Introduction to html5 game programming with ImpactJs

Luca GalliEmail: luca.galli@polimi.it
An Introduction to HTML5 Game Programming with ImpactJS

Mobile Applications Development –HTML5 Games
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Why choosing HTML5 for Web Games?

Why choosing HTML5 for Web Games?
●No need to install external plugins (e.g. Unity)
●Crossplatform Development & Deployment with no additional costs (Licencing for Unity, UDK and similars is a nightmare)
●Optional Installation: the distribution of HTML5 games depends solely upon the developer; live updates are possible if online distributed, cross deployment as hybrid applications
●...but still a futuristic technology, even if we are getting there

What if I want to start from scratch?
●Starting from scratch is certainly possible but not a good idea if you want to start developing a game quickly:
●Games require a lot of images, sound effects, libraries and other resources that are loaded asynchronously; the Javascript code will start executing before all the resources have been loaded. This results in popping images and sound effect not played when they should. Solution: create a preloaderthat defers script execution until all the resources have been downloaded
●Different machines and browsers will run the game at different speeds. Animations and movement speeds should be independent of the frame rate the game is running at
●You have to handle different resolutions targeting different devices (e.g. Desktop, Mobile...)
●Lesson: There is a lot of boilerplate code that every game needs to work properly. No need to reinvent the wheel: Use a Framework!
●Further details on: http://msdn.microsoft.com/en-us/hh563503

What is ImpactJs?
●JavaScript game framework.
●Impact takes advantage of the modern browser’s Canvas element in order to create high-performance 2D games on the Web and even mobile.
●Easy to pick up good code examples, active community, robust level editor
●Why choosing it if it’s commercial?
●Once you buy the license, the engine source code it’s yours to be modified at will.
●You can dive into the code and actually read it, without jumping back and forth between dozens of layers of abstractions. Other game engines are more convoluted and don't make it as clear what a particular piece of code is doing.
●Accessible and consistent API that does not make use of cumbersome and obscure abstractions.
●Performance monitoring tools integrated within the engine
●For these reasons, Impactis a good fit if you want to LEARN about game engines or even Javascript programming in general.

Let’s start...
●Must have:
●PHP
–The level editor has a php dependence for saving levels and baking the game, and it’s a core tool.
●Apache
–Necessary for hosting the game locally and testing it out while developing
●IDEs
–Impact has no IDE dependencies; you can create your games with any simple text editor.
–Hint: As Politecnico’s Students you have free access to the Jetbrains suite, use it!
●Browsers
–Any modern browser with canvas and audio tag support will do but Webkit based ones such as Chrome will offer better performances.

Let’s start...
●Handy Tools:
●IntelXdk: Development Framework for HTML5 Applications with devices emulation
●Shoebox: to handle spritesheets generation
●Spartan: Pixel Art animation software and procedurally generated textures
●Spriter: an editor for cutout animations
●Cxfr: Tool for dummies to create sound effects

Apache/Php/MySql
●There are a number of possible choices to have such a stack, I prefer a quick and easy environment just to start as soon as possible:

Download and deploy the engine
●Extract the zip file containing the engine and the editor, take the «Impact» folder, rename it as you wish (the name of your game could be a good idea) and place it in the root location of your server, for XAMPP:
"your installation location"xampphtdocs

Directory Structure
●media/Contains Images, Sounds, Musics
●lib/Hosts the Javascript Files
●lib/game/Hosts the code of the game
●lib/game/entities/Hosts the code for the entities
●lib/game/levels/Hosts the levels of the game
●lib/impact/Hosts the Game Engine Libraries
●lib/weltmeister/Hosts the Level Editor Libraries

Test the environment:
●http://localhost/impact

The Index File

The main.js file

Impact Modules
1 ig.module(
2 'game.main'
3 )
4 .requires(
5 'impact.game',
6 'impact.image',
7 'game.other-file'
8 )
9 .defines(function(){
10 // code for this module
11 });
Natively, javascript has not a way to load other javascript files within an object, for this reason impact has implemented its own include system, similar in a way to other Javascript libraries to load modules and dependeces (e.g. Require.js). Modules and their dependencies typically reside in the lib/ folder of your Impact project directory, and subdirectories are included in a path to these files using object-model dot syntax.
Defines the module name that corresponds to the file name. Therefore, the main.js file sits in the lib/game/main.js
Any additional files that have to be loaded at runtime, before the module’s body and define function. Are essentially the libraries that we need to use when creating a game
The last step the module takes is to execute the function passed to the .defines() method.

Classes in Impact
●Javascript does not have a real notion of a traditional class structure like the one you could have in other OOP languages, since it is a prototypal language; everything is an Object.
●This is the blessing and curse of Javascript, making it incredibly flexible but also extremely difficult to structure the code in a reusable way.
●Impact classes are derived from John Resig’s Javascript Inheritance code, extended with deep copying of properties and static instantations
●For further information regarding OOP in Javascript, refer to:
●https://developer.mozilla.org/en-US/docs/Web/JavaScript/A_re-introduction_to_JavaScript
●https://developer.mozilla.org/en-US/docs/Web/JavaScript/Introduction_to_Object- Oriented_JavaScript
●http://www.crockford.com/javascript/private.html
●http://phrogz.net/js/classes/OOPinJS.html

Classes in Impact
// Create a new class "Person"
varPerson = ig.Class.extend({
name: '',
init: function( name ) {
this.name = name;
}
});
// Instantiate an object of the first class
vare = new Person('John Doe');
e.name; // => John Doe
// Extend the "Person" class
varZombie = Person.extend({
init: function( name ) {
this.parent( 'Zombie: ' + name );
}
});
// Instantiate a Zombievarp = new Zombie('John Doe');
p.name; // => Zombie: John Doe

Core Classes: The Impact Engine Source Code

The Game Loop
●Agame loopruns continuously during gameplay. Each turn of the loop, it processes user inputwithout blocking,updates the game state, andrenders the game. It tracks the passage of time tocontrol the rate of gameplay.
●Actually more considerations could be done (like fixed or variable time steps) , but we will not dig through all the details…
http://gameprogrammingpatterns.com/game-loop.html

Impact.js -> ig namespace
●the igobject provides the Module definition and loading capabilities as well as some utility functions.
●igis not an instance of a ig.Class, but just a simple JavaScript object and thus can't be subclassed.
●Impact extends some of JavaScriptsnative objects, such as Number, Array and Function.

Animation
●An ig.Animationobject takes care of animating an Entity or Background Map tile. Frames from an animation sheet –an image with all animation frames –are drawn as specified by the animations frameTimeand sequence.

Background Map
●An ig.BackgroundMapdraws tiles from a Tileset, as indicated by its 2D data Array.
●The size of the tilesetimage must a multiple of the tilesize, otherwise Impact will get confused with the tile numbering. E.g. with a tilesizeof 32 the width and height of your tilesetimage should be one of: 32, 64, 96, 128…
•You can either create a BackgroundMapby hand, or use the ig.Gameclass' .loadLevel() method, which takes the save data from the level editor and creates BackgroundMapsaccording to it.

Collision Map
●An ig.Collisiontakes a 2D tilemapand allows tracing against it for collisions.
●The ig.Gameclass' .loadLevel() method takes the save data from Weltmeisterand creates a CollisonMapif present.
●By default, all entities trace against the ig.game.collisionMapas part of their update cycle.

Entity
●Interactive objects in the game world are typically subclassedfrom this base entity class. It provides animation, drawing and basic physics. Subclassingyour entities from ig.Entityensures that it can be added to the game world, react to the collision map and other entities and that it can be used in the editor.

Game
●ig.Gameis the main hub for your game. It hosts all currently active entities, background maps and a collision map. You can subclass your own Game Class from ig.Game.
●Its .update() and .draw() methods take care of collision detection, checking entities against each other and drawing everything to the screen.

Input
●ig.Inputhandles all Keyboard and Mouse input.
●You can bind keys to specific actions and then ask if one of these actions is currently held down (.state()) or was just pressed down after the last frame (.pressed()).
●Note that an instance of ig.Inputis automatically created at ig.input(lowercase) by the ig.main() function.
// On game start
ig.input.bind( ig.KEY.UP_ARROW, 'jump' );
// In your game's or entity's update() method
if( ig.input.pressed('jump') ) {
this.vel.y = -100;
}

System
●ig.Systemtakes care of starting and stopping the run loop and calls the .run() method on the current Game object. It also does the housekeeping for ig.Inputand provides some utility methods.
●An instance of ig.Systemis automatically created at ig.system(lowercase) by the ig.main() function.

And more...
Several other classes are present in the Engine:
●Loader: takes care of preloading the resources and showing a loading bar
●Music, Sound, Soundmanagerto handle sounds within the game
●Timerto handle timers and time intervals
●Fontto draw text based on bitmap fonts into the canvas

•The Sequence Diagram describes the necessary interactions between the basic classes to start a game loop
•Classes for handling sounds have been omitted for simplicity
ImpactJs Game Engine Flow: Sequence Diagram

Impact Game Engine Flow Summarized –Pt.1
●As soon as you start your game, Impact will set up an interval that calls the ig.system.run() method 60 times per second. This method does some housekeeping stuff and then calls your game's .run() method -which, by default, just calls .update() and .draw() on itself.
●The ig.Game's.draw() method is pretty boring: it just clears the screen and calls .draw() on each background layer and entity.
●The .update() method updates the background layer positions and -this is where it gets interesting -calls .update() on each entity. The default .update() method of an entity moves it according to it's physics properties (position, velocity, bounciness...) and takes the game's collision map into account.

Impact Game Engine Flow Summarized –Pt.2
●After all entities have been updated, the game's .checkEntities() method is called. This resolves all dynamic collisions -that is, Entity vs. Entity collisions. It also calls an entities .check() method, if it overlaps with another entity and "wants" checks (see the Class Reference for more details).
●You can overwrite any of these methods in your own ig.Entityand ig.Gamesub-classes -you can provide your own logic and then, if you want, call the original methods with this.parent().
●Remember, this all is happening for each and every frame. That is (if the browser can keep up) 60 times per second.

Test the editor
●http://localhost/test/weltmeister.html

Coding Magic!
Metroidvania Like Game

•Endless Swarm Game
•Kill the enemies by typing their name
•Exclusive Demofrom the Developer

Impact++
●Free Boilerplate for ImpactJs
●Dynamic Lights, AI, Pathfindingand optimizations for ImpactJs

Questions?

Mobile Applications Development – HTML5 Games
References, Bibliography, Useful Resources
http://impactjs.com/documentation
http://html5devstarter.enclavegames.com/
http://www.html5gamedevelopment.com/
http://gamemechanicexplorer.com/
https://hacks.mozilla.org/2013/09/getting-started-
with-html5-game-development/
http://www.pixelprospector.com/
http://html5gameengine.com/
http://www.jsbreakouts.org/
Free HTML5 Game Development Course from
Google’s Gurus:
https://www.udacity.com/course/cs255
Note: For the students of the course,
agreement for 10 ImpactJs licenses for
300$, 30$ per Licence! Email me for details

Introduction to html5 game programming with ImpactJs

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