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This chapter is from the book

This chapter is from the book

1.5 Snail Bait’s Humble Beginning

Figure 1.16 shows Snail Bait’s initial set of files. Throughout this book we add many more files, but for now all we need is an HTML file to define the structure of the game’s HTML elements, a CSS file to define the visual properties for those elements, a JavaScript file for the game’s logic, and two images, one for the background and another for the runner.

Figure 1.16

Figure 1.16 Snail Bait’s initial files

Figure 1.17 shows the starting point for the game, which simply draws the background and the runner. To start, the runner is not a sprite; instead, the game draws her directly.

Figure 1.17

Figure 1.17 Drawing the background and runner

Example 1.3 lists the starting point for the game’s HTML, which is just a distilled version of the HTML in Example 1.2.

Example 1.3 The starting point for Snail Bait’s HTML

<!DOCTYPE html>
<html>
   <head>
      <title>Snail Bait</title>
      <link rel='stylesheet' href='snailbait.css'/>
   </head>

   <body>
      <div id='snailbait-arena'>
         <canvas id='snailbait-game-canvas' width='800' height='400'>
            Your browser does not support HTML5 Canvas.
         </canvas>
      </div>

     <!-- JavaScript................................................-->

      <script src='snailbait.js'></script>
   </body>
</html>

Initially, the arena contains only the game’s canvas, which is 800 pixels wide by 400 pixels high and has a thin blue border. Example 1.4 shows the starting point for Snail Bait’s CSS.

Example 1.4 The starting point for Snail Bait’s CSS

body {
   background: cornflowerblue;
}

#snailbait-arena {
   margin: 0 auto;
   margin-top: 50px;
   width: 800px;
   height: 400px;
}

#snailbait-game-canvas {
   border: 1.5px solid blue;
}

Example 1.5 shows the starting point for Snail Bait’s JavaScript.

Example 1.5 The starting point for Snail Bait’s JavaScript

var canvas = document.getElementById('snailbait-game-canvas'),
   context = canvas.getContext('2d'),

   background  = new Image(),
   runnerImage = new Image();

function initializeImages() {
   background.src = 'images/background.png';
   runnerImage.src = 'images/runner.png';

   background.onload = function (e) {
      startGame();
   };
}

function startGame() {
   draw();
}

function draw() {
   drawBackground();
   drawRunner();
}

function drawBackground() {
   context.drawImage(background, 0, 0);
}

function drawRunner() {
   context.drawImage(runnerImage, 50, 280);
}

// Launch game.........................................................

initializeImages();

The preceding JavaScript accesses the canvas element and subsequently obtains a reference to the canvas’s 2D context. The code then draws the background and runner by using the three-argument variant of drawImage() to draw images at a particular location in the canvas.

The game starts when the background image loads. For now, starting the game entails simply drawing the background and the runner.

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