Loading HTML5 Game Assets

Before diving into HTML5 game development, I built a few conventional web sites (like this one - nice snapshots of the Blue Ridge). With conventional web design, page assets like images are loaded once the page loads, slowly filling out the blank screen with colorful pictures. This works well for web pages, since you can immediately begin reading the content and hardly notice the small delay as the images are painted in.

However, the wait for in-game assets can be especially long when downloading 10MB worth of images and sounds for a game like Entanglement. With Entanglement, the first issue I came across was that I couldn't draw a beautiful wooden tile to the gameboard canvas if the wooden tile image hadn't been downloaded yet.

My first attempt to solve this problem was to throw a JavaScript try/catch block around the drawImage calls so they wouldn't break the game if the image wasn't available yet. This resulted in the game starting immediately, but the gameboard slowly appeared as the images were downloaded. This is fine behavior for a web page but seemed a bit ugly for a web game.

My second attempt was a bit better: I included all the images in the html document as invisible <img> tags. This ensures that the images are indeed loaded before the game began but had the unfortunate side effect that JavaScript processing didn't start until the entire page was downloaded (including all the referenced images). This caused the long wait at a "Loading..." screen that couldn't be dynamically updated with a progress bar since JavaScript wasn't running yet.

It's doing something but we're not sure what nor how long it's going to take.

Enter the Asset Loader! This tool allows larger assets to be loaded after the page loads and the JavaScript processing begins. We're in the prototype stage of our next game, and I took this opportunity to build a proper asset loader. Seth Ladd wrote a wonderful tutorial on the basic steps to set up a simple asset manager. Using his insight and including a bit more code to handle audio assets, we created an asset loader that loads assets as well as calls two functions we can supply to track progress: what to do when a single asset is finished loading and what to do when all the assets are finished loading.

function loadAssets(assetList, onAssetLoaded, onAllAssetsLoaded){...}

The next piece we want is a nice progress bar. The <progress> HTML5 element has two properties we care about for keeping tabs on our loaded assets: max and value. Using these two properties we can create a very simple progress bar class to hand off to the asset loader that might look something like this:

function progressBar(parent){
 this.element = document.createElement('progress');
 parent.appendChild(this.element);
 
 // initialize the progress bar at zero percent
 this.element.setAttribute('value', 0);
 this.element.setAttribute('max', 1000);
 
 // for this example, progress will be some value [0,1]
 this.update = function (progress) {
  this.element.setAttribute('value', progress * 1000);
 };
};

We can then set up our asset loader to update the progress bar each time an asset is finished loading, by feeding the progressBar's update function the current state of loaded assets. In our asset loader, the "ratio" value shown below is simply the number of assets loaded divided by the total number of assets.

var assetProgress = new progressBar(document.getElementById('loading-screen'));
loadAssets(assetList, function(ratio){assetProgress.update(ratio);}, startGame);

We've already implemented the progress bar in the latest builds of both Thwack!! and Entanglement if you care to see it in action!

Thwack!! on html5rocks.com

HTML5Rocks.com has just published a tutorial I put together on resizing games to fit a browser's window size. I fleshed out some of my content from an earlier blog post and put together a few diagrams. Check it out!

We're working on online multiplayer for Thwack!! and are excited about releasing it soon.

Entanglement on html5rocks.com

Curious about how we made the hexagonal tiles for Entanglement? I go into all the technical details in our case study on html5rocks.com:

This past spring (2010) I took interest in the rapidly increasing support for HTML5 and related technologies. At the time, a friend and I had been challenging each other in two-week game development competitions to hone our programming and development skills as well as bring to life the game ideas we were constantly tossing at each other. So, I naturally began incorporating HTML5 elements into my competition entries to gain a better understanding for how they worked and be able to do things that were nearly impossible using earlier HTML specs.

Of the many new features in HTML5, the increasing support for the canvas tag offered me an exciting opportunity to implement interactive art using JavaScript, which led me to try implementing a puzzle game now called Entanglement. I had already created a prototype using the back of Settlers of Catan tiles, so using this as a blueprint of sorts, there are three essential parts to fashioning the hexagonal tile on the HTML5 canvas for web play: drawing the hexagon, drawing the paths, and rotating the tile. The following goes into detail outlining how I accomplished each of these in their current form. 

And then I dive into all the HTML5 goodness. Click through to read the rest of the article if you enjoy programming or seeing how one part of Entanglement is put together.