Step 5 - Add Gravity

The previous example great for top-down, but what if it is a side view with jumping like our platformer? We need to add gravity. First, let’s define a constant to represent the acceleration for gravity, and one for a jump speed.

05_add_gravity.py - Add Gravity
GRAVITY = 1
PLAYER_JUMP_SPEED = 20

At the end of the setup method, change the physics engine to PhysicsEnginePlatformer and include gravity as a parameter.

05_add_gravity.py - Add Gravity
        # Create the 'physics engine'
        self.physics_engine = arcade.PhysicsEnginePlatformer(
            self.player_sprite, gravity_constant=GRAVITY, walls=self.scene["Walls"]
        )

We are sending our SpriteList for the things the player should collide with to the walls parameter of the the physics engine. As we’ll see in later chapters, the platformer physics engine has a platforms and walls parameter. The difference between these is very important. Static non-moving spritelists should always be sent to the walls parameter, and moving sprites should be sent to the platforms parameter. Ensuring you do this will have extreme benefits to performance.

Adding static sprites via the platforms parameter is roughly an O(n) operation, meaning performance will linearly get worse as you add more sprites. If you add your static sprites via the walls parameter, then it is nearly O(1) and there is essentially no difference between for example 100 and 50,000 non-moving sprites.

We also see here some new syntax relating to our Scene object. You can access the scene like you would a Python dictionary in order to get your SpriteLists from it. There are multiple ways to access the SpriteLists within a Scene but this is the easiest and most straight forward. You could alternatively use scene.get_sprite_list("My Layer").

Then, modify the key down and key up event handlers. We’ll remove the up/down statements we had before, and make ‘UP’ jump when pressed.

05_add_gravity.py - Add Gravity
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    def on_key_press(self, key, modifiers):
        """Called whenever a key is pressed."""

        if key == arcade.key.UP or key == arcade.key.W:
            if self.physics_engine.can_jump():
                self.player_sprite.change_y = PLAYER_JUMP_SPEED
        elif key == arcade.key.LEFT or key == arcade.key.A:
            self.player_sprite.change_x = -PLAYER_MOVEMENT_SPEED
        elif key == arcade.key.RIGHT or key == arcade.key.D:
            self.player_sprite.change_x = PLAYER_MOVEMENT_SPEED

    def on_key_release(self, key, modifiers):
        """Called when the user releases a key."""

        if key == arcade.key.LEFT or key == arcade.key.A:
            self.player_sprite.change_x = 0
        elif key == arcade.key.RIGHT or key == arcade.key.D:
            self.player_sprite.change_x = 0

Note

You can change how the user jumps by changing the gravity and jump constants. Lower values for both will make for a more “floaty” character. Higher values make for a faster-paced game.

Source Code

05_add_gravity.py - Add Gravity
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"""
Platformer Game
"""
import arcade

# Constants
SCREEN_WIDTH = 1000
SCREEN_HEIGHT = 650
SCREEN_TITLE = "Platformer"

# Constants used to scale our sprites from their original size
CHARACTER_SCALING = 1
TILE_SCALING = 0.5

# Movement speed of player, in pixels per frame
PLAYER_MOVEMENT_SPEED = 5
GRAVITY = 1
PLAYER_JUMP_SPEED = 20


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

    def __init__(self):

        # Call the parent class and set up the window
        super().__init__(SCREEN_WIDTH, SCREEN_HEIGHT, SCREEN_TITLE)

        # Our Scene Object
        self.scene = None

        # Separate variable that holds the player sprite
        self.player_sprite = None

        # Our physics engine
        self.physics_engine = None

        arcade.set_background_color(arcade.csscolor.CORNFLOWER_BLUE)

    def setup(self):
        """Set up the game here. Call this function to restart the game."""

        # Initialize Scene
        self.scene = arcade.Scene()

        # Set up the player, specifically placing it at these coordinates.
        image_source = ":resources:images/animated_characters/female_adventurer/femaleAdventurer_idle.png"
        self.player_sprite = arcade.Sprite(image_source, CHARACTER_SCALING)
        self.player_sprite.center_x = 64
        self.player_sprite.center_y = 128
        self.scene.add_sprite("Player", self.player_sprite)

        # Create the ground
        # This shows using a loop to place multiple sprites horizontally
        for x in range(0, 1250, 64):
            wall = arcade.Sprite(":resources:images/tiles/grassMid.png", TILE_SCALING)
            wall.center_x = x
            wall.center_y = 32
            self.scene.add_sprite("Walls", wall)

        # Put some crates on the ground
        # This shows using a coordinate list to place sprites
        coordinate_list = [[512, 96], [256, 96], [768, 96]]

        for coordinate in coordinate_list:
            # Add a crate on the ground
            wall = arcade.Sprite(
                ":resources:images/tiles/boxCrate_double.png", TILE_SCALING
            )
            wall.position = coordinate
            self.scene.add_sprite("Walls", wall)

        # Create the 'physics engine'
        self.physics_engine = arcade.PhysicsEnginePlatformer(
            self.player_sprite, gravity_constant=GRAVITY, walls=self.scene["Walls"]
        )

    def on_draw(self):
        """Render the screen."""

        # Clear the screen to the background color
        arcade.start_render()

        # Draw our Scene
        self.scene.draw()

    def on_key_press(self, key, modifiers):
        """Called whenever a key is pressed."""

        if key == arcade.key.UP or key == arcade.key.W:
            if self.physics_engine.can_jump():
                self.player_sprite.change_y = PLAYER_JUMP_SPEED
        elif key == arcade.key.LEFT or key == arcade.key.A:
            self.player_sprite.change_x = -PLAYER_MOVEMENT_SPEED
        elif key == arcade.key.RIGHT or key == arcade.key.D:
            self.player_sprite.change_x = PLAYER_MOVEMENT_SPEED

    def on_key_release(self, key, modifiers):
        """Called when the user releases a key."""

        if key == arcade.key.LEFT or key == arcade.key.A:
            self.player_sprite.change_x = 0
        elif key == arcade.key.RIGHT or key == arcade.key.D:
            self.player_sprite.change_x = 0

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

        # Move the player with the physics engine
        self.physics_engine.update()


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


if __name__ == "__main__":
    main()