19. Moving Sprites

How do we get sprites to move?

To customize our sprite’s behavior, we need to subclass the Sprite class with our own child class. This is easy:

class Coin(arcade.Sprite):

We need to provide each sprite with a update method. The update method is automatically called to update the sprite’s position.

class Coin(arcade.Sprite):

    def update(self):
        # Code to move goes here

Wait! We have a new class called Coin, but we aren’t using it. Find in our original code this line:

coin = arcade.Sprite("coin_01.png", COIN_SPRITE_SCALING)

See how it is creating an instance of Sprite? We want to create an instance of our new Coin class instead:

coin = Coin("coin_01.png", COIN_SPRITE_SCALING)

Now, how do we get the coin to move?

19.1. Moving Sprites Down

To get the sprites to “fall” down the screen, we need to make their y location smaller. This is easy. Over-ride update in the sprite and subtract from y each frame:

class Coin(arcade.Sprite):

    def update(self):
        self.center_y -= 1

Next, create an instance of the Coin class instead of a Sprite class.

Sprite Sample Move Down
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""" Sprite Sample Program """

import random
import arcade

# --- Constants ---
SPRITE_SCALING_PLAYER = 0.5
SPRITE_SCALING_COIN = 0.2
COIN_COUNT = 50

SCREEN_WIDTH = 800
SCREEN_HEIGHT = 600


class Coin(arcade.Sprite):

    def update(self):
        self.center_y -= 1


class MyGame(arcade.Window):
    """ Our custom Window Class"""

    def __init__(self):
        """ Initializer """
        # Call the parent class initializer
        super().__init__(SCREEN_WIDTH, SCREEN_HEIGHT, "Sprite Example")

        # Variables that will hold sprite lists
        self.player_list = None
        self.coin_list = None

        # Set up the player info
        self.player_sprite = None
        self.score = 0

        # Don't show the mouse cursor
        self.set_mouse_visible(False)

        arcade.set_background_color(arcade.color.AMAZON)

    def setup(self):
        """ Set up the game and initialize the variables. """

        # Sprite lists
        self.player_list = arcade.SpriteList()
        self.coin_list = arcade.SpriteList()

        # Score
        self.score = 0

        # Set up the player
        # Character image from kenney.nl
        self.player_sprite = arcade.Sprite("character.png", SPRITE_SCALING_PLAYER)
        self.player_sprite.center_x = 50
        self.player_sprite.center_y = 50
        self.player_list.append(self.player_sprite)

        # Create the coins
        for i in range(COIN_COUNT):

            # Create the coin instance
            # Coin image from kenney.nl
            coin = Coin("coin_01.png", SPRITE_SCALING_COIN)

            # Position the coin
            coin.center_x = random.randrange(SCREEN_WIDTH)
            coin.center_y = random.randrange(SCREEN_HEIGHT)

            # Add the coin to the lists
            self.coin_list.append(coin)

    def on_draw(self):
        """ Draw everything """
        arcade.start_render()
        self.coin_list.draw()
        self.player_list.draw()

        # Put the text on the screen.
        output = f"Score: {self.score}"
        arcade.draw_text(output, 10, 20, arcade.color.WHITE, 14)

    def on_mouse_motion(self, x, y, dx, dy):
        """ Handle Mouse Motion """

        # Move the center of the player sprite to match the mouse x, y
        self.player_sprite.center_x = x
        self.player_sprite.center_y = y

    def update(self, delta_time):
        """ Movement and game logic """

        # Call update on all sprites (The sprites don't do much in this
        # example though.)
        self.coin_list.update()

        # Generate a list of all sprites that collided with the player.
        hit_list = arcade.check_for_collision_with_list(self.player_sprite,
                                                        self.coin_list)

        # Loop through each colliding sprite, remove it, and add to the score.
        for coin in hit_list:
            coin.kill()
            self.score += 1


def main():
    """ Main method """
    window = MyGame()
    window.setup()
    arcade.run()


if __name__ == "__main__":
    main()

This causes the coins to move down. But once they move off the screen they keep going into negative-coordinate land. We can’t see them any more. Sad.

../../_images/coins_down_1.gif

Coins moving down

19.1.1. Resetting to the Top

We can get around this by resetting the coins up to the top. Here’s how its done:

class Coin(arcade.Sprite):

    def update(self):
        self.center_y -= 1

        # See if we went off-screen
        if self.center_y < 0:
            self.center_y = SCREEN_HEIGHT

But this isn’t perfect. Because if your eyes are fast, you can see the coin ‘pop’ in and out of existence at the edge. It doesn’t smoothly slide off. This is because we move it when the center of the coin is at the edge. Not the top of the coin has slid off.

There are a couple ways we can do this. Here’s one. We’ll check at -20 instead of 0. As long as the coin radius is 20 or less, we are good.

class Coin(arcade.Sprite):

    def update(self):
        self.center_y -= 1

        # See if we went off-screen
        if self.center_y < -20:
            self.center_y = SCREEN_HEIGHT + 20

There’s another way. In addition to center_y, sprites have other members that are useful in these cases. They are top, bottom, left and right. So we can do this:

class Coin(arcade.Sprite):

    def update(self):
        self.center_y -= 1

        # See if we went off-screen
        if self.top < 0:
            self.bottom = SCREEN_HEIGHT

Doing this allows the coins to smoothly slide on and off the screen. But since they reappear at the top, we get repeating patters. See the image below:

../../_images/pattern.gif

Coins repeating in a pattern

Instead we can randomize it a bit:

def update(self):

    # Move the coin
    self.center_y -= 1

    # See if the coin has fallen off the bottom of the screen.
    # If so, reset it.
    if self.top < 0:
        # Reset the coin to a random spot above the screen
        self.center_y = random.randrange(SCREEN_HEIGHT + 20,
                                         SCREEN_HEIGHT + 100)
        self.center_x = random.randrange(SCREEN_WIDTH)

19.1.2. Never Ending Coins

This works, but when we we collect all the coins we are done. What if it was a never-ending set of coins? Instead of “killing” the coin, let’s reset it to the top of the screen.

def update(self, delta_time):
    """ Movement and game logic """

    self.coin_list.update()

    hit_list = arcade.check_for_collision_with_list(self.player_sprite, self.coin_list)

    for coin in hit_list:
        self.score += 1

        # Reset the coin to a random spot above the screen
        coin.center_y = random.randrange(SCREEN_HEIGHT + 20,
                                         SCREEN_HEIGHT + 100)
        coin.center_x = random.randrange(SCREEN_WIDTH)

We can even take that common code, and move it to a method. Here’s a full example:

Full Move Down Sprite Sample
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""" Sprite Sample Program """

import random
import arcade

# --- Constants ---
SPRITE_SCALING_PLAYER = 0.5
SPRITE_SCALING_COIN = 0.2
COIN_COUNT = 50

SCREEN_WIDTH = 800
SCREEN_HEIGHT = 600


class Coin(arcade.Sprite):
    """
    This class represents the coins on our screen. It is a child class of
    the arcade library's "Sprite" class.
    """

    def reset_pos(self):

        # Reset the coin to a random spot above the screen
        self.center_y = random.randrange(SCREEN_HEIGHT + 20,
                                         SCREEN_HEIGHT + 100)
        self.center_x = random.randrange(SCREEN_WIDTH)

    def update(self):

        # Move the coin
        self.center_y -= 1

        # See if the coin has fallen off the bottom of the screen.
        # If so, reset it.
        if self.top < 0:
            self.reset_pos()


class MyGame(arcade.Window):
    """ Our custom Window Class"""

    def __init__(self):
        """ Initializer """
        # Call the parent class initializer
        super().__init__(SCREEN_WIDTH, SCREEN_HEIGHT, "Sprite Example")

        # Variables that will hold sprite lists
        self.player_list = None
        self.coin_list = None

        # Set up the player info
        self.player_sprite = None
        self.score = 0

        # Don't show the mouse cursor
        self.set_mouse_visible(False)

        arcade.set_background_color(arcade.color.AMAZON)

    def setup(self):
        """ Set up the game and initialize the variables. """

        # Sprite lists
        self.player_list = arcade.SpriteList()
        self.coin_list = arcade.SpriteList()

        # Score
        self.score = 0

        # Set up the player
        # Character image from kenney.nl
        self.player_sprite = arcade.Sprite("character.png", SPRITE_SCALING_PLAYER)
        self.player_sprite.center_x = 50
        self.player_sprite.center_y = 50
        self.player_list.append(self.player_sprite)

        # Create the coins
        for i in range(COIN_COUNT):

            # Create the coin instance
            # Coin image from kenney.nl
            coin = Coin("coin_01.png", SPRITE_SCALING_COIN)

            # Position the coin
            coin.center_x = random.randrange(SCREEN_WIDTH)
            coin.center_y = random.randrange(SCREEN_HEIGHT)

            # Add the coin to the lists
            self.coin_list.append(coin)

    def on_draw(self):
        """ Draw everything """
        arcade.start_render()
        self.coin_list.draw()
        self.player_list.draw()

        # Put the text on the screen.
        output = f"Score: {self.score}"
        arcade.draw_text(output, 10, 20, arcade.color.WHITE, 14)

    def on_mouse_motion(self, x, y, dx, dy):
        """ Handle Mouse Motion """

        # Move the center of the player sprite to match the mouse x, y
        self.player_sprite.center_x = x
        self.player_sprite.center_y = y

    def update(self, delta_time):
        """ Movement and game logic """

        # Call update on all sprites (The sprites don't do much in this
        # example though.)
        self.coin_list.update()

        # Generate a list of all sprites that collided with the player.
        hit_list = arcade.check_for_collision_with_list(self.player_sprite,
                                                        self.coin_list)

        # Loop through each colliding sprite, remove it, and add to the score.
        for coin in hit_list:
            coin.kill()
            self.score += 1


def main():
    """ Main method """
    window = MyGame()
    window.setup()
    arcade.run()


if __name__ == "__main__":
    main()

19.2. Bouncing Coins

Instead of always adding one to the y-coordinate and have the sprites move down, we can keep a vector by using change_x and change_y. By using these, we can have the sprite bounce around the screen:

../../_images/sprites_bouncing.gif

Coins bouncing around

sprites_sample_move_bouncing.py
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""" Sprite Sample Program """

import random
import arcade

# --- Constants ---
SPRITE_SCALING_PLAYER = 0.5
SPRITE_SCALING_COIN = 0.2
COIN_COUNT = 50

SCREEN_WIDTH = 800
SCREEN_HEIGHT = 600


class Coin(arcade.Sprite):

    def __init__(self, filename, sprite_scaling):

        super().__init__(filename, sprite_scaling)

        self.change_x = 0
        self.change_y = 0

    def update(self):

        # Move the coin
        self.center_x += self.change_x
        self.center_y += self.change_y

        # If we are out-of-bounds, then 'bounce'
        if self.left < 0:
            self.change_x *= -1

        if self.right > SCREEN_WIDTH:
            self.change_x *= -1

        if self.bottom < 0:
            self.change_y *= -1

        if self.top > SCREEN_HEIGHT:
            self.change_y *= -1


class MyGame(arcade.Window):
    """ Our custom Window Class"""

    def __init__(self):
        """ Initializer """
        # Call the parent class initializer
        super().__init__(SCREEN_WIDTH, SCREEN_HEIGHT, "Sprite Example")

        # Variables that will hold sprite lists
        self.player_list = None
        self.coin_list = None

        # Set up the player info
        self.player_sprite = None
        self.score = 0

        # Don't show the mouse cursor
        self.set_mouse_visible(False)

        arcade.set_background_color(arcade.color.AMAZON)

    def setup(self):
        """ Set up the game and initialize the variables. """

        # Sprite lists
        self.player_list = arcade.SpriteList()
        self.coin_list = arcade.SpriteList()

        # Score
        self.score = 0

        # Set up the player
        # Character image from kenney.nl
        self.player_sprite = arcade.Sprite("character.png", SPRITE_SCALING_PLAYER)
        self.player_sprite.center_x = 50
        self.player_sprite.center_y = 50
        self.player_list.append(self.player_sprite)

        # Create the coins
        for i in range(COIN_COUNT):

            # Create the coin instance
            # Coin image from kenney.nl
            coin = Coin("coin_01.png", SPRITE_SCALING_COIN)

            # Position the coin
            coin.center_x = random.randrange(SCREEN_WIDTH)
            coin.center_y = random.randrange(SCREEN_HEIGHT)
            coin.change_x = random.randrange(-3, 4)
            coin.change_y = random.randrange(-3, 4)

            # Add the coin to the lists
            self.coin_list.append(coin)

    def on_draw(self):
        """ Draw everything """
        arcade.start_render()
        self.coin_list.draw()
        self.player_list.draw()

        # Put the text on the screen.
        output = f"Score: {self.score}"
        arcade.draw_text(output, 10, 20, arcade.color.WHITE, 14)

    def on_mouse_motion(self, x, y, dx, dy):
        """ Handle Mouse Motion """

        # Move the center of the player sprite to match the mouse x, y
        self.player_sprite.center_x = x
        self.player_sprite.center_y = y

    def update(self, delta_time):
        """ Movement and game logic """

        # Call update on all sprites (The sprites don't do much in this
        # example though.)
        self.coin_list.update()

        # Generate a list of all sprites that collided with the player.
        hit_list = arcade.check_for_collision_with_list(self.player_sprite,
                                                        self.coin_list)

        # Loop through each colliding sprite, remove it, and add to the score.
        for coin in hit_list:
            coin.kill()
            self.score += 1


def main():
    """ Main method """
    window = MyGame()
    window.setup()
    arcade.run()


if __name__ == "__main__":
    main()

TODO: Put in some text about spawning a sprite too close to the edge. Also make a refer to it from the final project.

Take what you’ve learned from the example above, and see if you can replicate this:

../../_images/Test_Pattern.gif

Test Pattern

19.3. Coins Moving In Circles

../../_images/sprites_circle.gif

Coins moving in a circle

sprites_circle.py
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import random
import arcade
import math

SPRITE_SCALING = 0.5

SCREEN_WIDTH = 800
SCREEN_HEIGHT = 600


class Coin(arcade.Sprite):

    def __init__(self, filename, sprite_scaling):
        """ Constructor. """
        # Call the parent class (Sprite) constructor
        super().__init__(filename, sprite_scaling)

        # Current angle in radians
        self.circle_angle = 0

        # How far away from the center to orbit, in pixels
        self.circle_radius = 0

        # How fast to orbit, in radians per frame
        self.circle_speed = 0.008

        # Set the center of the point we will orbit around
        self.circle_center_x = 0
        self.circle_center_y = 0

    def update(self):

        """ Update the ball's position. """
        # Calculate a new x, y
        self.center_x = self.circle_radius * math.sin(self.circle_angle) \
            + self.circle_center_x
        self.center_y = self.circle_radius * math.cos(self.circle_angle) \
            + self.circle_center_y

        # Increase the angle in prep for the next round.
        self.circle_angle += self.circle_speed


class MyGame(arcade.Window):
    """ Main application class. """

    def __init__(self, width, height):

        super().__init__(width, height)

        # Sprite lists
        self.player_list = None
        self.coin_list = None

        # Set up the player
        self.score = 0
        self.player_sprite = None

    def start_new_game(self):
        """ Set up the game and initialize the variables. """

        # Sprite lists
        self.player_list = arcade.SpriteList()
        self.coin_list = arcade.SpriteList()

        # Set up the player
        self.score = 0

        # Character image from kenney.nl
        self.player_sprite = arcade.Sprite("character.png", SPRITE_SCALING)
        self.player_sprite.center_x = 50
        self.player_sprite.center_y = 70
        self.player_list.append(self.player_sprite)

        for i in range(50):

            # Create the coin instance
            # Coin image from kenney.nl
            coin = Coin("coin_01.png", SPRITE_SCALING / 3)

            # Position the center of the circle the coin will orbit
            coin.circle_center_x = random.randrange(SCREEN_WIDTH)
            coin.circle_center_y = random.randrange(SCREEN_HEIGHT)

            # Random radius from 10 to 200
            coin.circle_radius = random.randrange(10, 200)

            # Random start angle from 0 to 2pi
            coin.circle_angle = random.random() * 2 * math.pi

            # Add the coin to the lists
            self.coin_list.append(coin)

        # Don't show the mouse cursor
        self.set_mouse_visible(False)

        # Set the background color
        arcade.set_background_color(arcade.color.AMAZON)

    def on_draw(self):

        # This command has to happen before we start drawing
        arcade.start_render()

        # Draw all the sprites.
        self.coin_list.draw()
        self.player_list.draw()

        # Put the text on the screen.
        output = "Score: " + str(self.score)
        arcade.draw_text(output, 10, 20, arcade.color.WHITE, 14)

    def on_mouse_motion(self, x, y, dx, dy):
        self.player_sprite.center_x = x
        self.player_sprite.center_y = y

    def update(self, delta_time):
        """ Movement and game logic """

        # Call update on all sprites (The sprites don't do much in this
        # example though.)
        self.coin_list.update()

        # Generate a list of all sprites that collided with the player.
        hit_list = arcade.check_for_collision_with_list(self.player_sprite,
                                                        self.coin_list)

        # Loop through each colliding sprite, remove it, and add to the score.
        for coin in hit_list:
            self.score += 1
            coin.kill()


def main():
    window = MyGame(SCREEN_WIDTH, SCREEN_HEIGHT)
    window.start_new_game()
    arcade.run()


if __name__ == "__main__":
    main()

Sprites can easily be rotated by setting their angle attribute. Angles are in degrees. So the following will flip the player upside down:

self.player_sprite.angle = 180

If you put this in the coin’s update method, it would cause the coins to spin:

self.angle += 1

# If we rotate past 360, reset it back a turn.
if self.angle > 359:
    self.angle -= 360