Javascript Canvas Animations

Question

I am creating an animation using javascript and canvas. How can I allow the user to specify the order in which to display the frames and change the speed at which they are displayed. This is what I have so far

(function() {

var lastTime = 0;
var vendors = ['ms', 'moz', 'webkit', 'o'];
for(var x = 0; x < vendors.length && !window.requestAnimationFrame; ++x) {
    window.requestAnimationFrame = window[vendors[x]+'RequestAnimationFrame'];
    window.cancelAnimationFrame = window[vendors[x]+'CancelAnimationFrame'] 
                               || window[vendors[x]+'CancelRequestAnimationFrame'];
}

if (!window.requestAnimationFrame)
    window.requestAnimationFrame = function(callback, element) {
        var currTime = new Date().getTime();
        var timeToCall = Math.max(0, 16 - (currTime - lastTime));
        var id = window.setTimeout(function() { callback(currTime + timeToCall); }, 
          timeToCall);
        lastTime = currTime + timeToCall;
        return id;
    };

if (!window.cancelAnimationFrame)
    window.cancelAnimationFrame = function(id) {
        clearTimeout(id);
    };
}());

(function () {

var coin,
    coinImage,
    canvas;                 

function gameLoop () {

  window.requestAnimationFrame(gameLoop);

  coin.update();
  coin.render();
}

function sprite (options) {

    var that = {},
        frameIndex = 0,
        tickCount = 0,
        ticksPerFrame = options.ticksPerFrame || 0,
        numberOfFrames = options.numberOfFrames || 1;

    that.context = options.context;
    that.width = options.width;
    that.height = options.height;
    that.image = options.image;

    that.update = function () {

        tickCount += 1;

        if (tickCount > ticksPerFrame) {

            tickCount = 0;

            // If the current frame index is in range
            if (frameIndex < numberOfFrames - 1) {  
                // Go to the next frame
                frameIndex += 1;
            } else {
                frameIndex = 0;
            }
        }
    };

    that.render = function () {

      // Clear the canvas
      that.context.clearRect(0, 0, that.width, that.height);

      // Draw the animation
      that.context.drawImage(
        that.image,
        frameIndex * that.width / numberOfFrames,
        0,
        that.width / numberOfFrames,
        that.height,
        0,
        0,
        that.width / numberOfFrames,
        that.height);
    };

    return that;
}

// Get canvas
canvas = document.getElementById("coinAnimation");
canvas.width = 500;
canvas.height = 500;


// Create sprite sheet
coinImage = new Image();    

// Create sprite
coin = sprite({
    context: canvas.getContext("2d"),
    width: 500,
    height:72,
    image: coinImage,
    numberOfFrames: 10,
    ticksPerFrame: 4
});

// Load sprite sheet
coinImage.addEventListener("load", gameLoop);
coinImage.src = "sprite.png";

 } ());

Answer 1

Rq : You might remove the code defining requestAnimationFrame for Browsers not supporting it since 1) setTimeout is too inaccurate and 2) a Browser supporting Canvas will moooost likely implement it, so the polyfill is enough.

To change your coin animation, you need to have a sprite update that takes a time parameter (dt, the time elapsed since last frame). So compute the time in your update loop : have it compute game time and current frame's dt (what you did for rAF can be reused here).

Then you need some way of storing the frames/frames duration data within your sprite.

You can use an array of { duration : , frame : } objects.
I personally like an array like this one : [ duration1, frameIndex1, duration2, frameIndex2, ...] (faster, and no reference issue if you copy it with slice(0). ).

Then during the update of the coin, you'll have to handle a current time position instead of a tick count, and 'eat-up' as many duration as you can with the dt you have at hand to find the next frame.

Now to come to your very question, to edit the animation, i would use a separate canvas, below your display canvas, and a bunch of buttons to changes things. i did a very simple jsbin, it looks like this :

the jsbin is here : http://jsbin.com/IjAtOxe/1/edit?js,output