· Hacking · 9 min read

Codelab - HTML5 Canvas and Game development

· Google Devfest 2015 HTML5 Canvas Game Codelab

Objective

This codelab will demonstrate how to build a game using HTML5 canvas. As an example, we will build a simple snake game.

We will walk you through the process of building the in a series of incremental steps. After each step you should have a working (not necessarily useful) version of the game. Each step is described in the form of a tag in the git repository.

Requirements

The requirements for this codelab are:

Why HTML5?

The most obvious advantage for creating a HTML5 game is that it is cross platform. The same codebase can be packaged to run on browsers, Andorid, iOS, Windows Phone or any other modern device.

Another advantage the pervasiveness of javascript. It is everywhere and very easy to get started with. Most people are already familiar with Javascript and that all that is needed to develop a HTML5 game. The development process is fairly easy. No waiting for compilation, updates and debugging in real-time, and once the game is done, you can push out the update immediately.

Step 0: Getting the Code

You can get the code from https://github.com/shivanshuag/codelab-snake Clone the repository using git clone https://github.com/shivanshuag/codelab-snake.

You can check out each tag using git checkout [tagname]

To run the code, simply open the index.html file in the browser of your choice.

Preliminaries

Step 1: The Canvas Tag

git checkout step1

The canvas tag is similar to the <div>, <a>, or <table> tag, with the exception that its contents are rendered with JavaScript. The width and height attributes are used to specify the dimensions of the canvas.

document.getElementById('myCanvas') gives the reference to the canvas DOM object in the html document.

canvas.getContext('2d') gives the canvas context which is an object with properties and methods that you can use to render graphics inside the canvas element.  The context can be 2d or webgl (3d).

Step 2: Drawing A Line

git checkout step2

All the properties of the line should be set before calling the stroke function.

Step 3: Arc

git checkout step3

Bonus Points

Step 4: Rectangle

git checkout step4

When setting both the fill and stroke for a shape, make sure that you use fill() before stroke().Otherwise, the fill will overlap half of the stroke.

Step 5: Images

git checkout step5

Bonus Point

Step 6: Text

git checkout step6

Snake Game

Now we will create a game using the tools we just learnt.

Step 7: Creating Arena for the Snake

git checkout step7

This will create a 450x450 canvas with a balck boundary.

Step 8: Creating the Snake

git checkout step8

function create_snake() { var length = 5; //Length of the snake snake_array = []; //Empty array to start with for(var i = length-1; i>=0; i--) { //This will create a horizontal snake starting from the top left snake_array.push({x: i, y:0}); } } create_snake(); // calling the create_snake function

Here, each element in the array represents a cell of the snake. Initially, the snake is made up of 5 cells. In a later step, we will write logic to increase the length of the snake when it eats food.

Step 9: Painting the Snake

git checkout step9

var cw = 10;//cell width function paint() { for(var i = 0; i < snake_array.length; i++) { var c = snake_array[i]; context.fillStyle  = "blue"; //Lets paint 10px wide cells context.fillRect(c.x*cw, c.y*cw, cw, cw); context.strokeStyle  = "white"; context.strokeRect(c.x*cw, c.y*cw, cw, cw); } } paint();

In the earlier step, we just created a array containing the coordinates of all the cells of the snake. In this step, we are painting all the cells on the canvas.

context.fillRect fills a rectangle and context.strokeRect creates its boundary.

Step 10: Moving the snake

git checkout step10

In this step, we will create the Game Loop. Game Loop contains the statements which will be called in every frame of the game. These instructions will be responsible for updating the state of the game (like the position of the snake, handling input).

game_loop = setInterval(paint, 60); //Put the following lines inside the paint function //Pop out the tail cell and place it infront of the head cell var nx = snake_array[0].x; var ny = snake_array[0].y; //These were the position of the head cell. //We will increment it to get the new head position nx++; var tail = snake_array.pop(); //pops out the last cell tail.x = nx; tail.y = ny; snake_array.unshift(tail); //puts back the tail as the first cell

Game loop is a javascript setInterval timer. It calls the paint function every 60 milliseconds.

We modify the paint function to include logic for moving the snake forward in the right direction by popping out its tail and pushing it ahead of the head in the snake_array.

After every 60 ms, paint() function will create new rectangle but will not delete the previous rectangles. This will leave a trail of the movement of the snake. So, we will first paint the background of the canvas white inside the paint() function.

// Put the following lines at the start of the paint function context.fillStyle= "white"; context.fillRect(0 , 0, w, h); context.strokeStyle = "white"; context.strokeRect(0, 0, w, h);

Step 11: Handling User Input

git checkout step11

First, we will add direction based movement to the snake. Instead of the line nx++ in the code, add the following snippet.

var d = "right"; //default direction. Add this line outside the paint function //add the following lines in the paint function replacing 'nx++' if(d == "right") nx++; else if(d == "left") nx--; else if(d == "up") ny--; else if(d == "down") ny++;

Add a key listener and change the direction of the snake accordingly.

document.onkeydown = function(e) { var key = e.which; if(key == "37" && d != "right") d = "left"; else if(key == "38" && d != "down") d= "up"; else if(key == "39" && d != "left") d= "right"; else if(key == "40" && d != "up") d= "down"; };

onkeydown function is called when the user presses a key and e.which contains the keycode of the key which the user has pressed. Refer to https://developer.mozilla.org/en-US/docs/Web/API/KeyboardEvent/keyCode for a list of the keycodes.

Step 12: Creating Food

git checkout step12

First, we will make a function to paint cells. This function can be used while painting the cells of the snake too.

function paint_cell(x,y) { context.fillStyle = "blue"; context.fillRect(*cw, *cw, cw, cw); context.strokeStyle = "white"; context.strokeRect(*cw, *cw, cw, cw); }

Then, we create a function which gives the coordinates of a random cell which is to be used as food.

var food; // global variable which contains food function create_food() { food = { x: Math.floor(Math.random()*(w-cw)/cw), y: Math.floor(Math.random()*(h-cw)/cw), }; //This will create a cell with x and y between 0-44 //Because there are 45(450/10) positions accross the rows and columns } create_food(); //Inside the paint function paint_cell(food.x, food.y);

Math.random() gives a random value between 0 and 1.

Step 13: Eating the Food

git checkout step13

//Inside the paint function //If the new head position matches with that of the food, //Create a new head instead of moving the tail if(nx == food.x && ny == food.y) { var tail = {x: nx, y: ny}; //Create new food create_food(); } else { var tail = snake_array.pop(); //pops out the last cell tail.x = nx; tail.y = ny; }

Step 14: Game Over Condition

git checkout step14

First create an init function which will restore the game state to the initial state. Move the code used to set the initial state of the game to the init function and call it when the page loads.

var game_loop; // Make gameloop a global variable similar to d, food and snake_array. function init() { d = "right"; //default direction create_snake(); create_food(); game_loop = setInterval(paint, 60); } init(); // call init when the script loads.

Add the following condition inside the paint function to detect collision with the boundaries.

if(nx==-1 || nx==w/cw || ny==-1 || ny==h/cw) { // clear the game loop clearInterval(game_loop); init(); return; }

clearInterval will clear the previous game_loop and the init function will create a new one.

Step 15: Checking Body Collision

git checkout step15

Create the check_collision function as follows.

function check_collision(x, y, array) { //This function will check if the provided x/y coordinates exist //in an array of cells or not, excluding the head for(var i = 1; i < array.length; i++) { if(array[i].x == x && array[i].y == y) return true; } return false; }

Modify the collision condition as follows

if(nx==-1 || nx==w/cw || ny==-1 || ny==h/cw || check_collision(nx, ny, snake_array)) { // clear the game loop clearInterval(game_loop); init(); return; }

Step 16: Implement Scoring

This is a DIY step!

Where to go From Here?

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