We Keep Coding

How to make a precise sleep function in Javascript, possibly using promises?

I am trying to make a sleep function in Javascript.
The function drawLinesToHtmlCanvas() is meant to draw random lines to a HTML canvas and the user is meant to be able to see the lines being drawn in real time.
For this example I use a delay of 500ms, but would like to be able to go to 1ms (or even less than 1ms resolution in the future)
Originally I followed a answer from this post: What is the JavaScript version of sleep()?
function sleep(ms) {
return new Promise(resolve => setTimeout(resolve, ms));
}
async function drawLinesToHtmlCanvas() {
// Get canvas and context here...
var drawSpeed = 500; // ms.
for (i=0; i<lines; i++) {
// Draw lines to canvas...
await sleep(drawSpeed);
}
}
And that worked very well (above). It was efficient, did not slow the browser down at all and let me have some control over the timing.
The issue was that setTimeout() cannot seem to go down to 1ms precision and this is something I require for this function.
So instead I tried my own approach as follows:
function sleep(ms) {
ms = parseInt(ms);
var now = new Date();
nowMs = now.valueOf();
var endMs = nowMs + ms;
while (endMs > nowMs) {
nowMs = new Date().valueOf();
}
return true;
}
function drawLinesToHtmlCanvas() {
// Get canvas and context here...
var drawSpeed = 500; // ms.
for (i=0; i<lines; i++) {
// Draw lines to canvas...
while (!sleep(drawSpeed));
}
}
This one is very slow, the while loop waiting for the right time uses up all the browsers resources, it's completely unusable. In addition as the function drawLinesToHtmlCanvas() is running, the lines are not being updated to the canvas element.
The promise solution with the setTimeout() was fantastic, it is just not precise enough for my requirements.
Can I make a promise that works similar to the first example? But instead of using setTimeout() it uses a similar algorithm to my Date() now vs end ms comparison, as that would be much more accurate?
The lines need to be able to be drawn down to 1 ms for now and have real time updates, the user needs to be able to see the lines being drawn to the canvas.

Even if setTimeout did work on such extremely small time frames this would probably not have worked out. When you use callbacks and/or promises you rely of JS runtime's event loop. This event loop only executes your callback as fast as it can. The architecture is going to impose lags that will become visible when you go below 1ms. The callback in setTimeout is not exactly executed after N ms passes. After N ms passes it only becomes eligible to be executed. And it gets invoked finally only when its turn comes on another event loop tick.
As for your second approach it does not exactly "use up resources". The thing is you no longer use event loop. But you must remember that JS is single-threaded. And because of it when JS-code executes non-stop it will not let user interact with UI at all. User can do something only between event callback executions. So don't ever use long running whiles in JS in browser unless you want to ruin user experience. Maybe unless you use Web workers because they will let you create new threads, but then you wouldn't be able to draw anything from there.
In general your approach to animation as "drawing something and then sleeping" is rather naive. Performant and smooth animations are what the video cards are made for although writing it in browser to efficiently utilize video card may be tricky. If you want to make animation in browser then you have to find specific browser function calls made specifically for animation on a Canvas or WebGL.
Maybe start here:
https://developer.mozilla.org/en-US/docs/Web/API/Canvas_API/Tutorial/Basic_animations
Also think about if you actually need so may frames per second. Over 1000fps? Can the monitor make it? What about performance impact?

Make an image appear and then grow in size in a html canvas

I was wondering if its possible to create an image using various lines and bez-curves that appears and then moves down the canvas and gets larger as it grows. The animation in question requires the room to fill with gas and so the the gas gets greater and greater as it moves around the canvas.
I had thought about just drawing the image and then using a for loop to move the image down until a certain y coordinate, but this doesnt help with the increasing part of the created image
Any help appreciated

There are different ways to make animations happen. What I normally do for my canvas games is that I have a gameloop that loops at a certain FPS that I decide at the start of my project. Each animation is normally controlled by time an speed.
var fps = 60;
var lastUpdateTime = +new Date(); //when did I last update the game?
function gameloop() {
var updateStartTime = +new Date();
update(updateStartTime-lastUpdateTime); //update the game for the according to the elapsed time since last update
lastUpdateTime = updateStartTime;
//Normally I also handle spawning stuff here
//I also remove old object in my gameloop
setTimeout(gameloop,1000/fps)
}
function update(elapsedTime) {
//This function update the locations of game elements such as player position, bullets, enemies, bananas etc. (or even gas clouds!)
//When I change a value, I use the time-parameter passed along with the call so that I get a smooth game even though the browser might lag.
playerX += velocity*timeElapsed; //as an example
}
Here's an example on your problem where I made a function to create a gas bubble object (yes, I do know how wrong it is to use gas bubbles =P). These objects travel downwards (physics?!) and increase in size (OK, this is starting to sound more and more crazy), just have a look:
http://jsfiddle.net/Niddro/ppa4xuw8/

HTML5 Canvas game loop delta time calculations

I'm new to game development. Currently I'm doing a game for js13kgames contest, so the game should be small and that's why I don't use any of modern popular frameworks.
While developing my infinite game loop I found several articles and pieces of advice to implement it. Right now it looks like this:
self.gameLoop = function () {
self.dt = 0;
var now;
var lastTime = timestamp();
var fpsmeter = new FPSMeter({decimals: 0, graph: true, theme: 'dark', left: '5px'});
function frame () {
fpsmeter.tickStart();
now = window.performance.now();
// first variant - delta is increasing..
self.dt = self.dt + Math.min(1, (now-lastTime)/1000);
// second variant - delta is stable..
self.dt = (now - lastTime)/16;
self.dt = (self.dt > 10) ? 10 : self.dt;
self.clearRect();
self.createWeapons();
self.createTargets();
self.update('weapons');
self.render('weapons');
self.update('targets');
self.render('targets');
self.ticks++;
lastTime = now;
fpsmeter.tick();
requestAnimationFrame(frame);
}
requestAnimationFrame(frame);
};
So the problem is in self.dt I've eventually found out that first variant is not suitable for my game because it increases forever and the speed of weapons is increasing with it as well (e.g. this.position.x += (Math.cos(this.angle) * this.speed) * self.dt;..
Second variant looks more suitable, but does it correspond to this kind of loop (http://codeincomplete.com/posts/2013/12/4/javascript_game_foundations_the_game_loop/)?

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 :)

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;
}
}

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/

I did not check the logic of the math in your code .. however here 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 idenpendant from the CPU-speed(ticks). Hope you can make use of it!

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.

Canvas element not updating

I am trying to visualise a reinforcement agent moving through a 2d grid. I coded up a visualisation using canvas, and everytime my agent makes a move I try to update the grid. I was hoping to see an animation, but instead I see nothing until the agent has completed all this moves and I see the final state. If I step through with Google Chromes Developer tools then I can see the individual steps. I do not think it is a problem of my code just running to fast, because each step takes a couple of seconds.
My implementation is as follows, with the function gridWorld() called once to create a new object and executeAction called every time I want to draw. As shown I have used ctx.save(), and ctx.restore(), but that is only an attempt to solve this problem, and it seems to have made no difference.
Thanks
var execute gridWorld = function(action) {
var canvas = document.getElementById("grid");
this.ctx = canvas.getContext("2d");
this.executeAction = function(action) {
this.ctx.save()
// ... Do reinforcement learning stuff
// For every cell in grid, do:
this.ctx.fillStyle = "rgb(244,0,0)"
this.ctx.fillRect(positionX, poisitonY, 10,10)
this.ctx.restore();
}
}

Even if the code takes a long time to execute, the browser will not update the display until there is an actual break in the code. Use setTimeout() to cause a break in code execution whenever you want the canvas to update.

Your not going to see animations because they are happening way too fast. You need to break them up like in the following example.
Live Demo
If I did something like this for example
for(x = 0; x < 256; x++){
player.x = x;
ctx.fillStyle = "#000";
ctx.fillRect(0,0,256,256);
ctx.fillStyle = "#fff";
ctx.fillRect(player.x,player.y,4,4);
}
You would only ever see the player at the end of the board every time that function is called, and you wouldn't see any of the animations in between, because the loop runs too fast. Thats why in my live demo I do it in small increments and call the draw every 15 milliseconds so you have a chance to actually see whats being put on the canvas.

Moving elements with javascript

What are the best practices for moving elements with javascript?
Do you use timeouts or intervals?
Is it bad to have timed events for 10 milliseconds, or will it be precise?
Do you move pixel by pixel, or a certain fraction of the total distance?
If you use intervals, how do you stop the interval when the element is in position?
The last two times I've seen motion in javascript have been with jQuery and Raphael.js, neither of which I can understand the source code of. Are there some good tutorials or code examples anywhere? Is there a simple explanation of the methods jQuery uses?

There is a recent function called requestAnimationFrame which runs a function as soon as possible. This is a good practice since it has been made for animation purposes.
The way it works in terms of coding is the same as setTimeout, e.g. when the function finishes you call requestAnimationFrame.
In the function, you fetch the current time to see how the object should be positioned at that time.
You can cancel a pending function it with cancelRequestAnimationFrame, passing the return value of requestAnimationFrame.
Currently this is not cross-browser and the functions are vendor-prefixed, e.g. webkitRequestAnimationFrame for Chrome.
E.g.: http://jsfiddle.net/pimvdb/G2ThU/1/.
var div = document.getElementById('div');
var animation;
function move() {
var time = Math.round((new Date()).getTime() / 10) % 200;
div.style.left = time + 'px';
animation = requestAnimationFrame(move);
}
document.getElementById("start").onclick = function() {
animation = requestAnimationFrame(move);
}
document.getElementById("stop").onclick = function() {
cancelRequestAnimationFrame(animation);
}

Here you can find a good Javascript Animation tutorial:
http://www.schillmania.com/content/projects/javascript-animation-1
But what you said is right. Jquery Animate uses setTimeout, moving the object based in calculations of duration, position and easing.

Intervals are preferable, I believe, because you only set it once in the code rather than once per frame. It only needs to read the code once and reuse it several times, rather than reading it every time it is created.
10ms is a bit short. The computer can natively support intervals of about 16ms, then more precise timers can be used for faster intervals, however these are very power-consuming. IE9 supports both, depending on the computer's power settings, but ideally you shouldn't need anything faster than 50ms (20 FPS).
I like to move a fraction of the total distance, based on the time that has passed since the animation started. This way, no matter what the speed of the computer and browser, the animation will always take the exact same amount of time. Guaranteed.
Something like:
interval = setInterval(function() {
// do stuff
if( /*animation ended*/) clearInterval(interval);
},time);
jQuery is easy for some, but personally I find nothing beats writing it yourself in plain, old JS. Much easier to understand what's going on exactly, rather than relying on some framework to get it right for you.