01219245/cocos2d/Sprites2

This is part of 01219245-56, 2nd semester 2556. For the recent version with Cocos-JS see 01219245/cocos2d-js/Sprites2.

In this tutorial, we will recreate a clone of a wonderful Flappy Bird. Let's call it Flappy Dot (as our player would look like a dot). We will develop basic game mechanics in this tutorial. We will try to add special effects to the game in the next tutorial.

Make sure you have completed Tutorial 100 on basic sprites.

Task breakdown

Before we start, make sure you know how this game works. You may want to try it for a bit. I guess many of your friends have it on their phones. This is how our game would look like:

As usual, let's start by thinking about the possible list of increments we would add to an empty project to get this game.

When you get your list, please see the steps that we plan to take here.

• Show the player on the screen.
• The player can jump and fall. (Implement player physics)
• Show a single pillar pair.
• Move the pillar pair across the screen.
• Let the pillar pair reappear.
• Check for player-pillar collision.
• Make the game with one pillar pair.
• Show more than one pillar pairs.

The player and its movement

You can start from our template 219245-template.zip.

Creating the sprite

In this step, we shall create a sprite for the player, and show it in the middle of the screen.

Use a graphic editor to create an image for our player. The image should be of size 40 pixels x 40 pixels. Save the image as images/dot.png and try to make it look cute.

We shall create class Player as src/Player.js.

var Player = cc.Sprite.extend({
    ctor: function() {
        this._super();
        this.initWithFile( 'images/dot.png' );
    }
});

We shall create the player in GameLayer.init. To do so add these lines:

        this.player = new Player();
        this.player.setPosition( new cc.Point( screenWidth / 2, screenHeight / 2 ) );
        this.addChild( this.player );
        this.player.scheduleUpdate();

Note that we use constants screenWidth and screenHight (which are 800 and 600, respectively). Don't forget to add this constant in main.js.

Try to refresh the game. You should see your sprite in the middle of the screen.

Commit your work.

Review of physics

You might forget all these, but if you want objects in your game to look and act a bit like real objects, you might have to recall stuffs you learned from mechanics.

Let's look at the basics. An object has a position, its position changes if it has non-zero velocity.

How can you change the player's position? Put something like this in Player.update:

this.setPosition( x, y );

If you want to apply the velocity, you can change the player position based on the velocity.

If there is an acceleration, the object's velocity also changes. The Sprite do not have velocity as its property, so we will add it. You can update the velocity based on the acceleration.

These properties (the position, the velocity, and the acceleration) all have directions. Sometimes, you see negative velocity; this means the object is moving in an opposite direction as the positive direction. We shall follow the standard co-ordinate system for Cocos2d, i.e., for the y-axis, we think of the direction as going upwards.

In Physics, everything is continuous. When writing games, we don't really need exact physics, so we can move objects in discrete steps. (In fact, method update is also called with parameter dt, the time period between this call and the last call, and you can use this to make your simulation more smooth.)

So the usual pseudo code for physics is as follows.

pos = pos + velocity;
velocity = velocity + acceleration

Falling dot

To simulate the player falls, we should maintain the player's current velocity, so that we can make it falls as close as the real object.

Let's add this line that initialize property vy in Player.ctor:

        this.vy = 15;

You may wonder why we put 15 here. It is just pure guess at this point. However, when you write games, you might want to try various possible values and pick the best one (i.e., the one that make the game fun).

The update method changes the player's position

    update: function( dt ) {
        var pos = this.getPosition();
        this.setPosition( new cc.Point( pos.x, pos.y + this.vy ) );
        this.vy += -1;
    }

Note that we update this.vy at the end of update. The constant -1 is the acceleration.

Try to run the program. You should see the player falling.

While our program works, don't just rush to commit right away. Let's try to get rid of the magic numbers first, by defining them explicitly.

Add these line at the end of Player.js

Player.G = -1;
Player.STARTING_VELOCITY = 15;

Then replace 15 and -1 in the code with the appropriate constants.

When your program looks good, commit it

Jumping dot

Now, let's make the dot jumps. Let's add method Player.jump that set the velocity to some positive amount.

    jump: function() {
        this.vy = Player.JUMPING_VELOCITY;
    }

Also, add this constant after the class is defined in the .extend block.

Player.JUMPING_VELOCITY = 15;

To jump, we have to call player.jump() in an appropriate time. We will response to keyboard inputs, so let's add this to GameLayer.init:

this.setKeyboardEnabled( true );

We will jump in any key input, so let's add the following onKeyDown method to GameLayer.

    onKeyDown: function( e ) {
        this.player.jump();
    }

To test this increment, you will have to click on the game canvas, and then quickly hit on any key to get the dot jumping. Try a few times to see how the dot moves. You can adjust the jumping velocity to make the movement nice.

After a few trials to make sure your code works, please commit.

Game states

It won't be nice to have the user start clicking right away after the game loads. Therefore we shouldn't let the player move before the game actually starts. To do so, we will add a state to the game. How various objects interact depends on the game state. In this tutorial, we shall simply add a property state to GameLayer, and perform various events and update methods according to this state. We will learn a cleaner way later on.

For now, the game has 2 states: FRONT and STARTED. After the game loads, its state is FRONT. In this state, nothing moves, and after the user hit any keyboard, it changes its state to STARTED with the dot start to jump. The dot falls and jumps as usual in the STARTED state.

Let's add the constants for states after the class GameLayer is defined (i.e., at the end of extend call).

GameLayer.STATES = {
    FRONT: 1,
    STARTED: 2
};

With this, we can refer to states as GameLayer.STATES.FRONT and GameLayer.STATES.STARTED.

Initial state

We let property state of GameLayer keeps the current game state. We initialize the state in GameLayer.init:

this.state = GameLayer.STATES.FRONT;

State transition

Our game changes its state when the user hits the keyboard, we rewrite method onKeyDown to

    onKeyDown: function( e ) {
        if ( this.state == GameLayer.STATES.FRONT ) {
            this.state = GameLayer.STATES.STARTED;
            // <--- some code to tell the player to start falling (TO BE ADDED LATER)
        }
        if ( this.state == GameLayer.STATES.STARTED ) {
            this.player.jump();
        }
    }

Note that in the code above, if the condition in the first if holds, you will also execute the body of the second if. Therefore, the two conditions are dependent. When the user first hits the key, we let the player starts and also jumps. However, if you don't look carefully, you might not notice the jump call because it is in the second if.

We might want to make it a bit clearly like this:

    onKeyDown: function( e ) {
        if ( this.state == GameLayer.STATES.FRONT ) {
            this.state = GameLayer.STATES.STARTED;
            // <--- some code to tell the player to start falling (TO BE ADDED LATER)
            this.player.jump();
        } else if ( this.state == GameLayer.STATES.STARTED ) {
            this.player.jump();
        }
    }

Or, you may want to use switch statement.

Player's state

Now, the Player shouldn't fall until the game tells it to get started. We shall add a state to the Player as well. Initialize property started in Player.ctor:

        this.started = false;

Add method start to update this state.

    start: function() {
        this.started = true;
    }

Finally, we only perform position update when the player is started.

 
    update: function( dt ) {
        if ( this.started ) {
            // ... old update code here
        }
    }

We shall call

this.player.start()  // <-- this is the code to tell the player to start falling.

in the onKeyDown method in GameLayer.

Try to see if the game works as expected. Commit your work after you are done.

The pillar pair

In this section, we will implement a moving pair of opposing pillars. Let's call them a pillar pair. Clearly, we will use one or more sprites to represent it. Before we think about how to implement it, let's think about what we want from this thing: (1) we want to move the pillar pair and (2) we want to check if the dot hits the pillars.

There are basically two approaches for this.

1. Use 1 sprite with transparent background in the middle. 2. Use 2 sprites (one for the top pillar, another for the bottom pillar).

Both approaches are shown below.

Question: Can you think of the advantages and disadvantages for using each approach? Expand to see some of the possible advantages and disadvantages.

Using 1 sprite is easier to deal with. However, the space between two opposing pillars are fixed. Using 2 sprites gives flexibility but it might be hard to to deal with two objects. It might be even worse if we have to deal with many pillar pairs.

Here, we will try to get the best out of both approaches. We will use 2 sprites, but we shall combine them into one object.

The blue object in the figure above contains two red sprite objects. Specifically, we shall create a cc.Node object that contains two sprites.

Sprite images

Draw two images for the top and bottom pillars. Since we can place each pillar at different height, we should make the images large enough so that the other end of the pillar is still outside the screen. The figure below shows the sprites and the screen; the red border shows the screen boundary.

So let's create two images pillar-top.png and pillar-bottom.png, each of width 80 pixels and height 600 pixels. Save them in directory images.

Node class

Let's create class PillarPair in src/PillarPair.js. In its constructor, we create two sprites from the images we have just created. Then we add the sprites as the child of this Node.

var PillarPair = cc.Node.extend({
    ctor: function() {
	this._super();
	this.topPillar = cc.Sprite.create( 'images/pillar-top.png' );
	this.topPillar.setAnchorPoint( new cc.Point( 0.5, 0 ) );
	this.topPillar.setPosition( new cc.Point( 0, 100 ) );
	this.addChild( this.topPillar );
	
	this.bottomPillar = cc.Sprite.create( 'images/pillar-bottom.png' );
	this.bottomPillar.setAnchorPoint( new cc.Point( 0.5, 1 ) );
	this.bottomPillar.setPosition( new cc.Point( 0, -100 ) );
	this.addChild( this.bottomPillar );
    }
}

To put both sprites in the correct position relative to the position of the PillarPair, we set two properties of each sprite: the AnchorPoint and the Position. To understand this, consider the top pillar. We call

this.topPillar.setAnchorPoint( new cc.Point( 0.5, 0 ) );

to say that when we talk about the position of the top pillar, that position is the middle point (0.5) of the bottom (0) of the image (see Figure below, blue co-ordinates). For anchor points, the co-ordinate of the bottom-left corner of the sprite is (0,0) and the top-right corner is (1,1). Position (0.5,0) is the middle point of the bottom of the sprite.

We then call

this.topPillar.setPosition( new cc.Point( 0, 100 ) );

to place the top pillar at position (0, 100) relative to the position of the pillar: this means the top pillar is 100 pixels above the Node position (see Figure below, red co-ordinates).

Let's see if the pillars appear. Add file src/PillarPair.js to the module list in cocos2d.js. Then in GameLayer, let's add the code that create a PillarPair in GameLayer.init.

        this.pillarPair = new PillarPair();
        this.pillarPair.setPosition( new cc.Point( 700, 300 ) );
        this.addChild( this.pillarPair );

If the pillar pair appears, commit your change.

Question: What is the width of the space between the top pillar and the bottom pillar?

200 pixels.

Moving the pillars

Let's add the code that move the pillars, PillarPair.update.

    update: function( dt ) {
        this.setPositionX( this.getPositionX() - 5 );
    }

Then in GameLayer.init, add a line

this.pillarPair.scheduleUpdate();

after the PillarPair is created.

Don't forget to commit.

Technicalities: preloading of resources

If you runs the game a few times, you might notice that sometimes some pillar does not appear. This is because when we create the sprites, the images we want to use are not completely loaded. To avoid this problem, we will tell the Cocos2d-html5 library to load all our resources before starting our Scene.

Create file src/resource.js with the following content:

var g_resources = [
    //image
    {src: 'images/dot.png' },
    {src: 'images/pillar-top.png' },
    {src: 'images/pillar-bottom.png' }

    //plist

    //fnt

    //tmx

    //bgm

    //effect
];

Note that this file lists all images that we want to use. There are other sections for other kinds of resources as well (plist, fnt, etc,...).

Add this file into file list in cocos2d.js:

         appFiles:[
+            'src/resource.js',
             'src/GameLayer.js',  
             ...

Notes: do not copy the green '+' signs or red '-' signs to your code; they are there to shows which lines are inserted and which lines are deleted. These are from git diff.

Finally, in the starting code in main.js, replace a call to runWithScene with a call to cc.LoaderScene.preload. (Replace the red line, with the green lines.)

         director.setAnimationInterval( 1.0 / this.config[ 'frameRate' ] );
 
-        director.runWithScene( new this.startScene() );
+        cc.LoaderScene.preload(g_resources, function () {
+            director.replaceScene( new this.startScene() );
+        }, this );
 
         return true;
     }

Now, when you start our game, you'll see the Cocos2d-html5 preloading start screen.

If your code works, commit the changes.

Moving the pillars after the game starts

Currently, we create the pillars right after the scene starts. We should synchronize this with the game state.

We will create the pillar pair often, so let's take the code for pillar creation out of GameLayer.init and place it in method createPillarPair:

    createPillarPair: function() {
        this.pillarPair = new PillarPair();
        this.pillarPair.setPosition( new cc.Point( 900, 300 ) );
        this.addChild( this.pillarPair );
        this.pillarPair.scheduleUpdate();
    }

In GameLayer.init, let's put this null initialization in place of the old pillar pair creation.

this.pillarPair = null;

Finally, let's call createPillarPair after the game state changes to GameLayer.STATES.STARTED in onKeyDown:

	if ( this.state == GameLayer.STATES.FRONT ) {
	    this.state = GameLayer.STATES.STARTED;
	    this.createPillarPair();
	    this.player.start();
            this.player.jump();
        } else if ( ... ) {
            // ...
        }

Try the game to see if the pillar pair starts moving after we hit a keyboard.

Commit your change.

The code for starting a new game currently lies in an event handler onKeyDown. This method will get longer and longer as we work on our game. So let's clean the method a bit. We shall extract the game initialization code into method startGame.

    onKeyDown: function( e ) {
	if ( this.state == GameLayer.STATES.FRONT ) {
            this.startGame();
	    this.state = GameLayer.STATES.STARTED;
        } else if ( ... ) {
            // ...
        }
    },

    // ...

    startGame: function() {
        this.createPillarPair();
        this.player.start();
        this.player.jump();
    },

Test the code, and commit.

Commit your change.

z-axis: Showing the player in front of the pillars

If you move the player through the pillars, you should notice that the pillars are drawn over the player. It would be better to have the player in front of the pillars. To do so, we assign the higher z co-ordinate to the player when we add the player to the GameLayer with method addChild.

        this.player = new Player();
        this.player.setPosition( new cc.Point( screenWidth / 2, screenHeight / 2 ) );
        this.addChild( this.player, 1 );

Note that the default z co-ordinate is 0; so the z-co-ordinates of the pillars is lower than the player's.

Test and commit the changes.

Exercise: Reusing the pillars

In this game, our player will have to fly passing a lot of pillar pairs. However, we will not always create a new pillar pair. Instead, we shall reuse the old pillar pair that recently disappear on the left side of the screen.

EXERCISE: modify method PillarPair.update so that right after the pillar pair move outside the screen, it re-enter at the right side of the screen.

Test and commit the changes.

Exercise: Collision detection

We will check if our dot hits the pillar pair. We will write method hit in class PillarPair:

    hit: function( player ) {
        // return true if the player hits the pillar pair.
    }

Unit testing

Question: When we write method hit, how can we test if the method is correct?

I'll have to test it. Right now, we have to run the game and try to hit and miss the pillars in as many cases as possible. This is rather hard to do because we need to control the dot as well. This make conventional testing for this logic hard to do.

We will try to use automated testing for this part of the game. The idea is simple: we will try to explicitly call hit in various ways to check if it returns the correct decision.

For normal functions, this is not very hard to do. However, in this case, it is fairly hard to directly call hit because it is a method in class derived from cc.Node for which we do not quite know how to create it outside the Cocos2d-html framework.

We will get away with that by extracting the decision function as a stand-alone function and test it independently of the Cocos2d-html framework. Let's finish method hit first.

    hit: function( player ) {
	var playerPos = player.getPosition();
	var myPos = this.getPosition();

        return checkPlayerPillarCollision( playerPos.x, playerPos.y, myPos.x, myPos.y );
    }

We will write function checkPlayerPillarCollision in a separated file, so that we can test it without having to deal with Cocos2d-html5 library.

Since this part is quite a detour, I'll give more detailed instructions.

Let's create a (broken) function checkPlayerPillarCollision in file src/coldetect.js.

var checkPlayerPillarCollision = function( playerX, playerY, pillarX, pillarY ) {
    return false;
};

Then, add this file (src/coldetect.js) to the appFiles config in cocos2d.js. (We might want to do this after we have finish writing the function, but let's do it now so that we won't forget.)

We will test this function in a separated main JavaScript program. We will put all these testing codes in a separated directory src/test.

Let's create directory src/test.

Add index.html in src/test:

<!doctype html>
<html lang="en">
  <body>
    <ul id="test-output">
    </ul>
    <script src="../coldetect.js"></script>
    <script src="test.js"></script>
  </body>
</html>

Our main JavaScript program that runs the unit testing will be in test.js. Create test.js in directory src/test:

var assert = function( actual, expected, message ) {
    var msg = "";

    if ( actual === expected ) {
	msg = '<span style="color: green">OK</span>';
    } else {
	msg = '<span style="color: red; font-weight: bold">Failed:</span> ' + message;
    }
    
    var elt = document.getElementById( 'test-output' );
    elt.innerHTML += "<li>" + msg + "</li>";
};

assert( checkPlayerPillarCollision( 100, 100, 300, 200 ), false,
	'when the dot is very far left of the pillar pair' );
assert( checkPlayerPillarCollision( 300, 300, 300, 200 ), true,
	'when the dot hit the middle of the top pillar' );

This test.js file contains function assert and a few lines of test cases. You can ignore function assert for now, but these two calls to assert checks the following two collision cases:

Let's access the test page that we have just added. Go to URL like:

file:///XXXXX/mygames/flappydot/src/test/

(I.e., add /src/test/ after the URL for the game.) You should see something like this:

If you look at test.js, each assert line is a single test case. Currently our function is broken int the second test case. You can start fixing the function in src/coldetect.js to make sure that you get all "OK"'s.

NOTES: You don't have to make the collision detection perfect to make the game fun. For example, you don't have to think of the dot as a perfect shape that you have drawn or even a circle, but you can think of it as a simple rectangle which is large enough. Our dot is of size 40 x 40, but we have some curve. Therefore we can view it as a rectangle of size, say, 30 x 30 centered at the position of the player.

Clearly these two test cases are not enough to ensure that your checkPlayerPillarCollision works correctly. Therefore, after your code for checkPlayerPillarCollision passes these two test cases, you might want to add more test cases. The figure below shows a few other test cases that you should try adding to the test script.

Don't forget to commit your work at this point.

Combining with the game mechanics

With method PillarPair.hit, we can regularly checks if the player hits the pillars. We create method update in GameLayer to do this.

    update: function( dt ) {
        if ( this.state == GameLayer.STATES.STARTED ) {
            if ( this.pillarPair && this.pillarPair.hit( this.player ) ) {
                // ... do something
            }
        }
    },

Note that we only check collision when the game is started. To get this method update running, don't forget to call

this.scheduleUpdate();

in GameLayer.init.

What should we do when the player hits a pillar?

First, the game state should change. Let's add another state constant:

GameLayer.STATES = {
    FRONT: 1,
    STARTED: 2,
    DEAD: 3
};

Also, we should stop the player and the pillars from moving. Let's write method endGame in GameLayer to do this:

    endGame: function() {
        this.player.stop();
        if ( this.pillarPair ) {
            this.pillarPair.unscheduleUpdate();
        }
    }

and add method Player.stop to class Player:

    stop: function() {
	this.started = false;
    }

With this, the update method becomes:

    update: function( dt ) {
        if ( this.state == GameLayer.STATES.STARTED ) {
            if ( this.pillarPair && this.pillarPair.hit( this.player ) ) {
                this.endGame();
                this.state = GameLayer.STATES.DEAD;
            }
        }
    },

Try the game. You might want to try a few hit/miss at the pillars to manually test the collision detection function as well.

The game should be somewhat playable. Don't forget to commit your work.

Exercise: Random the pillar heights

Let's create the pillar pairs with different heights. Our screen's height is 600 pixels, and we probably do not want the pillar to be too high or too low.

Exerciese: Add this method to PillarPair.

    randomPositionY: function() {
        // ... change the y position of the pillar pair.
    },

Don't forget to call this method when (1) you create a pillar pair, and (2) when the piilar pair wraps around the screen.

Notes: When you test this method, you may want to switch off the collision detection. This can be easily done by adding return false; at the top of checkPlayerPillarCollision function. When you are done, don't forget to remove this hack.

Commit your work. Don't forget to remove the hack to avoid checkPlayerPillarCollision.

Exercise: The series of pillar pairs

As we have most objects written nicely, adding more pillars can be done by creating more PillarPair's and spread them out nicely across the screen.

We only outline the changes that you need to make. It is your exercises to get all these steps done.

• PillarPair creation: Right now we only create one pillar pair. We should create more and keep them in an array. Make sure you spread them out nicely over the x co-ordinate. Don't forget to rename the method to createPillars (with the s).
• Collision detection: We can't just call this.pillarPair.hit( ... ). It would be nice to write a method for checking collision on all pillar pairs.
• endGame: Now we have to stop all the pillars.

For each of these step, don't forget to commit when you get a unit of work done.

Additional Exercises

There are many ways to make the game more fun like a real game. Here we list a few:

• After the player dies, the game stops. Implement the game restart feature, i.e., when the player hit any key after the player dies, the game should restart.
• The game shows no score. Add the scoring to the game.
• Flappy dot is extremely hard because you can hit only once. Add more lives to the player.
• In our game, the pillar pair can have different spacing. Implement this feature to make the game extremely hard.
• You can allow the dot to fire a few rockets to destroy the pillars.

Good luck. Have fun writing games!

Don't forget to regularly commit your work.