Prg2/arcade4 flappy dot

This is part of the course Programming 2, the material is originally from 01219245/cocos2d-js/Sprites2 from 01219245, 2nd semester 2557.

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.

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

Create a new project and set up a Git repository

We will start with an empty game template. Put the following code in our main program flappy.py.

import arcade
 
SCREEN_WIDTH = 800
SCREEN_HEIGHT = 600
 
class FlappyDotWindow(arcade.Window):
    def __init__(self, width, height):
        super().__init__(width, height)
 
        arcade.set_background_color(arcade.color.WHITE)

    def on_draw(self):
        arcade.start_render()
        
 
def main():
    window = FlappyDotWindow(SCREEN_WIDTH, SCREEN_HEIGHT)
    arcade.set_window(window)
    arcade.run()
 
if __name__ == '__main__':
    main()

Try to run the game to see if an empty white window appears. Then, create a git repository at the project directory and commit the code.

Create your git repository.

Creating the dot model and 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 will continue our model/window code structure. So let's create a dot model (called Player) and World in models.py as in our previous projects. Note that currently the Player do nothing in update

class Player:
    def __init__(self, world, x, y):
        self.world = world
        self.x = x
        self.y = y

    def update(self, delta):
        pass

class World:
    def __init__(self, width, height):
        self.width = width
        self.height = height
 
        self.player = Player(self, width // 2, height // 2)
 
     def update(self, delta):
        self.player.update(delta)

As in the previous projects, we then use ModelSprite to display the sprite. Add the class in flappy.py and update FlappyDotWindow accordingly.

class ModelSprite(arcade.Sprite):
    def __init__(self, *args, **kwargs):
        self.model = kwargs.pop('model', None)
 
        super().__init__(*args, **kwargs)
 
    def sync_with_model(self):
        if self.model:
            self.set_position(self.model.x, self.model.y)
 
    def draw(self):
        self.sync_with_model()
        super().draw()


class FlappyDotWindow(arcade.Window):
    def __init__(self, width, height):
        super().__init__(width, height)

        arcade.set_background_color(arcade.color.WHITE)

        self.world = World(SCREEN_WIDTH, SCREEN_HEIGHT)
        
        self.dot_sprite = ModelSprite('images/dot.png', model=self.world.player)

    def update(self, delta):
        self.world.update(delta)

    def on_draw(self):
        arcade.start_render()

        self.dot_sprite.draw()

Try to run 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? We can set its x and y attribute on the model.

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 Player currently does not have velocity as its attribute, so we will add it. Now, you can update the velocity based on the acceleration.

These attributes (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 arcade, i.e., for the y-axis, we think of the direction as going upwards.

While in Physics, everything is continuous, but 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 delta, 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:

File: src/Player.js

        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

File: src/Player.js

    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. The parameter dt represents delta time; we do not use it for now.

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

File: src/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.

File: src/Player.js

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

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

File: src/Player.js

Player.JUMPING_VELOCITY = 15;

To jump, we have to call player.jump() in an appropriate time. We will response to keyboard inputs. We shall follow the style we did in the last tutorial.

First, add these functions. Function addKeyboardHandler registers the event handlers: onKeyDown and onKeyUp.

File: src/GameLayer.js

    addKeyboardHandlers: function() {
        var self = this;
        cc.eventManager.addListener({
            event: cc.EventListener.KEYBOARD,
            onKeyPressed : function( keyCode, event ) {
                self.onKeyDown( keyCode, event );
            },
            onKeyReleased: function( keyCode, event ) {
                self.onKeyUp( keyCode, event );
            }
        }, this);
    },

    onKeyDown: function( keyCode, event ) {
    },

    onKeyUp: function( keyCode, event ) {
    }

Then, call addKeyboardHandlers in GameLayer.init

File: src/GameLayer.js

    init: function() {
        // ...
	this.addKeyboardHandlers();
        // ...
    },

We will jump in any key input, so we shall modify onKeyDown as follows.

File: src/GameLayer.js

    onKeyDown: function( keyCode, event ) {
        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.