Sprite Rotation Around a Tank

This example uses a player-controlled tank to demonstrate the difference between the right and wrong way of rotating a turret around an attachment point. In both modes, the tank’s barrel follows the mouse. The turret sprite is ommitted to keep the rotation visible.

See the docstring, comments, and on screen instructions for further info.

sprite_rotation_around_tank.py

"""
Sprite Rotation Around a Point, With A Tank

Games often include elements that rotate toward targets. Common
examples include gun turrets on vehicles and towers. In 2D games,
these rotating parts are usually implemented as sprites that move
relative to whatever they're attached to.

There's a catch to this: you have to rotate these parts around their
attachment points rather than the centers of their sprites. Otherwise,
the rotation will look wrong!

To illustrate the difference, this example uses a player-controllable
tank with a barrel that follows the mouse. You can press P to switch
between two ways of rotating the barrel:
1. Correctly, with the barrel's rear against the tank's center
2. Incorrectly, around the barrel's center pinned to the tank's 

Artwork from https://kenney.nl

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


TANK_SPEED_PIXELS = 64  # How many pixels per second the tank travels
TANK_TURN_SPEED_DEGREES = 70  # How fast the tank's body can turn


# This is half the length of the barrel sprite.
# We use it to ensure the barrel's rear sits in the middle of the tank
TANK_BARREL_LENGTH_HALF = 15  


SCREEN_WIDTH = 800
SCREEN_HEIGHT = 600
SCREEN_MIDDLE = (SCREEN_WIDTH // 2, SCREEN_HEIGHT // 2)


SCREEN_TITLE = "Rotating Tank Example"


# These paths are built-in resources included with arcade
TANK_BODY = ":resources:images/topdown_tanks/tankBody_dark_outline.png"
TANK_BARREL = ":resources:images/topdown_tanks/tankDark_barrel3_outline.png"


class RotatingSprite(arcade.Sprite):
    """
    Sprite subclass which can be rotated around a point.

    This version of the class always changes the angle of the sprite.
    Other games might not rotate the sprite. For example, moving
    platforms in a platformer wouldn't rotate.
    """
    def rotate_around_point(self, point: arcade.Point, degrees: float):
        """
        Rotate the sprite around a point by the set amount of degrees

        :param point: The point that the sprite will rotate about
        :param degrees: How many degrees to rotate the sprite
        """

        # Make the sprite turn as its position is moved
        self.angle += degrees

        # Move the sprite along a circle centered around the passed point
        self.position = arcade.rotate_point(
            self.center_x, self.center_y,
            point[0], point[1], degrees)


class ExampleWindow(arcade.Window):

    def __init__(self):
        super().__init__(SCREEN_WIDTH, SCREEN_HEIGHT, SCREEN_TITLE)

        # Set Background to be green
        self.background_color = arcade.csscolor.SEA_GREEN

        # The tank and barrel sprite
        self.tank = arcade.Sprite(TANK_BODY)
        self.tank.position = SCREEN_MIDDLE

        self.barrel = RotatingSprite(TANK_BARREL)
        self.barrel.position =\
            SCREEN_MIDDLE[0], SCREEN_MIDDLE[1] - TANK_BARREL_LENGTH_HALF

        self.tank_direction = 0.0  # Forward & backward throttle 
        self.tank_turning = 0.0  # Turning strength to the left or right

        self.mouse_pos = 0, 0

        self.tank_sprite_list = arcade.SpriteList()
        self.tank_sprite_list.extend([self.tank, self.barrel])

        self._correct = True
        self.correct_text = arcade.Text(
            "Turret Rotation is Correct, Press P to Switch",
            SCREEN_MIDDLE[0], SCREEN_HEIGHT - 25,
            anchor_x='center')

        self.control_text = arcade.Text(
            "WASD to move tank, Mouse to aim",
            SCREEN_MIDDLE[0], 15,
            anchor_x='center')

    def on_draw(self):
        self.clear()
        self.tank_sprite_list.draw()

        self.control_text.draw()
        self.correct_text.draw()

    def on_update(self, delta_time: float):
        self.move_tank(delta_time)

    def move_tank(self, delta_time):
        """
        Perform all calculations for moving the tank's body and barrel 
        """

        # Rotate the tank's body in place without changing position
        # We'll rotate the barrel after updating the entire tank's x & y
        self.tank.angle += TANK_TURN_SPEED_DEGREES\
            * self.tank_turning * delta_time

        # Calculate how much the tank should move forward or back
        move_magnitude = self.tank_direction * TANK_SPEED_PIXELS * delta_time
        x_dir = math.cos(self.tank.radians - math.pi / 2) * move_magnitude
        y_dir = math.sin(self.tank.radians - math.pi / 2) * move_magnitude

        # Move the tank's body
        self.tank.position =\
            self.tank.center_x + x_dir,\
            self.tank.center_y + y_dir

        # Move the barrel with the body 
        self.barrel.position =\
            self.barrel.center_x + x_dir,\
            self.barrel.center_y + y_dir

        # Begin rotating the barrel by finding the angle to the mouse 
        mouse_angle = arcade.get_angle_degrees(
            self.tank.center_y, self.tank.center_x,
            self.mouse_pos[1], self.mouse_pos[0])

        # Compensate for the vertical orientation of the barrel texture
        # This could be skipped if the texture faced right instead
        mouse_angle += 90

        if self.correct:
            # Rotate the barrel sprite with one end at the tank's center

            # Subtract the old angle to get the change in angle
            angle_change = mouse_angle - self.barrel.angle

            self.barrel.rotate_around_point(self.tank.position, angle_change)
        else:
            # Swivel the barrel with its center aligned with the body's 
            self.barrel.angle = mouse_angle

    def on_key_press(self, symbol: int, modifiers: int):
        if symbol == arcade.key.W:
            self.tank_direction += 1
        elif symbol == arcade.key.S:
            self.tank_direction -= 1
        elif symbol == arcade.key.A:
            self.tank_turning += 1
        elif symbol == arcade.key.D:
            self.tank_turning -= 1
        elif symbol == arcade.key.P:
            self.correct = not self.correct

            self.correct_text.text =\
                f"Turret Rotation is "\
                f"{'Correct' if self.correct else 'Incorrect'},"\
                f" Press P to Switch"

    def on_key_release(self, symbol: int, modifiers: int):
        if symbol == arcade.key.W:
            self.tank_direction -= 1
        elif symbol == arcade.key.S:
            self.tank_direction += 1
        elif symbol == arcade.key.A:
            self.tank_turning -= 1
        elif symbol == arcade.key.D:
            self.tank_turning += 1

    def on_mouse_motion(self, x: int, y: int, dx: int, dy: int):
        self.mouse_pos = x, y

    @property
    def correct(self):
        return self._correct

    @correct.setter
    def correct(self, correct: bool):
        """
        Move the tank's barrel between correct rotation and incorrect positions
        """
        self._correct = correct
        if correct:
            angle = arcade.get_angle_radians(
                self.tank.center_y, self.tank.center_x,
                self.mouse_pos[1], self.mouse_pos[0])

            self.barrel.position =\
                self.barrel.center_x + math.cos(angle) * TANK_BARREL_LENGTH_HALF,\
                self.barrel.center_y + math.sin(angle) * TANK_BARREL_LENGTH_HALF

        else:
            self.barrel.position = self.tank.position


def main():
    window = ExampleWindow()
    window.run()


if __name__ == '__main__':
    main()