花栗鼠 Servo 关节

你如何在Pymunk中实现“ Servo ”关节？

我'm trying to create a simple model where a box is balanced on a single thin 1385092 below it. I'已经能够创建一个盒子并使用PinJoint将其连接到地面，但是没有't seem to be any way to control the angle where the join attaches to the box. I want to be able to specify the angle of attachment. None of the other joints seem to support this. They all seem to be passive joints, with the exception of the SimpleMotor joint, but even that is only a constant spinning joint that you can't控制 .

我已经设法将一些东西拼凑在一起，通过使用PinJoint将两个薄盒子连接在一起，以及一个SimpleMotor，使它们相对于彼此旋转以响应用户按下“向上”和“向下” “ 方向键 . 以下是代码：

import sys

import pygame
from pygame.locals import USEREVENT, QUIT, KEYDOWN, KEYUP, K_s, K_r, K_q, K_ESCAPE, K_UP, K_DOWN
from pygame.color import THECOLORS

import pymunk
from pymunk import Vec2d
import pymunk.pygame_util

class Simulator(object):

    def __init__(self):
        self.display_flags = 0
        self.display_size = (600, 600)

        self.space = pymunk.Space()
        self.space.gravity = (0.0, -1900.0)
        self.space.damping = 0.999 # to prevent it from blowing up.

        # Pymunk physics coordinates start from the lower right-hand corner of the screen.
        self.ground_y = 100
        ground = pymunk.Segment(self.space.static_body, (5, self.ground_y), (595, self.ground_y), 1.0)
        ground.friction = 1.0
        self.space.add(ground)

        self.screen = None

        self.draw_options = None

    def reset_bodies(self):
        for body in self.space.bodies:
            if not hasattr(body, 'start_position'):
                continue
            body.position = Vec2d(body.start_position)
            body.force = 0, 0
            body.torque = 0
            body.velocity = 0, 0
            body.angular_velocity = 0
            body.angle = body.start_angle

    def draw(self):
        ### Clear the screen
        self.screen.fill(THECOLORS["white"])

        ### Draw space
        self.space.debug_draw(self.draw_options)

        ### All done, lets flip the display
        pygame.display.flip()

    def main(self):

        pygame.init()
        self.screen = pygame.display.set_mode(self.display_size, self.display_flags)
        width, height = self.screen.get_size()
        self.draw_options = pymunk.pygame_util.DrawOptions(self.screen)

        def to_pygame(p):
            """Small hack to convert pymunk to pygame coordinates"""
            return int(p.x), int(-p.y+height)
        def from_pygame(p):
            return to_pygame(p)

        clock = pygame.time.Clock()
        running = True
        font = pygame.font.Font(None, 16)

        # Create the torso box.
        box_width = 50
        box_height = 100
        leg_length = 100

        mass = 1
        points = [(-100, -1), (0, -1), (0, 1), (-100, 1)]
        moment = pymunk.moment_for_poly(mass, points)
        body1 = pymunk.Body(mass, moment)
        # body1.position = (0, 0)
        body1.position = (self.display_size[0]/2, self.ground_y+100)
        body1.start_position = Vec2d(body1.position)
        body1.start_angle = body1.angle
        shape1 = pymunk.Poly(body1, points)
        shape1.friction = 0.8
        self.space.add(body1, shape1)

        # Create bar 2 extending from the right to the origin.
        mass = 1
        points = [(100, -1), (0, -1), (0, 1), (100, 1)]
        moment = pymunk.moment_for_poly(mass, points)
        body2 = pymunk.Body(mass, moment)
        # body2.position = (0, 0)
        body2.position = (self.display_size[0]/2, self.ground_y+100)
        body2.start_position = Vec2d(body2.position)
        body2.start_angle = body2.angle
        shape2 = pymunk.Poly(body2, points)
        shape2.friction = 0.8
        self.space.add(body2, shape2)

        # Link bars together at end.
        pj = pymunk.PinJoint(body1, body2, (0, 0), (0, 0))
        self.space.add(pj)

        motor_joint = pymunk.SimpleMotor(body1, body2, 0)
        self.space.add(motor_joint)

        pygame.time.set_timer(USEREVENT+1, 70000) # apply force
        pygame.time.set_timer(USEREVENT+2, 120000) # reset
        pygame.event.post(pygame.event.Event(USEREVENT+1))
        pygame.mouse.set_visible(False)

        simulate = False
        while running:
            for event in pygame.event.get():
                if event.type == QUIT or (event.type == KEYDOWN and event.key in (K_q, K_ESCAPE)):
                    #running = False
                    sys.exit(0)
                elif event.type == KEYDOWN and event.key == K_s:
                    # Start/stop simulation.
                    simulate = not simulate
                elif event.type == KEYDOWN and event.key == K_r:
                    # Reset.
                    # simulate = False
                    self.reset_bodies()
                elif event.type == KEYDOWN and event.key == K_UP:
                    motor_joint.rate = 1
                elif event.type == KEYDOWN and event.key == K_DOWN:
                    motor_joint.rate = -1
                elif event.type == KEYUP:
                    motor_joint.rate = 0

            self.draw()

            ### Update physics
            fps = 50
            iterations = 25
            dt = 1.0/float(fps)/float(iterations)
            if simulate:
                for x in range(iterations): # 10 iterations to get a more stable simulation
                    self.space.step(dt)

            pygame.display.flip()
            clock.tick(fps)

if __name__ == '__main__':
    sim = Simulator()
    sim.main()

但是，这种行为有点奇怪 . 当你向上/向下按压时，它会动态设置SimpleMotor关节上的速率，使两个盒子在它们共同的“ Servo 关节”处转动，如：

除了随着时间的推移，两个杆将翻转到一端，否则无视重力，看起来像：

为什么是这样？我仍然是Pymunk / Chipmunk物理模拟器的新手，所以我不确定我是否正确使用这些关节 .