By Dominic Szablewski, creator of the Impact Game Engine

This post is part of Who's at Google I/O, a series of guest blog posts written by developers who are appearing in the Developer Sandbox at Google I/O.


Impact is a JavaScript game engine that uses the HTML5 Canvas and Audio elements for graphics and sound, instead of relying on any browser plugins. Impact's main focus is on classic 2D games. Examples include the Biolab Disaster Jump'n'Run game and the Z-Type Space Shooter. These games, like many other 2D games, draw sprites in front of multiple background layers.


Each background layer is drawn from a tileset, an image containing all the individual building blocks, and a tilemap, a 2D array that tells the renderer where to draw each of these tiles. Similarly, sprites are drawn from an animation sheet, an image with all the animation's frames.

This technique has proven so efficient and flexible that it was enforced in hardware on early game consoles: the Super Nintendo could not do anything other than draw tiled background maps and sprites. There was no way to directly access single pixels on the screen.

The HTML5 Canvas element is perfectly equipped for these kinds of games. Most importantly, the Canvas API's drawImage() method allows us to draw only a certain part of a tileset or animation sheet to the screen. In Impact, however, you don't have to deal with any of the Canvas API methods directly. Instead, you specify your tilemaps and animation sheets and let the engine handle the details.

This is how you'd create an animation from an animation sheet:


// Each animation frame is 16x16 pixels
var sheet = new ig.AnimationSheet( 'player.png', 16, 16 );

// This creates the "run" animation: it has 6 frames (the 2nd row
// in the image), with each one being shown for 0.07 seconds
var run = new ig.Animation( sheet, 0.07, [6,7,8,9,10,11] );
Similarly, here's the code needed to create and draw a background layer:
// Create a 2D tilemap
var map = [
[5, 3, 4],
[2, 7, 1],
[6, 0, 3]
];

// Specify a layer with a tilesize of 16px and our tilemap
var layer = new ig.BackgroundMap( 16, map, 'tileset.png' );
layer.draw();
You don't have to create these tilemaps by hand. Impact comes with a powerful level editor called Weltmeister, which makes it easy to draw background layers and position your entities (non-static objects in the game world) in your levels.


When drawing on a Canvas, the performance is mostly bounded by the number of draw calls. It is far more efficient to draw one or two very large images than to draw several hundred small ones. This means that drawing background layers tile by tile can be quite slow, especially on mobile devices.

The Impact engine therefore has a special "pre-render" mode that you can enable on background layers. This mode will first draw the tilemap into large chunks of 512x512 pixels when loading a level, and then use these chunks to fill the screen instead of drawing the layer tile by tile. With this technique, you can get good frame rates even for fast-paced games on Android and iOS devices.

Impact also handles sound, input, timing, and much more for you. It's by no means a game engine that can do everything – and it doesn't try to be one – but it's very good at the things it can do.


Come see Impact in the Developer Sandbox at Google I/O on May 10-11.

Dominic Szablewski is the creator of the Impact Game Engine. He recently finished his Bachelor Thesis about HTML5 Gaming and now lives the dream by selling Impact and making games.

Posted by Scott Knaster, Editor