Michal Budzynski: Tutorial: Simple game with HTML5 Canvas

Friday, September 10, 2010

Tutorial: Simple game with HTML5 Canvas - part 1

Check other language versions: [RUSSIAN]

Tutorial: Simple game with HTML5 Canvas
Part 1 - Introduction & Background
Part 2 - Character & Animation
Part 3 - Physics & Controls
Part 4 - Platforms & Collisions
Part 5 - Scrolling & Game States

If you are interested in news and updates of that tutorial just follow me on twitter: [MichalBe's Twitter]

Because 10KApart is closed now, and we are all waiting for the results, it is good time to remind my very simple "Stairs to heaven" game and explain in details how it was made.

INTRODUCTION

StH is very simple clone of Doodle Jump, but to be honest I was inspired by Icy Tower and discover DJ after I submit StH to the competition. Never mind.
The goal is to control little angel & jump on the two kinds of platforms - orange (regular ones) and green (super high jump springboards). The game ends when the angel falls down to the bottom of the screen. Try it: [Stairs to heaven].
I create that game in about 8hours and later, after playing more and more, I discover few bugs so in this tutorial I want to fix it all. Let's do it!

Part 1. BACKGROUND
Because whole game, including images and scripts, couldn't be over 10K, I didn't want to use image on the background. It was cheaper to draw some generic-like stuff using canvas drawing functions.
First of all we need little HTML, nothing special, just one canvas element with some unique id, little bit of CSS and include of not existing yet game.js:

<html>
  <head>
    <title>Simple game with HTML5 Canvas</title>
  <style>
  body {
    margin:0px;
    padding:0px;
    text-align:center;
  }

  canvas{
    outline:0;
    border:1px solid #000;
    margin-left: auto;
    margin-right: auto;
  }
  </style>
  </head>
  <body>
    <canvas id='c'></canvas>
    <script src="game.js"></script>
  </body>
</html>

That's all in HTML we will need during this tutorial.
Ok, so let's create some Javascript.
First of all we need to create few global (for now, I know that global = evil) variables & change canvas attributes. That will be enough:

var width = 320,
//width of the canvas
  height = 500,
//height of the canvas

  c = document.getElementById('c'), 
//canvas itself 

  ctx = c.getContext('2d');
//and two-dimensional graphic context of the
//canvas, the only one supported by all 
//browsers for now

c.width = width;
c.height = height;
//setting canvas size

First of all its important to understand one thing about canvas - it is not possible to move objects in the canvas surface. It's necessarily to clear it, whole or in the parts, on each frame. To achieve this, let's create clear() function.

var clear = function(){
  ctx.fillStyle = '#d0e7f9';
//set active color to #d0e... (nice blue)
//UPDATE - as 'Ped7g' noticed - using clearRect() in here is useless, we cover whole surface of the canvas with blue rectangle two lines below. I just forget to remove that line
//ctx.clearRect(0, 0, width, height);
//clear whole surface
  ctx.beginPath();
//start drawing
  ctx.rect(0, 0, width, height);
//draw rectangle from point (0, 0) to
//(width, height) covering whole canvas
  ctx.closePath();
//end drawing
  ctx.fill();
//fill rectangle with active
//color selected before
}

One colored clear background is boring as hell, so let's draw some clouds on it. Maybe not regular clouds, but simple, semitransparent circles imitating clouds. First we will draw some in random places of the canvas, each with different size and transparency. We will keep all the informations about circles in 2d array (there are no two-dimensional arrays in JS, best way to solve this is just put one Array into another).

var howManyCircles = 10, circles = [];

for (var i = 0; i < howManyCircles; i++) 
  circles.push([Math.random() * width, Math.random() * height, Math.random() * 100, Math.random() / 2]);
//add information about circles into
//the 'circles' Array. It is x & y positions, 
//radius from 0-100 and transparency 
//from 0-0.5 (0 is invisible, 1 no transparency)

var DrawCircles = function(){
  for (var i = 0; i < howManyCircles; i++) {
    ctx.fillStyle = 'rgba(255, 255, 255, ' + circles[i][3] + ')';
//white color with transparency in rgba
    ctx.beginPath();
    ctx.arc(circles[i][0], circles[i][1], circles[i][2], 0, Math.PI * 2, true);
//arc(x, y, radius, startAngle, endAngle, anticlockwise)
//circle has always PI*2 end angle
    ctx.closePath();
    ctx.fill();
  }
};

Nice, but boring less only a little. Why are the clouds standing still? Lets make a tiny little function with one Number type argument, which moves clouds down given number of pixels, and when particular circle disappears under the canvas, it will moves it on the top with changed position X, radius and transparency:

var MoveCircles = function(deltaY){
  for (var i = 0; i < howManyCircles; i++) {
    if (circles[i][1] - circles[i][2] > height) {
//the circle is under the screen so we change
//informations about it 
      circles[i][0] = Math.random() * width;
      circles[i][2] = Math.random() * 100;
      circles[i][1] = 0 - circles[i][2];
      circles[i][3] = Math.random() / 2;
    } else {
//move circle deltaY pixels down
      circles[i][1] += deltaY;
    }
  }
};

Now, last but not least, let's create main game loop and connect everything we create for now in there. Each frame will clear the screen, move circles 5px lower, draw them and after 1/50sec call another frame. I use two setTimeouts except one setInterval, but I'm not pretty sure why:). I know that there was some performance issues in IE back in the days or something. Also don't forget to add gLoop variable to that declared at the beginning.

var width = 320,  
//width of the canvas  
  height = 500,  
//height of the canvas  
  gLoop,
(...) //rest of the code goes here

var GameLoop = function(){
  clear();
  MoveCircles(5);
  DrawCircles();
  gLoop = setTimeout(GameLoop, 1000 / 50);
}
GameLoop();

According to Luis Giribone's comment below, I avoid Intervals and use Timeouts instead intentionally - Interval is called every 1000/fps seconds - even if the previous one disn't not finished yet. If you use Timeout, it will call another one only after previous was finished. I hope it is clear now. I also want to thanks Ped7g, author of Whiskas & Pedigree Javascript ad game for catching mistakes. Final result of that part should looks like this: [Simple game with HTML5 Canvas part 1], and sources are available on my Github account: [MichalBe] Tutorial: Simple game with HTML5 Canvas Part 1 - Introduction & Background Part 2 - Character & Animation Part 3 - Physics & Controls Part 4 - Platforms & Collisions Part 5 - Scrolling & Game States