PhysicsDirectBodyState2D: Reported values of get_contact_impulse not as expected

[stephanbogner]

Tested versions

Tested in:

• v4.2.1.stable.official [b09f793]
• v4.0.4.stable.official [fc0b241]

System information

Godot v4.2.1.stable - macOS 14.1.2 - Vulkan (Forward+) - integrated Apple M2 Pro - Apple M2 Pro (10 Threads)

Issue description

Context:

I try to make breakable objects and wanted to use get_contact_impulse for this case do detect "hits".

Problem:

How strong the impulses are seem almost random ... but is hard to test in a reproducible way

My physics lab setup 😂:

• All boxes are the same (mass, gravity scale, etc.)
• The top text shows the highest recorded accumulated impulse (sum of all impulses in one _integrate_forces-cycle)
• The lower text shows the highest recorded speed (length of linear-velocity)
• Distances setup:
  • The first box has no distance to the ground
  • The second has 1px distance
  • The next three are arbitrary
  • The last two have the same distance to the ground

Outcome:

• Velocity does not linearly correlate to impulse (but it should ... I think?):
  • Box 1 has no speed but an impulse
  • Box 2 has a impulse/speed ratio of 1.55
  • Box 3 has a impulse/speed ratio of 1.19
  • Box 6 and 7 don't have the same impulse but I guess they should as they have the same fall height and weight (since I sum impulses)

Note

I am not 100% sure if these are bugs or I am misunderstanding the concept of what get_contact_impulse is reporting.

Steps to reproduce

I recommend making a similar physics setup and playing around. Make sure you turn on contact_monitoring and set a value for max_contacts_reported.

I used the following code to track values:

extends RigidBody2D

var highest_speed := 0.0
var highest_impulse := 0.0

func _physics_process(delta):
	var speed = linear_velocity.length()
	if speed > highest_speed:
		highest_speed = speed
		$Velocity.text = str(snapped(highest_speed, 0.1))

func _integrate_forces(state):
	var total_impulse := 0.0
	for i in range(state.get_contact_count()):
		var collided_obj = state.get_contact_collider_object(i)
		var impulse = state.get_contact_impulse(i).length()
		total_impulse += impulse
	
	if total_impulse > highest_impulse:
		highest_impulse = total_impulse
		$Impulse.text = str(snapped(highest_impulse, 0.1))

Minimal reproduction project (MRP)

bug-contact-impulse.zip

[stephanbogner]

A different setup (bug-contact-impulse.zip):

• All objects have the exact same wonky shape, weight and mass ...
• ... but are rotated a bit (between 0° and 5°)...
• ... which makes the impulses differ widely on impact

[rburing]

Thanks for the MRP!

I haven't looked into this myself but others have made PRs for it. Can you test the PR #81654 and comment on whether it fixes the issue?

[stephanbogner]

@rburing Thanks for the reply.

I built from source (the other approaches mentioned on the page didn't work) and now generally it's much more in line with what I'd expect as an outcome ... but since I don't know enough about actual physics it's hard to say whether it's correct now or not though, but below are the results from my tests.

If you have a more clever idea for how to test properly I'd be happy to make more tests.