godot-docs/tutorials/step_by_step/simple_2d_game.rst

.. _doc_simple_2d_game:

Simple 2D game
==============

Pong
~~~~

In this simple tutorial, a basic game of Pong will be created. There are
plenty of more complex examples in the demos included with the engine,
but this should get one introduced to basic functionality for 2D Games.

Run Godot Engine and start a new project.

Assets
~~~~~~

Some assets are included for this tutorial:
:download:`pong_assets.zip </files/pong_assets.zip>`. Unzip its content
in your project folder.

Scene setup
~~~~~~~~~~~

For the sake of the old times, the game will be in 640x400 pixels
resolution. This can be configured in the Project Settings (see
:ref:`doc_scenes_and_nodes-configuring_the_project`) under Scene/Project
settings menu. The default background color should be set to black:

.. image:: /img/clearcolor.png

Create a :ref:`class_Node2D` node for the project root. Node2D is the
base type for the 2D engine. After this, add some sprites
(:ref:`class_Sprite` node) for the left and right paddles, the separator
and ball. You can set a custom name for each node, and set the texture
for each sprite in the Inspector. 

.. image:: /img/pong_nodes.png

Set nodes positions:
 - "left" node: (67, 183)
 - "right" node: (577, 187)
 - "separator" node: (320, 200)
 - "ball" node: (320, 188)


The final scene layout should look similar to this (note: the ball is in
the middle!):

.. image:: /img/pong_layout.png


Save the scene as "pong.tscn" and set it as the main scene in the
project
properties.

.. _doc_simple_2d_game-input_actions_setup:

Input actions setup
~~~~~~~~~~~~~~~~~~~

Video games can be played using various input methods: Keyboard, Joypad,
Mouse, Touchscreen (multitouch)... Godot is able to use them all.
However, it would be interesting to define the inputs as "Input Actions"
instead of hardware actions that you'd manage separately. This way, any
input method can be used: each of them only require the user to connect
buttons to game actions that you defined. 

This is Pong. The only input that matters is for the pads going up and
down.

Open the project properties dialog again (Scene/Project settings), but
this time move to the
"Input Map" tab.

In this tab, add 4 actions:
``left_move_up``, ``left_move_down``, ``right_move_up``,
``right_move_down``.
Assign the keys that you desire. A/Z (for the left player) and Up/Down
(for the right player) as keys
should work in most cases.

.. image:: /img/inputmap.png

Script
~~~~~~

Create a script for the root node of the scene and open it (as explained
in :ref:`doc_scripting-adding_a_script`). This script inherits Node2D:

::

    extends Node2D

    func _ready():
        pass
        
        
First things first, we need to define some members for our script so it
can store useful values. Such values are the dimensions of the screen, the pad
and the initial direction of the ball.

::

    extends Node2D
    
    # Member variables
    var screen_size
    var pad_size
    var direction = Vector2(1.0, 0.0)

    func _ready():
        pass


As you know, the ``_ready()`` function is the first function called
(after ``_enter_tree()`` which we don't need here). In this function,
two things have to be done. The first one is to enable
processing: this is the purpose of the ``set_process(true)`` function.
The second one is to initalize our two member variables.

::

    extends Node2D

    # Member variables
    var screen_size
    var pad_size

    func _ready():
        screen_size = get_viewport_rect().size
        pad_size = get_node("left").get_texture().get_size()
        set_process(true)
        
We initialize the ``pad_size`` variable by getting one of the pads nodes
(the left one here), and obtain its texture size. The ``screen_size`` is
initialized using the ``get_viewport_rect()`` which returns a Rect
object corresponding to the game window, and we store its size.


Now, we need to add some other members to our script in order to make
our ball move.

::

    extends Node2D

    # Member variables
    var screen_size
    var pad_size
    
    # Constant for pad speed (in pixels/second)
    const INITIAL_BALL_SPEED = 80
    # Speed of the ball (also in pixels/second)
    var ball_speed = INITIAL_BALL_SPEED
    # Constant for pads speed
    const PAD_SPEED = 150

    func _ready():
        screen_size = get_viewport_rect().size
        pad_size = get_node("left").get_texture().get_size()
        set_process(true)

    

Finally, the ``_process()`` function. All the code below is contained by
this function.

We have to init some useful values for computation. The first one is the
ball position (from the node), the second one is the rectangle
(``Rect2``) for each pad. These rectangles will be used for collisions
tests between the ball and the pads. Sprites center their textures by
default, so a small adjustment of ``pad_size / 2`` must be added.

::

    func _process(delta):
        var ball_pos = get_node("ball").get_pos()
        var left_rect = Rect2( get_node("left").get_pos() - pad_size*0.5, pad_size )
        var right_rect = Rect2( get_node("right").get_pos() - pad_size*0.5, pad_size )

Now, let's add some movement to the ball in the ``_process()`` function.
Since the ball position is stored in the ``ball_pos`` variable,
integrating it is simple:

::

        # Integrate new ball postion
        ball_pos += direction * ball_speed * delta

This code line is called at each iteration of the ``_process()``
function. That means the ball position will be updated at each new
frame.

Then, now that the ball has a new position, we need to test if it
collides with anything, that is the window borders and the pads. First,
the floor and the roof:

::

        # Flip when touching roof or floor
        if ((ball_pos.y < 0 and direction.y < 0) or (ball_pos.y > screen_size.y and direction.y > 0)):
            direction.y = -direction.y

Second, the pads: if one of the pads was touched, we need to invert the
direction of the ball on the X axis so it goes back, and define a new
random Y direction using ``randf()`` function. We also increase its
speed a little.

::

        # Flip, change direction and increase speed when touching pads
        if ((left_rect.has_point(ball_pos) and direction.x < 0) or (right_rect.has_point(ball_pos) and direction.x > 0)):
            direction.x = -direction.x
            direction.y = randf()*2.0 - 1
            direction = direction.normalized()
            ball_speed *= 1.1

If the ball went out of the screen, it's game over. That is, we test if
the X position of the ball is less than 0 or greater than the screen
width. If so, the game then restarts:

::

        # Check gameover
        if (ball_pos.x < 0 or ball_pos.x > screen_size.x):
            ball_pos = screen_size*0.5
            ball_speed = INITIAL_BALL_SPEED
            direction = Vector2(-1, 0)

Once everything was done with the ball, the node is updated with the new
position which was computed before:

::

        get_node("ball").set_pos(ball_pos)

Pads movement: we only update the pads according to player input. This
is done using the Input class:

::

        # Move left pad
        var left_pos = get_node("left").get_pos()

        if (left_pos.y > 0 and Input.is_action_pressed("left_move_up")):
            left_pos.y += -PAD_SPEED * delta
        if (left_pos.y < screen_size.y and Input.is_action_pressed("left_move_down")):
            left_pos.y += PAD_SPEED * delta

        get_node("left").set_pos(left_pos)

        # Move right pad
        var right_pos = get_node("right").get_pos()

        if (right_pos.y > 0 and Input.is_action_pressed("right_move_up")):
            right_pos.y += -PAD_SPEED * delta
        if (right_pos.y < screen_size.y and Input.is_action_pressed("right_move_down")):
            right_pos.y += PAD_SPEED * delta

        get_node("right").set_pos(right_pos)
        
We use the 4 actions previously defined in the Input actions setup
section. When the player activates the according key, the corresponding
action is triggered. When the action is triggered, we simply compute a
new position for the pad in the wished direction. Finally, we set this
new position to the node.

And that's it! A simple Pong was written with a few lines of code.