01219245/cocos2d/Sprites2

This is part of 01219245.

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 start with basic game mechanics, then we will try to add special effects to the game.

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.
• Move the pillar across the screen.
• Check for player-pillar collision.
• Let the pillar reappear.
• Show more than one pillars.

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.

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.

Moving the pillars

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

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

To test our code