X-Type – Making Of

For this year's Google IO, Google asked me to do a Chrome experiment for Mobile for them. They initially wanted me to vamp up Biolab Disaster – it's still a good game, but because of it's retro style it wouldn't be that impressive. Modern mobile browsers can do a lot more.

I suggested I would try to take another game of mine – X-Type – and make it work on mobile browsers. The game was made with my JavaScript Game Engine, so it mostly "just worked" on mobile browsers already. Yet, I still had a lot of work to do.

Have a look at X-Type over at chromeexperiments.com

Screen Size

One of the most difficult things for HTML5 games is dealing with the vast amount of different screen sizes and aspect ratios out there. I experimented a lot with different solutions and ended with a fairly simple one: the internal width the game's viewport is always 480px. These 480px get scaled to whatever is available on the device. The height of the viewport is variable, so that it fills the screen with the same scaling factor as the width.

// The internal width for our canvas is fixed at 480px.
// The internal height is set so that it fills the screen when scaled
canvas.width = 480;
canvas.height = window.innerHeight * (canvas.width / window.innerWidth);

// Scale the canvas via CSS to fill the screen 
canvas.style.width = window.innerWidth + 'px';
canvas.style.height = window.innerHeight + 'px';

In older browsers (Mobile and Desktop), scaling the <canvas> element was a horrible idea – it decreased performance to a tenth of what it would be unscaled. I'm happy to report that this is no longer true; Mobile Safari on iOS5 and the Chrome Beta on Android work just fine with scaled canvas. It still makes the game unplayable in Android's "Browser", though.

I also took care to only display the game in portrait mode and show a "Please Rotate the Device" message otherwise. Mobile Safari and Chrome both support the orientationchange event, which makes this easy. However, we can not rely on window.orientation, which reports the rotation in degrees (0, 90, 180 or 270), because some devices report 0° for portrait mode, while others report 0° for landscape. How convenient!

The solution is to just check if the window height is bigger than the width – if so, we're obviously in portrait mode! But as this would be too easy, Chrome's Browser offers another challenge for us: it only updates the window dimensions after it has fired the orientationchange event. So we listen for orientationchange and resize events. Sigh.

var wasPortrait = -1;
var checkOrientation = function() {
    var isPortrait = (window.innerHeight > window.innerWidth);
    if( isPortrait === wasPortrait ) { return; // Nothing to do here }
    wasPortrait = isPortrait;

    // Do your stuff...
};
window.addEventListener( 'orientationchange', checkOrientation, false );
window.addEventListener( 'resize', checkOrientation, false );

Performance

Since iOS 5 the <canvas> element is hardware accelerated and it really shows. You can draw hundreds of sprites on the screen without any slowdowns at all. The same is true for Chrome on Android – to a certain degree.

All drawing is scheduled via requestAnimationFrame and thus bound to the display's refresh rate. This works nicely on iOS, but Chrome refuses to draw at the 60hz even for the simplest scenes. You can use setInterval to process more frames, but only a portion of them is really presented on the screen.

So while Chrome's JavaScript engine is fast enough to process and render 60 FPS, it fails to display all of the rendered frames. I have no doubt that this bug(?) will get fixed.

While the game works in a multitude of other browser's on Android, such as Firefox, Dolphin or Opera, none of them provided good performance. I suspect the <canvas> element is not hardware acclerated in any of these, but didn't investigate further.

Controls

In the desktop version of the game you move the player with the arrow keys and aim and shoot with the mouse. Of course this doesn't work on touch screens, so I opted for dual virtual analog sticks. This worked out surprisingly well – with a bit of practice, you can control your spaceship quite precisely.

I also tried to make the analog sticks appear where you touch the screen first, so that you can always change the position. This made everything quite a bit confusing; it's easier to grasp the concept when the position of the analog sticks is fixed. Providing the dynamic positioning is probably more of "pro gamer" feature that should be optional, if at all.

Sound

This is the sad part. I complained about support for the <audio> element in mobile browsers last year already – and guess what: it's still the same shit. Apple hasn't done anything at all to improve the situation; same goes for Android's Browser. The Chrome Beta on Android seems to have some support for Audio, but it's not really usable for real time games at the moment. I'll investigate this further.

As always, I have high hopes though. Never give up, never surrender!

All in all, I'm very pleased with the results. Rendering performance in modern mobile browsers is really awesome and the quirks I encountered were workaroundable.

I learned a lot with this project and will use this new gained knowledge to make mobile browser support much more easy in the next version of Impact.