Rewritten some parts of pong tutorial to make them clearer (hopefully).

(cherry picked from commit faea70955d8a470c6c47437a080dbdd074228ce7)

tutorials/step_by_step/simple_2d_game.rst

@@ -10,17 +10,19 @@ 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>`.
+: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`). The default background color should be set to black:
+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
 
@@ -31,13 +33,23 @@ a custom name for each node, and set the texture for each sprite in the
 Inspector. The result should look similar to this (note: the ball is in
 the middle!):
 
-.. image:: /img/pong_layout.png
+.. image:: /img/pong_nodes.png
 
-The scene tree should, then, look similar to this:
+Name them "left", "right", "separator" and "ball". Set the each sprite node the corresponding texture available in the ZIP archive previously downloaded. 
+
+Set nodes positions :
+ - "left" node : (67.6875, 183.2079)
+ - "right" node : (577, 187)
+ - "separator" node : (320, 200)
+ - "ball" node : (320.2829, 188)
 
-.. image:: /img/pong_nodes.png
 
-Save the scene as "pong.scn" and set it as the main scene in the project
+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:
@@ -45,19 +57,14 @@ properties.
 Input actions setup
 ~~~~~~~~~~~~~~~~~~~
 
-There are so many input methods for video games... Keyboard, Joypad,
-Mouse, Touchscreen (Multitouch). Yet this is pong. The only input that
-matters is for the pads going up and down.
+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. 
 
-Handling all possible input methods can be very frustrating and take a
-lot of code. The fact that most games allow controller customization
-makes this worse. For this, Godot created the "Input Actions". An action
-is defined, then input methods that trigger it are added.
+This is Pong. The only input that matters is for the pads going up and down.
 
-Open the project properties dialog again, but this time move to the
+Open the project properties dialog again (Scene/Project settings), but this time move to the
 "Input Map" tab.
 
-On it, add 4 actions:
+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.
@@ -68,127 +75,152 @@ Script
 ~~~~~~
 
 Create a script for the root node of the scene and open it (as explained
-in :ref:`doc_scripting-adding_a_script`). The script will inherit Node2D:
+in :ref:`doc_scripting-adding_a_script`). This script inherits Node2D:
 
 ::
 
-    extends Node2D
+	extends Node2D
 
-    func _ready():
-        pass
-
-In the constructor, two things will be done. The first is to enable
-processing, and the second to store some useful values. Such values are
-the dimensions of the screen and the pad:
+	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 and the pad.
 
 ::
 
+	extends Node2D
+    
+	# Member variables
+	var screen_size
+	var pad_size
 
-    extends Node2D
-
-    var screen_size
-    var pad_size
+	func _ready():
+		pass
 
-    func _ready():
-        screen_size = get_viewport_rect().size
-        pad_size = get_node("left").get_texture().get_size()
-        set_process(true)
 
-Then, some variables used for in-game processing will be added:
+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.
 
 ::
 
-    #speed of the ball (in pixels/second)
+	extends Node2D
 
-    var ball_speed = 80
-    #direction of the ball (normal vector)
+	# Member variables
+	var screen_size
+	var pad_size
 
-    var direction = Vector2(-1, 0)
-    #constant for pad speed (also in pixels/second)
+	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.
 
-    const PAD_SPEED = 150
 
-Finally, the process function:
+Now, we need to add some other members to our script in order to make our ball move.
 
 ::
 
-    func _process(delta):
+	extends Node2D
 
-Get some useful values for computation. The first is the ball position
-(from the node), the second is the rectangle (``Rect2``) for each of the pads.
-Sprites center their textures by default, so a small adjustment of ``pad_size / 2``
-must be added.
+	# 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.
 
 ::
 
-        var ball_pos = get_node("ball").get_pos()
-        var left_rect = Rect2( get_node("left").get_pos() - pad_size/2, pad_size )
-        var right_rect = Rect2( get_node("right").get_pos() - pad_size/2, pad_size )
+	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 )
 
-Since the ball position was obtained, integrating it should be simple:
+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:
 
 ::
 
-        ball_pos += direction * ball_speed * delta
+		# 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, it should be tested against
-everything. First, the floor and the roof:
+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:
 
 ::
 
-        if ( (ball_pos.y < 0 and direction.y < 0) or (ball_pos.y > screen_size.y and direction.y > 0)):
-            direction.y = -direction.y
+		# 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
 
-If one of the pads was touched, change direction and increase speed a
-little.
+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.
 
 ::
 
-        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
-            ball_speed *= 1.1
-            direction.y = randf() * 2.0 - 1
-            direction = direction.normalized()
+		# 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. Game restarts:
+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:
 
 ::
 
-        if (ball_pos.x < 0 or ball_pos.x > screen_size.x):
-            ball_pos = screen_size * 0.5  # ball goes to screen center
-            ball_speed = 80
-            direction = Vector2(-1, 0)
+		# 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:
+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)
+		get_node("ball").set_pos(ball_pos)
 
-Only update the pads according to player input. The Input class is
-really useful here:
+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()
+		#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
+		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)
+		get_node("left").set_pos(left_pos)
 
-        #move right pad 
-        var right_pos = get_node("right").get_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
+		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)
+		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.