Pymunk hitreg issue

Question

I want to create a pinball game but the ball sometimes doesn't collide with other objects. example: https://youtu.be/HwSXwJ4-d2w

Here's the code

import pyglet, pymunk
from pymunk.pyglet_util import DrawOptions

win = pyglet.window.Window(1280, 720, resizable=False)
options = DrawOptions()

space = pymunk.Space()
space.gravity = 0, -1000



def Ball(mass, radius, coords):
    circle_moment = pymunk.moment_for_circle(mass, 0, radius)
    circle_body = pymunk.Body(mass, circle_moment)
    circle_shape = pymunk.Circle(circle_body, radius)    
    circle_shape.elasticity = 1.0    
    circle_body.position= coords
    space.add(circle_body, circle_shape)

def BouncyCircle(mass, coords, radius):
    circle_moment = pymunk.moment_for_circle(mass, 0, radius)
    circle_body = pymunk.Body(mass, circle_moment, pymunk.Body.STATIC)
    circle_shape = pymunk.Circle(circle_body, radius)    
    circle_shape.elasticity = 2.0
    circle_body.position= coords
    space.add(circle_body, circle_shape)

def Segment(mass, PointA, PointB, thickness):
    segment_moment = pymunk.moment_for_segment(mass, PointA, PointB, thickness)
    segment_body = pymunk.Body(mass, segment_moment, pymunk.Body.STATIC)
    segment_shape = pymunk.Segment(segment_body, PointA, PointB, thickness)
    segment_shape.elasticity = 0.7
    segment_body.position = 0,0
    space.add(segment_body, segment_shape)



Ball(0.1, 15, (640, 550)) #Falling Ball

BouncyCircle(1, (650,100), 40) #Ball

Segment(10, (105,55), (1195,55), 5) #Border
Segment(10, (100, 50), (100,680), 5)
Segment(10, (105, 675), (1195,675), 5)
Segment(10, (1200, 50), (1200, 680), 5)


@win.event

def on_draw():
    win.clear()
    space.debug_draw(options)

def update(dt):
    space.step(dt)


if __name__ == "__main__":
    pyglet.clock.schedule_interval(update, 1/60)
    pyglet.app.run()

It also happens when it's very slow and sometimes it bounces 10 times until it stops working. Does anybody know how to fix it?

by the way this is not exactly the same code, I lost it but this is the same scenario with the same problem occuring. — Stefan Förster, Oct 15 '18 at 17:51

score 0 · Answer 1 · answered May 20 '19 at 20:50

Its most likely because the ball is moving too fast. If a object, such as the ball, moves from one side of the wall to the other side of the wall in a single simulation step (a call to space.step()) the ball will tunnel through.

There are several ways to mitigate this problem. Sometimes it might be a good idea to do more than one of these.

Make sure the velocity of objects never get too high. One way to do that is to use a custom velocity function with a limit built in on the bodies that have a tendency to move too fast:

def limit_velocity(body, gravity, damping, dt):
    max_velocity = 1000
    pymunk.Body.update_velocity(body, gravity, damping, dt)
    l = body.velocity.length
    if l > max_velocity:
        scale = max_velocity / l
        body.velocity = body.velocity * scale

body_to_limit.velocity_func = limit_velocity

Depending on the requirements it might make more sense to clamp the velocity over multiple frames instead. Then the limit function could look like this instead:

def limit_velocity(body, gravity, damping, dt):
    max_velocity = 1000
    pymunk.Body.update_velocity(body, gravity, damping, dt)
    if body.velocity.length > max_velocity:
        body.velocity = body.velocity * 0.99

Use a smaller value for dt in the call to space.step. A simple way is to call space.step multiple times each frame in your application. This will also help to make the overall simulation more stable.

I think 1 or 2 is the best option for you.

I updated the pymunk docs with this explanation + a little more here: http://www.pymunk.org/en/latest/overview.html#object-tunneling

Pymunk hitreg issue

1 Answers1