Acceleration and Friction

Screen shot of moving a sprite by keyboard
sprite_move_keyboard_accel.py
  1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
"""
Show how to use acceleration and friction

Artwork from http://kenney.nl

If Python and Arcade are installed, this example can be run from the command line with:
python -m arcade.examples.sprite_move_keyboard_accel
"""

import arcade
import os

SPRITE_SCALING = 0.5

SCREEN_WIDTH = 800
SCREEN_HEIGHT = 600
SCREEN_TITLE = "Better Move Sprite with Keyboard Example"

# Important constants for this example

# Speed limit
MAX_SPEED = 3.0

# How fast we accelerate
ACCELERATION_RATE = 0.1

# How fast to slow down after we letr off the key
FRICTION = 0.02

class Player(arcade.Sprite):

    def update(self):
        self.center_x += self.change_x
        self.center_y += self.change_y

        # Check to see if we hit the screen edge
        if self.left < 0:
            self.left = 0
            self.change_x = 0 # Zero x speed
        elif self.right > SCREEN_WIDTH - 1:
            self.right = SCREEN_WIDTH - 1
            self.change_x = 0

        if self.bottom < 0:
            self.bottom = 0
            self.change_y = 0
        elif self.top > SCREEN_HEIGHT - 1:
            self.top = SCREEN_HEIGHT - 1
            self.change_y = 0


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

    def __init__(self, width, height, title):
        """
        Initializer
        """

        # Call the parent class initializer
        super().__init__(width, height, title)

        # Set the working directory (where we expect to find files) to the same
        # directory this .py file is in. You can leave this out of your own
        # code, but it is needed to easily run the examples using "python -m"
        # as mentioned at the top of this program.
        file_path = os.path.dirname(os.path.abspath(__file__))
        os.chdir(file_path)

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

        # Set up the player info
        self.player_sprite = None

        # Track the current state of what key is pressed
        self.left_pressed = False
        self.right_pressed = False
        self.up_pressed = False
        self.down_pressed = False

        # Set the background color
        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()

        # Set up the player
        self.player_sprite = Player(":resources:images/animated_characters/female_person/femalePerson_idle.png", SPRITE_SCALING)
        self.player_sprite.center_x = 50
        self.player_sprite.center_y = 50
        self.player_list.append(self.player_sprite)

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

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

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

        # Display speed
        arcade.draw_text(f"X Speed: {self.player_sprite.change_x:6.3f}", 10, 50, arcade.color.BLACK)
        arcade.draw_text(f"Y Speed: {self.player_sprite.change_y:6.3f}", 10, 70, arcade.color.BLACK)

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

        # Add some friction
        if self.player_sprite.change_x > FRICTION:
            self.player_sprite.change_x -= FRICTION
        elif self.player_sprite.change_x < -FRICTION:
            self.player_sprite.change_x += FRICTION
        else:
            self.player_sprite.change_x = 0

        if self.player_sprite.change_y > FRICTION:
            self.player_sprite.change_y -= FRICTION
        elif self.player_sprite.change_y < -FRICTION:
            self.player_sprite.change_y += FRICTION
        else:
            self.player_sprite.change_y = 0

        # Apply acceleration based on the keys pressed
        if self.up_pressed and not self.down_pressed:
            self.player_sprite.change_y += ACCELERATION_RATE
        elif self.down_pressed and not self.up_pressed:
            self.player_sprite.change_y += -ACCELERATION_RATE
        if self.left_pressed and not self.right_pressed:
            self.player_sprite.change_x += -ACCELERATION_RATE
        elif self.right_pressed and not self.left_pressed:
            self.player_sprite.change_x += ACCELERATION_RATE

        if self.player_sprite.change_x > MAX_SPEED:
            self.player_sprite.change_x = MAX_SPEED
        elif self.player_sprite.change_x < -MAX_SPEED:
            self.player_sprite.change_x = -MAX_SPEED
        if self.player_sprite.change_y > MAX_SPEED:
            self.player_sprite.change_y = MAX_SPEED
        elif self.player_sprite.change_y < -MAX_SPEED:
            self.player_sprite.change_y = -MAX_SPEED

        # Call update to move the sprite
        # If using a physics engine, call update on it instead of the sprite
        # list.
        self.player_list.update()

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

        if key == arcade.key.UP:
            self.up_pressed = True
        elif key == arcade.key.DOWN:
            self.down_pressed = True
        elif key == arcade.key.LEFT:
            self.left_pressed = True
        elif key == arcade.key.RIGHT:
            self.right_pressed = True

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

        if key == arcade.key.UP:
            self.up_pressed = False
        elif key == arcade.key.DOWN:
            self.down_pressed = False
        elif key == arcade.key.LEFT:
            self.left_pressed = False
        elif key == arcade.key.RIGHT:
            self.right_pressed = False


def main():
    """ Main method """
    window = MyGame(SCREEN_WIDTH, SCREEN_HEIGHT, SCREEN_TITLE)
    window.setup()
    arcade.run()


if __name__ == "__main__":
    main()