HTML5 Canvas game loop delta time calculations

The modern version of requestAnimationFrame now sends in a timestamp that you can use to calculate elapsed time. When your desired time interval has elapsed you can do your update, create and render tasks.

Here's example code:

var lastTime;
var requiredElapsed = 1000 / 10; // desired interval is 10fps

requestAnimationFrame(loop);

function loop(now) {
    requestAnimationFrame(loop);
    
    if (!lastTime) { lastTime = now; }
    var elapsed = now - lastTime;

    if (elapsed > requiredElapsed) {
        // do stuff
        lastTime = now;
    }
    
}

Here' an implementation of an HTML5 rendering system using a fixed time step with a variable rendering time:

http://jsbin.com/ditad/10/edit?js,output

It's based on this article:

http://gameprogrammingpatterns.com/game-loop.html

Here is the game loop:

    //Set the frame rate
var fps = 60,
    //Get the start time
    start = Date.now(),
    //Set the frame duration in milliseconds
    frameDuration = 1000 / fps,
    //Initialize the lag offset
    lag = 0;

//Start the game loop
gameLoop();

function gameLoop() {
  requestAnimationFrame(gameLoop, canvas);

  //Calcuate the time that has elapsed since the last frame
  var current = Date.now(),
      elapsed = current - start;
  start = current;
  //Add the elapsed time to the lag counter
  lag += elapsed;

  //Update the frame if the lag counter is greater than or
  //equal to the frame duration
  while (lag >= frameDuration){  
    //Update the logic
    update();
    //Reduce the lag counter by the frame duration
    lag -= frameDuration;
  }
  //Calculate the lag offset and use it to render the sprites
  var lagOffset = lag / frameDuration;
  render(lagOffset);
}

The render function calls a render method on each sprite, with a reference to the lagOffset

function render(lagOffset) {
  ctx.clearRect(0, 0, canvas.width, canvas.height);
  sprites.forEach(function(sprite){
    ctx.save();
    //Call the sprite's `render` method and feed it the
    //canvas context and lagOffset
    sprite.render(ctx, lagOffset);
    ctx.restore();
  });
}

Here's the sprite's render method that uses the lag offset to interpolate the sprite's render position on the canvas.

o.render = function(ctx, lagOffset) {
    //Use the `lagOffset` and previous x/y positions to
    //calculate the render positions
    o.renderX = (o.x - o.oldX) * lagOffset + o.oldX;
    o.renderY = (o.y - o.oldY) * lagOffset + o.oldY;

    //Render the sprite
    ctx.strokeStyle = o.strokeStyle;
    ctx.lineWidth = o.lineWidth;
    ctx.fillStyle = o.fillStyle;
    ctx.translate(
      o.renderX + (o.width / 2),
      o.renderY + (o.height / 2)
     );
    ctx.beginPath();
    ctx.rect(-o.width / 2, -o.height / 2, o.width, o.height);
    ctx.stroke();
    ctx.fill();

    //Capture the sprite's current positions to use as 
    //the previous position on the next frame
    o.oldX = o.x;
    o.oldY = o.y;
  };

The important part is this bit of code that uses the lagOffset and the difference in the sprite's rendered position between frames to figure out its new current canvas position:

o.renderX = (o.x - o.oldX) * lagOffset + o.oldX;
o.renderY = (o.y - o.oldY) * lagOffset + o.oldY;

Notice that the oldX and oldY values are being re-calculated each frame at the end of the method, so that they can be used in the next frame to help figure out the difference.

o.oldX = o.x;
o.oldY = o.y;

I'm actually not sure if this interpolation is completely correct or if this is best way to do it. If anyone out there reading this knows that it's wrong, please let us know :)

This isn't really an answer to your question, and without knowing more about the particular game I can't say for sure if it will help you, but do you really need to know dt (and FPS)?

In my limited forays into JS game development I've found that often you don't really need to to calculate any kind of dt as you can usually come up with a sensible default value based on your expected frame rate, and make anything time-based (such as weapon reloading) simply work based on the number of ticks (i.e. a bow might take 60 ticks to reload (~1 second @ ~60FPS)).

I usually use window.setTimeout() rather than window.requestAnimationFrame(), which I've found generally provides a more stable frame rate which will allow you to define a sensible default to use in place of dt. On the down-side the game will be more of a resource hog and less performant on slower machines (or if the user has a lot of other things running), but depending on your use case those may not be real concerns.

Now this is purely anecdotal advice so you should take it with a pinch of salt, but it has served me pretty well in the past. It all depends on whether you mind the game running more slowly on older/less powerful machines, and how efficient your game loop is. If it's something simple that doesn't need to display real times you might be able to do away with dt completely.

At some point you will want to think about decoupling your physics from your rendering. Otherwise your players could have inconsistent physics. For example, someone with a beefy machine getting 300fps will have very sped up physics compared to someone chugging along at 30fps. This could manifest in the first player cruising around in a mario-like scrolling game at super speed and the other player crawling at half speed (if you did all your testing at 60fps). A way to fix that is to introduce delta time steps. The idea is that you find the time between each frame and use that as part of your physics calculations. It keeps the gameplay consistent regardless of frame rate. Here is a good article to get you started: http://gafferongames.com/game-physics/fix-your-timestep/

requestAnimationFrame will not fix this inconsistency, but it is still a good thing to use sometimes as it has battery saving advantages. Here is a source for more info http://www.chandlerprall.com/2012/06/requestanimationframe-is-not-your-logics-friend/

A great solution to your game engine would be to think in objects and entities. You can think of everything in your world as objects and entities. Then you want to make a game object manager that will have a list of all your game objects. Then you want to make a common communication method in the engine so game objects can make event triggers. The entities in your game for example a player would not need to inherent from anything to get the ability to render to the screen or have collision detection. You would simple make common methods in the entity that the game engine is looking for. Then let the game engine handle the entity as it would like. Entities in your game can be created or destroyed at anytime in the game so you should not hard-code any entities at all in the game loop.

You will want other objects in your game engine to respond to event triggers that the engine has received. This can be done using methods in the entity that the game engine will check to see if the method is available and if it is would pass the events to the entity. Do not hard code any of your game logic into the engine it messes up portability and limits your ability to expand on the game later on.

The problem with your code is first your calling different objects render and updates not in the correct order. You need to call ALL your updates then call ALL your renders in that order. Another is your method of hard coding objects into the loop is going to give you a lot of problems, when you want one of the objects to no longer be in the game or if you want to add more objects into the game later on.

Your game objects will have an update() and a render() your game engine will look for that function in the object/entity and call it every frame. You can get very fancy and make the engine work in a way to check if the game object/entity has the functions prior to calling them. for example maybe you want an object that has an update() but never renders anything to the screen. You could make the game object functions optional by making the engine check prior to calling them. Its also good practice to have an init() function for all game objects. When the game engine starts up the scene and creates the objects it will start by calling the game objects init() when first creating the object then every frame calling update() that way you can have a function that you only run one time on creation and another that runs every frame.

delta time is not really needed as window.requestAnimationFrame(frame); will give you ~60fps. So if you're keeping track of the frame count you can tell how much time has passed. Different objects in your game can then, (based off of a set point in the game and what the frame count was) determine how long its been doing something based off its new frame count.

window.requestAnimationFrame = window.requestAnimationFrame || function(callback){window.setTimeout(callback,16)};
gameEngine = function () {
        this.frameCount=0;
        self=this;

        this.update = function(){
           //loop over your objects and run each objects update function
        }

        this.render = function(){
          //loop over your objects and run each objects render function
        }

        this.frame = function() {
            self.update();
            self.render();
            self.frameCount++;
            window.requestAnimationFrame(frame);
        }
        this.frame();
};

I have created a full game engine located at https://github.com/Patrick-W-McMahon/Jinx-Engine/ if you review the code at https://github.com/Patrick-W-McMahon/Jinx-Engine/blob/master/JinxEngine.js you will see a fully functional game engine built 100% in javascript. It includes event handlers and permits action calls between objects that are passed into the engine using the event call stack. check out some of the examples https://github.com/Patrick-W-McMahon/Jinx-Engine/tree/master/examples where you will see how it works. The engine can run around 100,000 objects all being rendered and executed per frame at a rate of 60fps. This was tested on a core i5. different hardware may vary. mouse and keyboard events are built into the engine. objects passed into the engine just need to listen for the event passed by the engine. Scene management and multi scene support is currently being built in for more complex games. The engine also supports high pixel density screens.

Reviewing my source code should get you on the track for building a more fully functional game engine.

I would also like to point out that you should have requestAnimationFrame() called when you're ready to repaint and not prior (aka at the end of the game loop). One good example why you should not call requestAnimationFrame() at the beginning of the loop is if you're using a canvas buffer. If you call requestAnimationFrame() at the beginning, then begin to draw to the canvas buffer you can end up having it draw half of the new frame with the other half being the old frame. This will happen on every frame depending on the time it takes to finish the buffer in relation to the repaint cycle (60fps). But at the same time you would end up overlapping each frame so the buffer will get more messed up as it loops over its self. This is why you should only call requestAnimationFrame() when the buffer is fully ready to draw to the canvas. by having the requestAnimationFrame() at the end you can have it skip a repaint if the buffer is not ready to draw and so every repaint is drawn as it is expected. The position of requestAnimationFrame() in the game loop has a big difference.

I did not check the logic of the math in your code... however here's what works for me:

GameBox = function()
{
    this.lastFrameTime = Date.now();
    this.currentFrameTime = Date.now();
    this.timeElapsed = 0;
    this.updateInterval = 2000;       //in ms
}

GameBox.prototype.gameLoop = function()
{
   window.requestAnimationFrame(this.gameLoop.bind(this));
   this.lastFrameTime = this.currentFrameTime;
   this.currentFrameTime = Date.now();
   this.timeElapsed +=  this.currentFrameTime - this.lastFrameTime ;
   if(this.timeElapsed >= this.updateInterval)
   {
      this.timeElapsed = 0;
      this.update(); //modify data which is used to render
   }
   this.render();
}

This implementation is independent from the CPU-speed (ticks). Hope you can make use of it!