Real-Time Physics Simulation Forum

General Bullet Physics Support and Feedback • info/help about capsule controller and 3d Animated Model

2026-02-26T17:30:19+00:00

Hi,
I'm working on a small 3d game engine project, OpenGL based. At present, I'm experimenting a little bit to integrate Bullet as physics subsystem.
My scene is composed of a skinned model with a bunch of animations (from Mixamo) and a box, scaled to resamble a ground plane.
In my Physics class I implemented a function, called AddCapsuleController(), within I create a capsule collision shape, set up radius and height, then I "link" rigid body and 3d object, using an std::map data structure.
In the main loop, after all the systems are updated, I invoke the SynchPhysAndRender function to allign physics with graphics.
But I'm encountering some issue with the Capsule Collider and the Animated model. There is something wrong in my attempt to put in synch rigidbody and the graphical rapresentation of the model. Infact, my player character is shifted some unit upward from the ground.
So I'm stuck, but I've discovered a naive solution. It consists in an offset value (glm::vec3), which is used into the aformentioned SynchPhysAndRender(): the updated position of the associated rigidbody is integrated by the offset, so that the model is rendered in the correct spot, from the point of view of my 3d Scene.
Obviously, I think this isn't the right solution.
So I'd like to ask if someone could help me to solve that kind of issue. What could be wrong, conceptually speaking, in this kind of approach?
Thanks in advance and sorry if I can't be able to describe the problem quite good as I should like.

Cosmo

Statistics: Posted by Cosmo — Thu Feb 26, 2026 5:30 pm

General Bullet Physics Support and Feedback • Re: m_collisionFlags !=btCollisionObject::CF_STATIC_OBJECT ?

2025-05-28T03:58:40+00:00

You should not set the linear velocity for static objects. Only dynamic and/or kinematic objects can ever move.
Thanks,
Erwin

I have a question. What is the meaning of setting velocity on a kinematic body? The world will not move this body, only user code, correct?

Thanks,
Morgan

Statistics: Posted by goosesensor — Wed May 28, 2025 3:58 am

General Bullet Physics Support and Feedback • Re: URDF importing correctly on one machine, incorrectly on another. Same code.

2025-05-02T07:25:34+00:00

Sprunki Game Have you tried compiling Bullet from source with debug flags on Machine B to trace the import process?

Statistics: Posted by otis — Fri May 02, 2025 7:25 am

General Bullet Physics Support and Feedback • Re: Incorrect triangle-triangle collision detected in GI

2025-03-14T03:14:16+00:00

Hi Otis,

We found that if we modify

Code:

btPrimitiveTriangle::overlap_test_conservative

- it currently only checks if all points of one triangle lie one on the positive side of the plane formed by the other (line 82). When we modify this check to also check of the points are all on the negative side of the plane, this functions as expected.

I.e. change:

Code:

if (dis0 > 0.0f && dis1 > 0.0f && dis2 > 0.0f) return false;

Code:

if (dis0 > 0.0f && dis1 > 0.0f && dis2 > 0.0f) return false;if (dis0 < 0.0f && dis1 < 0.0f && dis2 < 0.0f) return false;

This then seems to function as expected. We haven't been able to rigorously test this yet, and also it wouldn't take margins into account. Another potential fix would be to just check if any of the 3 points are within "margin" distance of the plane formed by the other triangle?

Statistics: Posted by ajshort — Fri Mar 14, 2025 3:14 am

General Bullet Physics Support and Feedback • Re: crash at btConvexHullShape::localGetSupportingVertexWithoutMargin

2025-03-05T01:55:54+00:00

I downloaded the latest version and now it seems to be ok. I no longer get the crash Mapquest Directions

that's awesome i will try to download it.

Statistics: Posted by otis — Wed Mar 05, 2025 1:55 am

General Bullet Physics Support and Feedback • Re: SDF link collisions change robot behavior, despite no interaction with robot

2025-03-05T01:52:11+00:00

Is PyBullet recalculating physics differently when objects are moved manually in GUI mode?
Geometry Dash

Statistics: Posted by lorde — Wed Mar 05, 2025 1:52 am

General Bullet Physics Support and Feedback • Re: Incorrect triangle-triangle collision detected in GI

2025-03-04T09:06:29+00:00

Hey ajshort, in your minimal code isolating the false positive collision with those two triangles,Retro Bowl have you tried tweaking the margin values or plane equations in `buildTriPlane()` to see if it resolves the SAT method’s over-detection?

Statistics: Posted by otis — Tue Mar 04, 2025 9:06 am

General Bullet Physics Support and Feedback • Incorrect triangle-triangle collision detected in GI

2025-01-29T10:17:10+00:00

Hi all! We're using gimpact for mesh-to-mesh collisions and have come across a case where two triangles are detecting a false positive collision. I've managed to isolate this pretty well to the two triangles in the STL file linked and pictured below - as you can see these aren't colliding, but are reported as a collision.

https://gist.github.com/ajshort/f032837 ... 60d66acffa

I've made some minimal code inside the btGImpactCollisionAlgorithm class below which reports a collision between these two triangles manually defined. Does anyone have any advice or idea as to the cause? Also if I define "BULLET_TRIANGLE_COLLISION" so the GJK rather than SAT implementation is used, the result is as expected.

Code:

btPrimitiveTriangle ta;ta.m_vertices[0] = btVector3(4049.92, 12169.6, 2252.3);ta.m_vertices[1] = btVector3(4083.47, 12166.3, 2258.52);ta.m_vertices[2] = btVector3(4050.78, 12191, 2238.16);ta.buildTriPlane();btPrimitiveTriangle tb;tb.m_vertices[0] = btVector3(5189.17, 11358, 500.004);tb.m_vertices[1] = btVector3(3162.31, 11358, 2724.61);tb.m_vertices[2] = btVector3(3162.31, 12642, 2724.61);tb.buildTriPlane();GIM_TRIANGLE_CONTACT gtc;if (ta.overlap_test_conservative(tb) && ta.find_triangle_collision_clip_method(tb, gtc)) {std::cout << "A collision has been detected using SAT method" << std::endl;} else {std::cout << "No collision detected using SAT method" << std::endl;}

Statistics: Posted by ajshort — Wed Jan 29, 2025 10:17 am

General Bullet Physics Support and Feedback • Re: crash at btConvexHullShape::localGetSupportingVertexWithoutMargin

2024-01-12T01:28:31+00:00

I don't use btStaticPlaneShape but in the latest update this error has been fixed.

geometry dash world

Statistics: Posted by shallshe — Fri Jan 12, 2024 1:28 am

General Bullet Physics Support and Feedback • SDF link collisions change robot behavior, despite no interaction with robot

2024-01-05T19:17:28+00:00

I created a robot that sends motor values to joints at each step of the simulation. I also generate a grid of SDF links. For some reason, the behavior changes when if I drag the SDF links in GUI mode, despite no interaction between them and the robot. Why could this be? I've included relevant code:

generate.py:
--------------------------------------------
import pyrosim.pyrosim as pyrosim
def Create_World():
pyrosim.Start_SDF("world.sdf")

for i in range(10):
for j in range(10):
pyrosim.Send_Cube(name=f"Box{i}", pos=[-10+i*1.5,10+j*1.5,.51] , size=[1,1,1])
pyrosim.End()

def Create_Robot():
pyrosim.Start_URDF("body.urdf")
pyrosim.Send_Cube(name="Torso", pos=[0,0,1.5] , size=[1,1,1])
pyrosim.Send_Joint( name = "Torso_FrontLeg" , parent= "Torso" , child = "FrontLeg" , type = "revolute", position = [0.5,0,1])
pyrosim.Send_Cube(name="FrontLeg", pos=[0.5,0,-0.5] , size=[1,1,1])

pyrosim.Send_Joint( name = "Torso_BackLeg" , parent= "Torso" , child = "BackLeg" , type = "revolute", position = [-0.5,0,1])
pyrosim.Send_Cube(name="BackLeg", pos=[-0.5,0,-0.5] , size=[1,1,1])

pyrosim.End()

Create_World()
Create_Robot()

simulate.py:
----------------------------------
import pybullet_data
import pybullet as p
import time
import pyrosim.pyrosim as pyrosim
import numpy as np
import random

physicsClient = p.connect(p.GUI)
p.setAdditionalSearchPath(pybullet_data.getDataPath())

p.configureDebugVisualizer(p.COV_ENABLE_GUI,0)

p.setGravity(0,0,-9.

planeId = p.loadURDF("plane.urdf")
p.loadSDF("world.sdf")
robotId = p.loadURDF("body.urdf")

pyrosim.Prepare_To_Simulate(robotId)

backLegSensorValues = np.zeros(1000)
frontTargetAngles = np.zeros(1000)
backTargetAngles = np.zeros(1000)

frontAmplitude = np.pi/4
frontFrequency = 10
frontPhaseOffset = 0

backAmplitude = np.pi/4
backFrequency = 10
backPhaseOffset = np.pi/3

stateOfLinkZero = p.getBasePositionAndOrientation(robotId)
positionOfLinkZero = stateOfLinkZero[0]
xCoord = positionOfLinkZero[0]
print('Initial X Coordinate= ', xCoord)

for i in range(1000):

p.stepSimulation()
frontTargetAngles = frontAmplitude * np.sin(frontFrequency * i + frontPhaseOffset)
backTargetAngles = backAmplitude * np.sin(backFrequency * i + backPhaseOffset)

pyrosim.Set_Motor_For_Joint(
bodyIndex = robotId,
jointName = "Torso_FrontLeg",
controlMode = p.POSITION_CONTROL,
targetPosition = frontTargetAngles,
maxForce = 500)

pyrosim.Set_Motor_For_Joint(
bodyIndex = robotId,
jointName = "Torso_BackLeg",
controlMode = p.POSITION_CONTROL,
targetPosition = backTargetAngles,
maxForce = 500)

time.sleep(1/4000)

if i%100 == 0 or i == 999:
stateOfLinkZero = p.getBasePositionAndOrientation(robotId)
positionOfLinkZero = stateOfLinkZero[0]
xCoord = positionOfLinkZero[0]
print('X Coordinate= ', xCoord)

p.disconnect()

Statistics: Posted by joro4o — Fri Jan 05, 2024 7:17 pm