Sprite Rotation Around a Point

Screen shot of 2 uses for rotating a sprite around a point

This non-interactive example demonstrates two applications of rotating a sprite around a point:

  1. On the left, a beam spinning around a point in a circle

  2. On the right, a platform rotating around a point while staying flat

Although this behavior is inspired by platformers, it could be useful for other types of games.

The base class used in this example offers circular movement with finer control than the one in Collect Coins that are Moving in a Circle.

For a related example demonstrating how to mix rotation around a point with user input, see Move By Keyboard, Fire Towards Mouse.

sprite_rotate_around_point.py
  1"""
  2Rotating Sprites Around Points
  3
  4Two minimal examples demonstrating how to rotate sprites around points
  5and how you might apply them in a game.
  6
  7Artwork from https://kenney.nl
  8
  9If Python and Arcade are installed, this example can be run from the command line with:
 10python -m arcade.examples.sprite_rotate_around_point
 11"""
 12import arcade
 13from arcade.math import rotate_point
 14
 15SCREEN_WIDTH = 800
 16SCREEN_HEIGHT = 600
 17QUARTER_WIDTH = SCREEN_WIDTH // 4
 18HALF_HEIGHT = SCREEN_HEIGHT // 2
 19
 20
 21SCREEN_TITLE = "Rotating Sprites Around Points"
 22
 23
 24class RotatingSprite(arcade.Sprite):
 25    """
 26    This sprite subclass implements a generic rotate_around_point method.
 27    """
 28
 29    def rotate_around_point(self, point, degrees, change_angle=True):
 30        """
 31        Rotate the sprite around a point by the set amount of degrees
 32
 33        You could remove the change_angle keyword and/or angle change
 34        if you know that sprites will always or never change angle.
 35
 36        :param point: The point that the sprite will rotate about
 37        :param degrees: How many degrees to rotate the sprite
 38        :param change_angle: Whether the sprite's angle should also be adjusted.
 39        """
 40
 41        # If change_angle is true, change the sprite's angle
 42        if change_angle:
 43            self.angle += degrees
 44
 45        # Move the sprite along a circle centered on the point by degrees
 46        self.position = rotate_point(
 47            self.center_x, self.center_y,
 48            point[0], point[1], degrees)
 49
 50
 51class ExampleWindow(arcade.Window):
 52
 53    def __init__(self):
 54        super().__init__(SCREEN_WIDTH, SCREEN_HEIGHT, SCREEN_TITLE)
 55
 56        self.sprites = arcade.SpriteList()
 57
 58        # This example is based off spinning lines of fire from Mario games
 59        # See https://www.mariowiki.com/Fire_Bar for more information
 60        self.left_rotating_laser_sprite = RotatingSprite(
 61            ":resources:images/space_shooter/laserBlue01.png",
 62            center_x=QUARTER_WIDTH + 26, center_y=HALF_HEIGHT)
 63
 64        self.laser_base_sprite = arcade.Sprite(
 65            ":resources:images/tiles/boxCrate.png", scale=0.25,
 66            center_x=QUARTER_WIDTH, center_y=HALF_HEIGHT)
 67
 68        self.laser_text = arcade.Text(
 69            "change_angle = True",
 70            QUARTER_WIDTH, SCREEN_HEIGHT // 2 - 150,
 71            anchor_x='center')
 72
 73        # This example demonstrates how to make platforms rotate around a point
 74        self.right_rotating_platform_sprite = RotatingSprite(
 75            ":resources:images/tiles/grassHalf.png", scale=0.25,
 76            center_x=3 * QUARTER_WIDTH + 50, center_y=HALF_HEIGHT)
 77
 78        self.platform_base_sprite = arcade.Sprite(
 79            ":resources:images/tiles/boxCrate.png", scale=0.25,
 80            center_x=3 * QUARTER_WIDTH, center_y=HALF_HEIGHT)
 81
 82        self.platform_text = arcade.Text(
 83            "change_angle = False",
 84            3 * QUARTER_WIDTH, HALF_HEIGHT - 150,
 85            anchor_x='center')
 86
 87        self.sprites.extend([
 88            self.laser_base_sprite,
 89            self.left_rotating_laser_sprite,
 90            self.platform_base_sprite,
 91            self.right_rotating_platform_sprite])
 92
 93    def on_update(self, delta_time: float):
 94        # Rotate the laser sprite and change its angle
 95        self.left_rotating_laser_sprite.rotate_around_point(
 96            self.laser_base_sprite.position,
 97            120 * delta_time)
 98
 99        # Rotate the platform sprite but don't change its angle
100        self.right_rotating_platform_sprite.rotate_around_point(
101            self.platform_base_sprite.position,
102            60 * delta_time, False)
103
104    def on_draw(self):
105        # Draw the sprite list.
106        self.clear()
107        self.sprites.draw()
108
109        self.laser_text.draw()
110        self.platform_text.draw()
111
112
113def main():
114    window = ExampleWindow()
115    window.run()
116
117
118if __name__ == '__main__':
119    main()