(Game&, Entity&, Entity&, std::pair<double, double>) {}
 };
@@ -50,17 +50,14 @@ void Game::execute(GLFWwindow* window)
   Texture buffer(GAME_WIDTH, GAME_HEIGHT);
 
   double lastTime = glfwGetTime();
-  int nbFrames = 0;
+  const double dt = 0.01;
+  double accumulator = 0.0;
+  
   while (!(shouldQuit || glfwWindowShouldClose(window)))
   {
     double currentTime = glfwGetTime();
-    nbFrames++;
-    if (currentTime - lastTime >= 1.0)
-    {
-      printf("%f ms/frame\n", 1000.0/double(nbFrames));
-      nbFrames = 0;
-      lastTime += 1.0;
-    }
+    double frameTime = currentTime - lastTime;
+    lastTime = currentTime;
     
     // Should we load a new world?
     if (newWorld)
@@ -74,23 +71,29 @@ void Game::execute(GLFWwindow* window)
     glfwPollEvents();
     
     // Tick!
-    for (auto entity : entities)
+    accumulator += frameTime;
+    while (accumulator >= dt)
     {
-      entity->tick(*this);
+      for (auto entity : entities)
+      {
+        entity->tick(*this, dt);
+      }
+      
+      accumulator -= dt;
     }
     
     // Do any scheduled tasks
     for (auto& task : scheduled)
     {
-      task.first--;
+      task.first -= frameTime;
       
-      if (task.first == 0)
+      if (task.first <= 0)
       {
         task.second();
       }
     }
     
-    scheduled.remove_if([] (std::pair<int, std::function<void ()>> value) { return value.first == 0; });
+    scheduled.remove_if([] (std::pair<double, std::function<void ()>> value) { return value.first <= 0; });
   
     // Do rendering
     buffer.fill(buffer.entirety(), 0, 0, 0);
@@ -133,9 +136,9 @@ void Game::saveGame(const Map& map, std::pair<double, double> position)
   save = {&map, position};
 }
 
-void Game::schedule(int frames, std::function<void ()>&& callback)
+void Game::schedule(double time, std::function<void ()>&& callback)
 {
-  scheduled.emplace_front(frames, callback);
+  scheduled.emplace_front(time, callback);
 }
 
 void Game::playerDie(Entity& player, const Map& curMap)
@@ -145,7 +148,7 @@ void Game::playerDie(Entity& player, const Map& curMap)
   
   playSound("../res/Hit_Hurt5.wav", 0.25);
   
-  schedule(FRAMES_PER_SECOND * 0.75, [&] () {
+  schedule(0.75, [&] () {
     if (&curMap != save.map)
     {
       loadMap(*(save.map));
@@ -30,7 +30,7 @@ class Game {
     void loadMap(const Map& map);
     void detectCollision(Entity& collider, std::pair<double, double> old_position);
     void saveGame(const Map& map, std::pair<double, double> position);
-    void schedule(int frames, std::function<void ()>&& callback);
+    void schedule(double time, std::function<void ()>&& callback);
     void playerDie(Entity& player, const Map& curMap);
     
   private:
@@ -43,7 +43,7 @@ class Game {
     Map m{"../maps/embarass.txt"};
     Map m2{"../maps/second.txt"};
     Savefile save;
-    std::list<std::pair<int, std::function<void ()>>> scheduled;
+    std::list<std::pair<double, std::function<void ()>>> scheduled;
     bool shouldQuit = false;
 };