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.
เนื้อหา
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's initialization code:
class Player:
def __init__(self, world, x, y):
# ... [old code hidden]
self.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
class Player:
# ...
def update(self, delta):
self.y += self.vy
self.vy -= 1
Note that we update self.vy at the end of update. The constant -1 is the acceleration. The parameter delta 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 contants at the beginning of Player in models.py. These are class variables.
class Player:
GRAVITY = 1
STARTING_VELOCITY = 15
# ...
Then replace 15 and 1 in the code with the appropriate constants as follows. Note that we can refer to these variables as self.GRAVITY or Player.GRAVITY.
class Player:
# ...
def __init__(self, world, x, y):
# ...
self.vy = Player.STARTING_VELOCITY
def update(self, delta):
# ...
self.vy -= Player.GRAVITY
- When your program looks good, commit it
Jumping dot
Now, let's make the dot jumps. Let's add method jump that set the velocity to some positive amount. Let's create a constant JUMPING_VELOCITY to represent this magic number as well.
class Player:
# ...
JUMPING_VELOCITY = 15
# ...
def jump(self):
self.vy = Player.JUMPING_VELOCITY
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 method on_key_press in FlappyDotWindow to forward the call to the world.
class FlappyDotWindow(arcade.Window):
# ...
def on_key_press(self, key, key_modifiers):
self.world.on_key_press(key, key_modifiers)
Then, in World for any key pressed, call jump.
class World:
# ...
def on_key_press(self, key, key_modifiers):
self.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 World, 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: FROZEN and STARTED. After the game loads, its state is FROZEN. 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 in class World:
class World:
STATE_FROZEN = 1
STATE_STARTED = 2
# ...
With this, we can refer to states as World.STATE_FRONT and World.STATE_STARTED.
Initial state
We let attribute state of World keeps the current game state. We initialize the state in __init__:
class World:
# ...
def __init__(self, width, height):
# ...
self.state = World.STATE_FROZEN
State transition
We add methods start and freeze to World so that FlappyDotWindow can tell the world to start working or to get frozen. We also include method is_started for querying the world's state.
class World:
# ...
def start(self):
self.state = World.STATE_STARTED
def freeze(self):
self.state = World.STATE_FROZEN
def is_started(self):
return self.state == World.STATE_STARTED
Our game changes its state when the user hits the keyboard. We let FlappyDotWindow inform the world to get started.
Rewrite method on_key_press in FlappyDotWindow to do that.
def on_key_press(self, key, key_modifiers):
if not self.world.is_started():
self.world.start()
self.world.on_key_press(key, key_modifiers)
The world's state
We have to change how the world behave according to its state. Basically, it should not update anything when it is frozen. Add the following if at the beginning of World.update
class World:
# ...
def update(self, delta):
if self.state == World.STATE_FROZEN:
return
# ...
- Try to see if the game works as expected. Commit your work after you are done.
