What Are We Building?
Here’s what the game looked like in the original XNA version:
We won’t go through adding all of these features, but we’ll get close!
Adding the Assets
- Download and extract the “Catapult Wars Lab” sample (CatapultWars_4_0.zip)
- From a Windows Explorer view of the folder, select and drag all four folders from the /Assets/Media/Textures folder and in Visual Studio, place them under the images folder. (This will copy and add them to the project.)
- Create a new folder called sounds in the root of the project.
- Copy the files from /Assets/Media/Sounds to the new sounds folder.
Your project should look like this:
Now that we have some assets, let’s put them to use.
The Splash Screen & Logos
Notice that when you run the game, you first see an “X” in a square? That’s the splash screen, by default showing the /images/splashscreen.png image, but we can do better. To adjust the splash screen, double-click the package.appxmanifest:
The file /images/Backgrounds/gameplay_screen.png is what we want to use, but the image must be 620×300 pixels. So, open the image in your favorite editor, resize and save as title_screen_620x300.png. Add that new file into the project.
Now we can set the Splash screen field to images\Backgrounds\title_screen_620x300.png. While we’re here, pick whatever background color you’d like to complement the image (e.g. darkGray). Now when run, the game greets us with a new splash screen:
We can also adjust the app’s tile, which by default looks like this:
Also in the app manifest, we see a number of places for logos. We can add 150×150, 310×150, and 30×30 logos for use in various places.
Now we have square and wide format custom tiles:
Looks good! Now if only we had a game to play…
Adding the HTML5 Canvas
First, we’re going to need something to display the game. The HTML5 canvas element is essentially a sandbox of pixels that you can draw to dynamically. We’re going to use a canvas to render the game, so we need to add it to our HTML page. Open default.html and replace Line 17 (the Content goes here line) with a canvas tag, so it looks like this:
Normally, you’d specify width & height and add fallback content in case canvas isn’t supported, but we’ll set width/height later and we know canvas will be supported. However, this is just one of the many times you should consider coding practices in case you want to reuse some of your app code as a traditional web application – but that’s a story for another series of posts…
Making Things Easier with CreateJS
We’ll use PreloadJS to help load assets before to use in the game and EaselJS to make it easier to manage the game loop and the drawing of image assets.
Starting the Game
To start the game, we need to know when the page is ready to run. For that, we use the DOMContentLoaded event to tell us when the page structure has been loaded and scripts are ready to run. This is different from the onload event, which waits for all referenced content to be downloaded.
In default.js, add an initialize() function and have it called by DOMContentLoaded. While we’re at it, let’s add the basis of the game loop as well:
Note: the app.oncheckpoint function is collapsed to make things easier to read.
To work with the canvas, store images, and create bitmaps, we’re going to need a bunch of variables. Also, because the original game assumed a 800×480 screen, we need to scale the images we draw to the actual screen size.
Add the following variables to default.js:
Initializing Canvas and Using PreloadJS
Update initialize() as follows:
Now we need to load our images so we can draw them to the canvas. There are many ways to do this, but PreloadJS is helpful because we can list what we’ll use and it ensures they are loaded before we reference them. If we don’t do this, we may not reliably get details like image sizes at runtime, creating bugs.
PreloadJS works by reading an array of resources, then calling a function when complete. We’ll specify all of the images we’ll be using.
Extend the initialize() function as follows:
When PreloadJS has readied our assets, the prepareGame() method will be called.
Using EaselJS to Create and Draw Images
Now we need to get those images to the screen (via the canvas). Fortunately, EaselJS has a number of features we’ll find useful:
- A Stage class that manages the canvas and the scene we’re drawing
- Bitmap, Text, and SpriteSheet classes, useful for representing items to draw
- Point class to help position items on the canvas
- A Ticker class to help manage the game loop (think of it as the heartbeat of the game)
We’ll get to the Ticker a bit later, but now let’s add the Stage so we can start populating it with content. In default.js, add the following to the initialize() function:
This creates the stage and connects it to our game’s canvas element. Now we can add items (called children) to the stage.
Right below the initialize() function, add a prepareGame() function. (Remember we told PreloadJS to call prepareGame when it’s done loading assets.) For now, let’s just add one item – the background:
What’s going on here?
- Line 62 - preload.getResult() is asking PreloadJS for the image it has already loaded for us
- Line 63 – Create an EaselJS Bitmap instance, using the image as it’s source
- Lines 64 & 65 – Scale the Bitmap to the resolution of our screen (relative to 800×480 of the original assets)
- Line 66 – Add the Bitmap to the Stage as a child
- Line 68 – Ask the Stage to tell the canvas about everything it knows
Let’s run the game. After the splash screen, we now see:
A Quick Change with CSS
As you can see, the background image we added is transparent, so our background color is showing through. The black background is spooky, but quite not what we’re looking for.
One thing we can do is to change which WinJS CSS base we’re using. By default, we use ui-dark.css, but a quick change in default.html to point to ui-light.css, and things automatically pick up new styles:
A quick run now shows:
However, let’s try for a more sky-like color… say, “azure”. We can override the WinJS background color by setting our own via CSS. Open /css/default.css and change the body style as shown:
A beautiful sky, ready for war!
Adding the Remaining Assets
Now that you’ve seen how to add the background. It’s mostly a matter of repetition to include the others (with a bit more math thrown in.) Head back to default.js and include the following in prepareGame():
A few notes on this:
- The catapults appear at “ground level” which we need to scale along with the overall size of the images
- Drawing player 2’s catapult is tricky because we need it to face the other direction. Using regX to set a transform point and setting a negative scale gets the job done.
- We create and add the ammo (boulder) image, but hide it until it’s fired later.
Adding Some Text
To wrap things up for this post, let’s use EaselJS’s Text class to add a game title along with indicators for each player’s remaining catapults. First, we’ll need a few variables near the top of default.js:
Then, add the following to prepareGame();
To the Stage, Text instances are children just like the Bitmaps we added earlier.
What does the game look like now?
Things are looking pretty good, but unfortunately that’s about it – nothing’s moving. In the next post, we’ll dive in to the game’s mechanics, fleshing out the game loop by adding motion, collision detection, scorekeeping, and the endgame.
(Note: Opinions expressed in this article and its replies are the opinions of their respective authors and not those of DZone, Inc.)