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Can static bodies apply collision impulses to dynamic bodies?

Here's a little recording of what my code does:

Recording

As you can see, the two dynamic triangles collide with each other and are stopped by the static line. however, the behaviour is not what I want. If I had only one triangle, it would be skidding down the ledge on just one vertex --> the static body does not inflict any sort of torque or counter forces (I'm no physicist) on the triangles.

Does this mean I should just use dynamic bodies as obstacles, with really high mass? My reasoning for using static ones is that I plan on having lots of obstacles in my sim, with dynamic bodies crashing into them. Is it feasible to construct the entire environment using dynamic bodies? What am I missing?

I'm using pymunk and pygame for this btw. Appreciate any help that I can get

import pygame
import pymunk
import pymunk.pygame_util

pygame.init()
screen = pygame.display.set_mode((1000, 500))

space = pymunk.Space()
space.gravity = 0, -0.07

# triangle creation func, takes position arguments
def create_tri(x, y):
    pos = pygame.math.Vector2(x, y)
    points = (0, 0), (50, 0), (25, 50)

    moment = pymunk.moment_for_poly(1, points)
    body = pymunk.Body(1, moment)
    body.position = pos

    shape = pymunk.Poly(body, points)
    return body, shape

# creating 2 triangles
tri2 = create_tri(100, 400)
space.add(tri2[0], tri2[1])

# temporary obstacle setup
line_moment = pymunk.moment_for_segment(0, (0, 0), (600, -300), 10)
line_body = pymunk.Body(10, line_moment, body_type=pymunk.Body.STATIC)
line_body.position = (0, 300)

line_shape = pymunk.Segment(line_body, (0, 0), (600, -300), 10)
space.add(line_shape)

# Main loop
game_running = True
while game_running:
   ev = pygame.event.poll()
   if ev.type == pygame.QUIT:
       pygame.quit()
   screen.fill((255, 255, 255))
   draw_options = pymunk.pygame_util.DrawOptions(screen)
   space.debug_draw(draw_options)
   space.step(0.02)
   pygame.display.flip()
lepsch
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Paul Straberger
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  • Can you rerun with just one triangle to see how it behaves? And, can you add a small example code to reproduce it, then its easier to understand what is wrong and how/if to fix it. – viblo Oct 29 '19 at 15:05
  • @viblo , awesome, thanks for answering! [Here's](https://imgur.com/goCKCq7) a short video of what happens with just one triangle, and I edited the entire code into the post. – Paul Straberger Oct 30 '19 at 12:48

1 Answers1

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The problem is that the center of gravity of the triangle is in the corner (0,0). Meaning all its mass is in that point which is why it doesnt turn and slide down on the side.

One way to fix it is to adjust so that (0,0) is in the middle of the triangle:

points = (-25, -25), (25, -25), (0, 25)

Another way is to transform the vertices of the triangle when creating the shape, by translating them -25 in each direction:

shape = pymunk.Poly(body, points, pymunk.Transform(tx=-25,ty=-25))

viblo
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  • Wow, that worked haha. Thank you so much, I would not have thought about that... If I had initially tested it with the sled facing the other way, I might have gotten a clue. Just one last question, again I don't understand much about physics, but shouldn't the body fall down with the COG vertex pointing down? Anyhow, you solved my question, so this is just a theoretical question. Great answer! – Paul Straberger Oct 31 '19 at 11:58
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    I think it doesnt rotate in air because there's no "air friction" in pymunk. Even IRL a light and a heavy object will fall with the same speed in a vacuum, its the air resistance that make a feather fall more slowly than a stone normally. – viblo Oct 31 '19 at 14:47