Car Game on HTML5

This is continuation to my previous blog post, that discussed how to make simple Slots machine with HTML5. The basic principle in this driving game demo is similar, but in addition to that it adds dynamic graphics using canvas.

Try it out here: http://ikonen.me/examples/drive/

Game road is div with background image that is translated down and back up to create illusion of moving road. This same method is used in Slots machine.

Cars are drawn on canvas on top of the road so while road moves cars appear moving forward. Each click starts moving the player car from current point to the click point using smooth  Bezier curve. Car acceleration is increased or decreased depending on click location. Player car also has smoke plume animation which intensity depends on acceleration.

function _bezier_quad(t, p0, p1, p2) {
     return {
         x: (1 - t)*( (1 -t) * p0.x + t*p1.x) + t * ( (1 - t) * p1.x + t * p2.x),
         y: (1 - t)*( (1 -t) * p0.y + t*p1.y) + t * ( (1 - t) * p1.y + t * p2.y)
     }
 }

After car has travelled long enough, the update loop starts slowing down road speed and moves in sync  div that holds Finish text and flags.

Collision detection is done on each update. In case player hits on other cars, the other cars may blurb random bubbles that are added as normal div elements with rounded borders. Element location is updated based on car location.

In collision both cars are bumped so player can make room by hitting to other cars. Game loop is run by animation frames, so it stops if browser window goes in background. This is not optimal but makes implementation simpler. Better option is to run update with setInterval and draw on animation frames.

...
    var that = this;
    (function gameLoop() {
        that.clear(); // clear canvas objects
        that.update(); // update game state
        that.draw(); // draw game
        requestAnimFrame( gameLoop );
    })();
...

Update and draw in separate loops. This is better solution for complex games because browser will always call setInterval callback but requestAnimFrame callback only if browser window is visible and active. Callback is not executed if browser window is minimized or it’s tab is on background. If you rely only on requestAnimFrame the game is paused on background.

    var that = this;
    setInterval(function() {
        that.update(); // update game state
    }, 1000/30); // 30 frames

    (function gameLoop() {
        that.clear(); // clear canvas objects
        that.draw(); // draw game
        requestAnimFrame( gameLoop );
    })();

Game code is available in GitHub.