How to keep one body on top of another with Pymunk

Question

I have a circle with a box on top:

A circle is a simple engine. I want the box to stay right above the circle. I tried various restrictions, but most of my attempts force the box to stand aside.

My most successful attempt was to set box body.moment to pymunk.inf and snap the field to a circle. This is getting closer, but the box is still moving side to side when I want to right above the center of the circle. I could manually install it there, but it seems to me that I can do this with some kind of restriction.

Any ideas? The following is sample code using the Pymunk and Arcade libraries.

import arcade
import pymunk
import math

SCREEN_WIDTH = 1200
SCREEN_HEIGHT = 800
BOX_SIZE = 45

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

    def __init__(self, width, height):
        super().__init__(width, height)
        arcade.set_background_color(arcade.color.DARK_SLATE_GRAY)

        # -- Pymunk space
        self.space = pymunk.Space()
        self.space.gravity = (0.0, -900.0)

        # Create the floor
        body = pymunk.Body(body_type=pymunk.Body.STATIC)
        self.floor = pymunk.Segment(body, [0, 10], [SCREEN_WIDTH, 10], 0.0)
        self.floor.friction = 10
        self.space.add(self.floor)

        # Create the circle
        player_x = 300
        player_y = 300
        mass = 2
        radius = 25
        inertia = pymunk.moment_for_circle(mass, 0, radius, (0, 0))
        circle_body = pymunk.Body(mass, inertia)
        circle_body.position = pymunk.Vec2d(player_x, player_y)
        self.circle_shape = pymunk.Circle(circle_body, radius, pymunk.Vec2d(0, 0))
        self.circle_shape.friction = 1

        self.space.add(circle_body, self.circle_shape)

        # Create the box
        size = BOX_SIZE
        mass = 5
        moment = pymunk.moment_for_box(mass, (size, size))
        moment = pymunk.inf
        body = pymunk.Body(mass, moment)
        body.position = pymunk.Vec2d(player_x, player_y + 49)
        self.box_shape = pymunk.Poly.create_box(body, (size, size))
        self.box_shape.friction = 0.3
        self.space.add(body, self.box_shape)

        # Create a joint between them
        constraint = pymunk.constraint.PinJoint(self.box_shape.body, self.circle_shape.body)
        self.space.add(constraint)

        # Make the circle rotate
        constraint = pymunk.constraint.SimpleMotor(self.circle_shape.body, self.box_shape.body, -3)
        self.space.add(constraint)

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

        # Draw circle
        arcade.draw_circle_outline(self.circle_shape.body.position[0],
                                   self.circle_shape.body.position[1],
                                   self.circle_shape.radius,
                                   arcade.color.WHITE,
                                   2)

        # Draw box
        arcade.draw_rectangle_outline(self.box_shape.body.position[0],
                                      self.box_shape.body.position[1],
                                      BOX_SIZE,
                                      BOX_SIZE,
                                      arcade.color.WHITE, 2,
                                      tilt_angle=math.degrees(self.box_shape.body.angle))

        # Draw floor
        pv1 = self.floor.body.position + self.floor.a.rotated(self.floor.body.angle)
        pv2 = self.floor.body.position + self.floor.b.rotated(self.floor.body.angle)
        arcade.draw_line(pv1.x, pv1.y, pv2.x, pv2.y, arcade.color.WHITE, 2)

    def animate(self, delta_time):

        # Update physics
        self.space.step(1 / 80.0)

window = MyApplication(SCREEN_WIDTH, SCREEN_HEIGHT)

arcade.run()

Answer 1

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# Create a joint between them
constraint = pymunk.constraint.PinJoint(self.box_shape.body, self.circle_shape.body, (-20,0))
self.space.add(constraint)

constraint = pymunk.constraint.PinJoint(self.box_shape.body, self.circle_shape.body, (20,0))
self.space.add(constraint)

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