Tutorial 3: Better Pong

In the last tutorial, we made a simple Pong game that was kind of boring. We’re going to make it better by adding scores, sounds, and joystick support.

Adding Scoring

Adding a score system will make our Pong game feel more like a game and less like a toy. Every time a player wins a round, they will get one point. When a player gets ten points, they will win the game, and a new game can be started by pressing the Enter key.

Making Enter Restart the Game

We are going to need a new global variable: game_in_progress. This variable will indicate whether or not a game is currently going and will be used to determine whether to start a new game or pause when the Enter key is pressed. Set it to True by default.

To make pressing Enter start a new game, we will check game_in_progress. If a game is in progress, we will pause the game, as we had it do previously. Otherwise, we will set game_in_progress to True and restart the room.

If you look through the documentation for sge.dsp.Room, you may notice that no “restart” method exists. In fact, this is a design choice; earlier versions of the SGE did have a method to restart rooms, but it was removed because this feature is overly difficult to maintain properly. But how do we restart the room, then? Well, we technically don’t. Instead, we create a new room which is exactly like the one we wanted to restart, and immediately start it. We will put the creation of the room into a new function, create_room(). Our definition of Game.event_key_press() becomes:

def event_key_press(self, key, char):
    global game_in_progress

    if key == 'f8':
        sge.gfx.Sprite.from_screenshot().save('screenshot.jpg')
    elif key == 'f11':
        self.fullscreen = not self.fullscreen
    elif key == 'escape':
        self.event_close()
    elif key in ('p', 'enter'):
        if game_in_progress:
            self.pause()
        else:
            game_in_progress = True
            create_room().start()

Now, we need to define create_room(). This is very simple; we just copy and paste the code we used at the bottom to create the room into it, but specify that player and player2 are global. Our function is as follows:

def create_room():
    global player1
    global player2
    player1 = Player(1)
    player2 = Player(2)
    ball = Ball()
    return sge.dsp.Room([player1, player2, ball], background=background)

Of course, this makes the identical code at the bottom redundant, so we will replace it with a call to create_room().

Giving Points to the Players

We now need to add score attributes to the Player objects. We will initialize the new attribute, score, in Player.event_create() as 0.

Now, in Ball.event_step(), add lines to increase player1.score and player2.score whenever the respective player wins a round.

Displaying the Scores

The players have points, but can’t see the score! We need to add a HUD (heads-up display) to show the score to the players.

There are a couple of ways we can do this. Most obviously, we can use sge.dsp.Game.project_text() or sge.dsp.Room.project_text(). However, there is a much better way: have a dynamically generated sprite that represents the look of the HUD at any given time, and displaying that sprite.

New Resources

We need to add a new global variable called hud_sprite. Assign a new sprite to this variable with a width of 320, a height of 120, an origin_x of 160, and an origin_y of 0.

To draw text, we need a font. Create a new sge.gfx.Font object and assign it to hud_font. For now, we will use a system font. I am choosing "Droid Sans Mono", but you can choose whatever font you prefer. Pass your choice as the first argument to sge.gfx.Font.__init__(). Set the size keyword argument to 48.

Note

We are using system fonts for simplicity, but it is generally a bad idea to rely on them. There is no standard for what fonts are available on the system, and the set of fonts available on the system varies widely. In real projects, it is better to distribute a font file with the game and use that.

Drawing the HUD

There are a few times we need to redraw the HUD: when the game starts, when player 1 scores, and when player 2 scores. Therefore, we will put the redrawing code into a function, refresh_hud(). This function needs to clear the HUD sprite, draw Player 1’s score, and then draw Player 2’s score.

Another constant is needed: TEXT_OFFSET, which we will define as 16.

We clear the HUD sprite with sge.gfx.Sprite.draw_clear().

To draw the text, we use sge.gfx.Sprite.draw_text(). Both calls have a few arguments in common: font is set to hud_font, y is set to TEXT_OFFSET, color is set to white, and valign is set to "top".

The rest of the arguments are different between the two. text is set to the respective player’s score, converted to a string. x is set to hud_sprite.width / 2 - TEXT_OFFSET for player 1’s score, and hud_sprite.width / 2 + TEXT_OFFSET for player 2’s score. halign is set to "right" for player 1’s score, and "left" for player 2’s score.

refresh_hud() ends up something like this:

def refresh_hud():
    # This fixes the HUD sprite so that it displays the correct score.
    hud_sprite.draw_clear()
    x = hud_sprite.width / 2
    hud_sprite.draw_text(hud_font, str(player1.score), x - TEXT_OFFSET,
                         TEXT_OFFSET, color=sge.gfx.Color("white"),
                         halign="right", valign="top")
    hud_sprite.draw_text(hud_font, str(player2.score), x + TEXT_OFFSET,
                         TEXT_OFFSET, color=sge.gfx.Color("white"),
                         halign="left", valign="top")

Add calls to refresh_hud() in the three places where a Player.score value changes, right after the change. These places are in Player.event_create() and Ball.event_step().

we have one more problem. refresh_hud() requires player1 and player2 to each have an attribute called score, but the first time it is called, one of the player objects has not had a chance to initialize this attribute. To work around this, add a class attribute to Player called score, and set it to 0. This will cause player1.score and player2.score to be 0 in the event that the respective object’s score has not been initialized yet.

Displaying the HUD

At this point, we have our HUD, but it isn’t displayed. We will fix this simply by adding a step event to Game which projects the HUD sprite onto the screen:

def event_step(self, time_passed, delta_mult):
    self.project_sprite(hud_sprite, 0, self.width / 2, 0)

Unlike sge.dsp.Room projections, sge.dsp.Game projections are relative to the screen. Additionally, these projections are always on top of everything else on the screen. This is usually how we want a HUD to be displayed, which is why we are using a sge.dsp.Game projection instead of a sge.dsp.Room projection or sge.dsp.Object object.

Note

You may notice that, when you pause the game, the HUD disappears. This is not a bug! This happens because the step event doesn’t occur while the game is paused. If you want the HUD to show up while the game is paused, project it in the paused step event, defined by sge.dsp.Game.event_paused_step(), as well.

Giving Victory

At this point, we have scores, but no one ever officially wins. We need to end the game when someone gets 10 points. We will go a little further and replace the scores with text that says “WIN” and “LOSE” for the winner and loser, respectively.

Define a new constant called POINTS_TO_WIN as 10.

In our case, the most convenient place to check for victory is within Ball.serve(). Specifically, put the code that sets the speed of the ball under a conditional that checks whether the score values of both players are less than POINTS_TO_WIN. Add an else block below that. This is where a player has won the game.

Since the game is over, stop the movement of the ball by setting xvelocity and yvelocity to 0. We don’t want any more scoring to happen.

Now, draw the new text onto the HUD. We do this using the same call to sge.gfx.Sprite.draw_text() we used in refresh_hud(), except instead of drawing the scores converted to strings, we draw "WIN" or "LOSE" depending on whether or not the respective player’s score is greater than the other player’s score.

Finally, set game_in_progress to False. Don’t forget to declare it with global first.

The new Ball.serve() looks something like this:

def serve(self, direction=None):
    global game_in_progress

    if direction is None:
        direction = random.choice([-1, 1])

    self.x = self.xstart
    self.y = self.ystart

    if (player1.score < POINTS_TO_WIN and
            player2.score < POINTS_TO_WIN):
        # Next round
        self.xvelocity = BALL_START_SPEED * direction
        self.yvelocity = 0
    else:
        # Game Over!
        self.xvelocity = 0
        self.yvelocity = 0
        hud_sprite.draw_clear()
        x = hud_sprite.width / 2
        p1text = "WIN" if player1.score > player2.score else "LOSE"
        p2text = "WIN" if player2.score > player1.score else "LOSE"
        hud_sprite.draw_text(hud_font, p1text, x - TEXT_OFFSET,
                             TEXT_OFFSET, color=sge.gfx.Color("white"),
                             halign="right", valign="top")
        hud_sprite.draw_text(hud_font, p2text, x + TEXT_OFFSET,
                             TEXT_OFFSET, color=sge.gfx.Color("white"),
                             halign="left", valign="top")
        game_in_progress = False

Adding Sounds

We have a complete Pong game now, but it’s still a little quiet. Let’s make it more lively by adding some sounds.

Getting the Sounds

I would normally go to a database like OpenGameArt for sound effects, but in this case, we are instead going to use a nice free/libre program called Sfxr. This program makes it easy to generate retro-sounding sound effects, so it’s perfect for Pong sounds. Generate three sounds: one for the ball bouncing off a paddle (“bounce.wav”), one for the ball bouncing off a wall (“bounce_wall.wav”), and one for the ball passing by a player (“score.wav”). Alternatively, you can copy the sounds I generated from examples/data. Create a folder in your project directory with the name “data” and put your sounds in this folder.

Note

Some file systems, like FAT32 and NTFS, are case-insensitive and will allow you to treat “bounce.wav” and “Bounce.wav” as if they are the same file name, but some, such as pretty much every Linux file system, are case-sensitive, meaning that “bounce.wav” and “Bounce.wav” are two completely different names; requesting one will never give you the other. If you have a case-insensitive file system, be careful to not get the case wrong, or some people who play the game will face a crash that will be completely invisible to you!

Loading the Sounds

Sounds in the SGE are stored in sge.snd.Sound objects. As the only argument, indicate the full path to the file. There are two ways to indicate the path: using the current working directory as a base, and using the directory of pong.py as a base. Both of methods require the os module, so be sure to add this to your list of imports.

The easiest way to get the path of the file is to use the current working directory as a base, on the assumption that the current working directory is also the directory that the “data” folder is located in. This method is very simple; assuming we want the file called “spam.wav”, we would use this code:

os.path.join("data", "spam.wav")

However, it is not always the case that the current working directory is the appropriate location to search for the “data” folder. It could be that the current working directory is the user’s home directory, for instance. To prevent the game from crashing in this case, define a constant called DATA, indicating the “data” directory relative to the location of pong.py:

DATA = os.path.join(os.path.dirname(__file__), "data")

__file__ is a special variable indicating the full path to the current file, i.e. pong.py in this case. By getting the directory name of the current file, we can be certain of where to look for the “data” folder. DATA now indicates the appropriate path to the “data” folder, so from now on, if we want a file called “spam.wav” located in this directory, we use this code:

os.path.join(DATA, "spam.wav")

Assign the appropriate sge.snd.Sound objects to bounce_sound, bounce_wall_sound, and score_sound.

Playing the Sounds

Sounds are played with sge.snd.Sound.play(). Call this method in the appropriate places: when a player scores, when the ball bounces off an edge of the screen, and when the ball hits a paddle. There are five places in total.

With that, our Pong game now has sound effects.

Adding Joystick Support

Joystick support is a nice thing to have in a game, so we are going to add it. We are going to support analog sticks and trackballs. Mouse control would actually be even better, but this would put one of the players at an unfair advantage.

First, we will add an attribute to Player indicating what joystick to use, called joystick. Set it to 0 (which is the first joystick) for player 1, and 1 (which is the second joystick) for player 2.

Axis Movement

Adding movement based on a joystick axis is easy. For this, we use sge.joystick.get_axis() in the step event of Player. Pass self.joystick as the first argument, and 1 (which is the Y-axis) as the second argument. Assign it to a variable called axis_motion. Later, we will be modifying the code that sets yvelocity so that it is chosen based on axis position, trackball movement, or key presses, whichever one would cause it to move fastest.

Trackball Movement

Since trackball motion is relative, it is a little trickier. We need to store the amount of movement it makes each frame. We will use an attribute called trackball_motion for that; initialize it as 0 in the create event.

We now need to define the trackball move event, which is defined by sge.dsp.Object.event_joystick_trackball_move(). Within this event, if the joystick argument is the same as self.joystick, add y to self.trackball_motion. We are adding to it, rather than replacing it, because the trackball might move multiple times in the same frame.

Applying the Joystick Controls

Currently, we have this line:

self.yvelocity = key_motion * PADDLE_SPEED

This line uses the state of the keys to determine how to move the paddle. We need to change this so that the joystick controls we defined can be used as well. It will be replaced with the following:

  • If the absolute value of axis_motion is greater than the absolute value of both key_motion and trackball_motion, set yvelocity to axis_motion * PADDLE_SPEED.
  • Otherwise, if trackball_motion is greater than key_motion, set yvelocity to self.trackball_motion * PADDLE_SPEED
  • Otherwise, use the line we have been using up until this point.

After this, we must set trackball_motion to 0.

The Final Result

Our final Pong game now has scores, sounds, and even joystick support:

#!/usr/bin/env python3

# Pong Example
# Written in 2013, 2014, 2015 by Julian Marchant <onpon4@riseup.net>
#
# To the extent possible under law, the author(s) have dedicated all
# copyright and related and neighboring rights to this software to the
# public domain worldwide. This software is distributed without any
# warranty.
#
# You should have received a copy of the CC0 Public Domain Dedication
# along with this software. If not, see
# <http://creativecommons.org/publicdomain/zero/1.0/>.

import random

import sge

DATA = os.path.join(os.path.dirname(__file__), "data")
PADDLE_XOFFSET = 32
PADDLE_SPEED = 4
PADDLE_VERTICAL_FORCE = 1 / 12
BALL_START_SPEED = 2
BALL_ACCELERATION = 0.2
BALL_MAX_SPEED = 15
POINTS_TO_WIN = 10
TEXT_OFFSET = 16

game_in_progress = True


class Game(sge.dsp.Game):

    def event_step(self, time_passed, delta_mult):
        self.project_sprite(hud_sprite, 0, self.width / 2, 0)

    def event_key_press(self, key, char):
        global game_in_progress

        if key == 'f8':
            sge.gfx.Sprite.from_screenshot().save('screenshot.jpg')
        elif key == 'f11':
            self.fullscreen = not self.fullscreen
        elif key == 'escape':
            self.event_close()
        elif key in ('p', 'enter'):
            if game_in_progress:
                self.pause()
            else:
                game_in_progress = True
                self.current_room.start()

    def event_close(self):
        self.end()

    def event_paused_key_press(self, key, char):
        if key == 'escape':
            # This allows the player to still exit while the game is
            # paused, rather than having to unpause first.
            self.event_close()
        else:
            self.unpause()

    def event_paused_close(self):
        # This allows the player to still exit while the game is paused,
        # rather than having to unpause first.
        self.event_close()


class Player(sge.dsp.Object):

    score = 0

    def __init__(self, player):
        if player == 1:
            self.joystick = 0
            self.up_key = "w"
            self.down_key = "s"
            x = PADDLE_XOFFSET
            self.hit_direction = 1
        else:
            self.joystick = 1
            self.up_key = "up"
            self.down_key = "down"
            x = sge.game.width - PADDLE_XOFFSET
            self.hit_direction = -1

        y = sge.game.height / 2
        super().__init__(x, y, sprite=paddle_sprite, checks_collisions=False)

    def event_create(self):
        self.score = 0
        refresh_hud()
        self.trackball_motion = 0

    def event_step(self, time_passed, delta_mult):
        # Movement
        key_motion = (sge.keyboard.get_pressed(self.down_key) -
                      sge.keyboard.get_pressed(self.up_key))
        axis_motion = sge.joystick.get_axis(self.joystick, 1)

        if (abs(axis_motion) > abs(key_motion) and
                abs(axis_motion) > abs(self.trackball_motion)):
            self.yvelocity = axis_motion * PADDLE_SPEED
        elif abs(self.trackball_motion) > abs(key_motion):
            self.yvelocity = self.trackball_motion * PADDLE_SPEED
        else:
            self.yvelocity = key_motion * PADDLE_SPEED

        self.trackball_motion = 0

        # Keep the paddle inside the window
        if self.bbox_top < 0:
            self.bbox_top = 0
        elif self.bbox_bottom > sge.game.current_room.height:
            self.bbox_bottom = sge.game.current_room.height

    def event_joystick_trackball_move(self, joystick, ball, x, y):
        if joystick == self.joystick:
            self.trackball_motion += y


class Ball(sge.dsp.Object):

    def __init__(self):
        x = sge.game.width / 2
        y = sge.game.height / 2
        super().__init__(x, y, sprite=ball_sprite)

    def event_create(self):
        self.serve()

    def event_step(self, time_passed, delta_mult):
        # Scoring
        if self.bbox_right < 0:
            player2.score += 1
            refresh_hud()
            score_sound.play()
            self.serve(-1)
        elif self.bbox_left > sge.game.current_room.width:
            player1.score += 1
            refresh_hud()
            score_sound.play()
            self.serve(1)

        # Bouncing off of the edges
        if self.bbox_bottom > sge.game.current_room.height:
            self.bbox_bottom = sge.game.current_room.height
            self.yvelocity = -abs(self.yvelocity)
            bounce_wall_sound.play()
        elif self.bbox_top < 0:
            self.bbox_top = 0
            self.yvelocity = abs(self.yvelocity)
            bounce_wall_sound.play()

    def event_collision(self, other, xdirection, ydirection):
        if isinstance(other, Player):
            if other.hit_direction == 1:
                self.bbox_left = other.bbox_right + 1
            else:
                self.bbox_right = other.bbox_left - 1

            self.xvelocity = min(abs(self.xvelocity) + BALL_ACCELERATION,
                                 BALL_MAX_SPEED) * other.hit_direction
            self.yvelocity += (self.y - other.y) * PADDLE_VERTICAL_FORCE
            bounce_sound.play()

    def serve(self, direction=None):
        global game_in_progress

        if direction is None:
            direction = random.choice([-1, 1])

        self.x = self.xstart
        self.y = self.ystart

        if (player1.score < POINTS_TO_WIN and
                player2.score < POINTS_TO_WIN):
            # Next round
            self.xvelocity = BALL_START_SPEED * direction
            self.yvelocity = 0
        else:
            # Game Over!
            self.xvelocity = 0
            self.yvelocity = 0
            hud_sprite.draw_clear()
            x = hud_sprite.width / 2
            p1text = "WIN" if player1.score > player2.score else "LOSE"
            p2text = "WIN" if player2.score > player1.score else "LOSE"
            hud_sprite.draw_text(hud_font, p1text, x - TEXT_OFFSET,
                                 TEXT_OFFSET, color=sge.gfx.Color("white"),
                                 halign="right", valign="top")
            hud_sprite.draw_text(hud_font, p2text, x + TEXT_OFFSET,
                                 TEXT_OFFSET, color=sge.gfx.Color("white"),
                                 halign="left", valign="top")
            game_in_progress = False


def create_room():
    global player1
    global player2
    player1 = Player(1)
    player2 = Player(2)
    ball = Ball()
    return sge.dsp.Room([player1, player2, ball], background=background)


def refresh_hud():
    # This fixes the HUD sprite so that it displays the correct score.
    hud_sprite.draw_clear()
    x = hud_sprite.width / 2
    hud_sprite.draw_text(hud_font, str(player1.score), x - TEXT_OFFSET,
                         TEXT_OFFSET, color=sge.gfx.Color("white"),
                         halign="right", valign="top")
    hud_sprite.draw_text(hud_font, str(player2.score), x + TEXT_OFFSET,
                         TEXT_OFFSET, color=sge.gfx.Color("white"),
                         halign="left", valign="top")


# Create Game object
Game(width=640, height=480, fps=120, window_text="Pong")

# Load sprites
paddle_sprite = sge.gfx.Sprite(width=8, height=48, origin_x=4, origin_y=24)
ball_sprite = sge.gfx.Sprite(width=8, height=8, origin_x=4, origin_y=4)
paddle_sprite.draw_rectangle(0, 0, paddle_sprite.width, paddle_sprite.height,
                             fill=sge.gfx.Color("white"))
ball_sprite.draw_rectangle(0, 0, ball_sprite.width, ball_sprite.height,
                           fill=sge.gfx.Color("white"))
hud_sprite = sge.gfx.Sprite(width=320, height=120, origin_x=160, origin_y=0)

# Load backgrounds
layers = [sge.gfx.BackgroundLayer(paddle_sprite, sge.game.width / 2, 0, -10000,
                                  repeat_up=True, repeat_down=True)]
background = sge.gfx.Background(layers, sge.gfx.Color("black"))

# Load fonts
hud_font = sge.gfx.Font("Droid Sans Mono", size=48)

# Load sounds
bounce_sound = sge.snd.Sound(os.path.join(DATA, 'bounce.wav'))
bounce_wall_sound = sge.snd.Sound(os.path.join(DATA, 'bounce_wall.wav'))
score_sound = sge.snd.Sound(os.path.join(DATA, 'score.wav'))

# Create rooms
sge.game.start_room = create_room()

sge.game.mouse.visible = False


if __name__ == '__main__':
    sge.game.start()