# Bullet Physics Collision is Not Detected

I have created two btConvexTriangleMeshShape objects. One is a cube of size
(0.1 x 0.1 x 0.1), and the other is is a plane of size (1.0, 0.1, 1.0). The plane is static, and the cube is dynamic, and should fall onto the plane under gravity.

Now, if I set the world position of the plane to be at (0, 0, 0), and the cube to be at (0, 0.3, 0), then the cube falls onto the plane and rests there, as would be expected. However, I am getting some strange behaviour when I change the position of the plane, even slightly.

For example, if I set the world position of the plane to be at (-0.1, 0, -0.1), with the cube still at (0, 0.3, 0), then the cube falls right through the plane, and there is no collision. But the plane is still directly beneath the cube as it falls, and so a collision should be detected. In fact, if I set the plane to be positioned at anything other than (0, 0, 0), then there is no collision.

The plane's position is set in the code:

btDefaultMotionState* plane_start_motion_state = new btDefaultMotionState(
btTransform(
btQuaternion(0, 0, 0, 1),
btVector3(0, 0, 0)
)
);


And no collision is detected when I use:

btDefaultMotionState* plane_start_motion_state = new btDefaultMotionState(
btTransform(
btQuaternion(0, 0, 0, 1),
btVector3(dx, 0, dz)
)
);


for even small values of dx and dz, other than zero. Any ideas on what is causing this? The minimum code to repeat this is below:

#include <iostream>

#include <Eigen/Eigen>

#include "btBulletDynamicsCommon.h"
#include "BulletCollision/Gimpact/btGImpactShape.h"
#include "BulletCollision/Gimpact/btGImpactCollisionAlgorithm.h"

int main(int argc, char** argv)
{
// Set up Bullet

btDefaultCollisionConfiguration* collision_configuration = new btDefaultCollisionConfiguration();
btCollisionDispatcher* dispatcher = new btCollisionDispatcher(collision_configuration);
btSequentialImpulseConstraintSolver* solver = new btSequentialImpulseConstraintSolver;
btDiscreteDynamicsWorld* dynamics_world = new btDiscreteDynamicsWorld(dispatcher, broadphase, solver, collision_configuration);
dynamics_world->setGravity(btVector3(0, -9.81, 0));

// Make the plane

float plane_dx = 1.0;
float plane_dy = 0.1;
float plane_dz = 1.0;

int plane_num_vertices = 8;
int plane_num_triangles = 12;
int plane_num_indices = plane_num_triangles * 3;

std::vector<Eigen::Vector3f> plane_vertex_positions;
std::vector<int> plane_vertex_indices;

plane_vertex_positions.resize(plane_num_vertices);

plane_vertex_positions[0] << 0, 0, 0;
plane_vertex_positions[1] << 0, -plane_dy, 0;
plane_vertex_positions[2] << 0, -plane_dy, plane_dz;
plane_vertex_positions[3] << 0, 0, plane_dz;
plane_vertex_positions[4] << plane_dx, 0, 0;
plane_vertex_positions[5] << plane_dx, -plane_dy, 0;
plane_vertex_positions[6] << plane_dx, -plane_dy, plane_dz;
plane_vertex_positions[7] << plane_dx, 0, plane_dz;

plane_vertex_indices.resize(plane_num_indices);

plane_vertex_indices[0] = 0;
plane_vertex_indices[1] = 3;
plane_vertex_indices[2] = 2;
plane_vertex_indices[3] = 2;
plane_vertex_indices[4] = 1;
plane_vertex_indices[5] = 0;
plane_vertex_indices[6] = 4;
plane_vertex_indices[7] = 0;
plane_vertex_indices[8] = 1;
plane_vertex_indices[9] = 1;
plane_vertex_indices[10] = 5;
plane_vertex_indices[11] = 4;
plane_vertex_indices[12] = 7;
plane_vertex_indices[13] = 4;
plane_vertex_indices[14] = 5;
plane_vertex_indices[15] = 5;
plane_vertex_indices[16] = 6;
plane_vertex_indices[17] = 7;
plane_vertex_indices[18] = 3;
plane_vertex_indices[19] = 7;
plane_vertex_indices[20] = 6;
plane_vertex_indices[21] = 6;
plane_vertex_indices[22] = 2;
plane_vertex_indices[23] = 3;
plane_vertex_indices[24] = 2;
plane_vertex_indices[25] = 6;
plane_vertex_indices[26] = 5;
plane_vertex_indices[27] = 5;
plane_vertex_indices[28] = 1;
plane_vertex_indices[29] = 2;
plane_vertex_indices[30] = 0;
plane_vertex_indices[31] = 4;
plane_vertex_indices[32] = 7;
plane_vertex_indices[33] = 7;
plane_vertex_indices[34] = 3;
plane_vertex_indices[35] = 0;

btTriangleIndexVertexArray* plane_mesh = new btTriangleIndexVertexArray(plane_num_triangles, plane_vertex_indices.data(), 0, plane_num_vertices, (btScalar*)plane_vertex_positions.data(), 0);
btConvexTriangleMeshShape* plane_shape = new btConvexTriangleMeshShape(plane_mesh);
plane_shape->setLocalScaling(btVector3(1., 1., 1.));
plane_shape->setMargin(0.04f);
btVector3 plane_inertia(0, 0, 0);
btScalar plane_mass(0.0f);
plane_shape->calculateLocalInertia(plane_mass, plane_inertia);
btDefaultMotionState* plane_start_motion_state = new btDefaultMotionState(btTransform(btQuaternion(0, 0, 0, 1), btVector3(0, 0, 0)));
btRigidBody* plane = new btRigidBody(plane_mass, plane_start_motion_state, plane_shape, plane_inertia);
plane->setFriction(0.5);
plane->setRestitution(0.5);

// Make the Cube

float cube_dx = 0.1;
float cube_dy = 0.1;
float cube_dz = 0.1;

int cube_num_vertices = 8;
int cube_num_triangles = 12;
int cube_num_indices = cube_num_triangles * 3;

std::vector<Eigen::Vector3f> cube_vertex_positions;
std::vector<int> cube_vertex_indices;

cube_vertex_positions.resize(cube_num_vertices);

cube_vertex_positions[0] << 0, 0, 0;
cube_vertex_positions[1] << 0, cube_dy, 0;
cube_vertex_positions[2] << 0, cube_dy, cube_dz;
cube_vertex_positions[3] << 0, 0, cube_dz;
cube_vertex_positions[4] << cube_dx, 0, 0;
cube_vertex_positions[5] << cube_dx, cube_dy, 0;
cube_vertex_positions[6] << cube_dx, cube_dy, cube_dz;
cube_vertex_positions[7] << cube_dx, 0, cube_dz;

cube_vertex_indices.resize(cube_num_indices);

cube_vertex_indices[0] = 0;
cube_vertex_indices[1] = 3;
cube_vertex_indices[2] = 2;
cube_vertex_indices[3] = 2;
cube_vertex_indices[4] = 1;
cube_vertex_indices[5] = 0;
cube_vertex_indices[6] = 4;
cube_vertex_indices[7] = 0;
cube_vertex_indices[8] = 1;
cube_vertex_indices[9] = 1;
cube_vertex_indices[10] = 5;
cube_vertex_indices[11] = 4;
cube_vertex_indices[12] = 7;
cube_vertex_indices[13] = 4;
cube_vertex_indices[14] = 5;
cube_vertex_indices[15] = 5;
cube_vertex_indices[16] = 6;
cube_vertex_indices[17] = 7;
cube_vertex_indices[18] = 3;
cube_vertex_indices[19] = 7;
cube_vertex_indices[20] = 6;
cube_vertex_indices[21] = 6;
cube_vertex_indices[22] = 2;
cube_vertex_indices[23] = 3;
cube_vertex_indices[24] = 2;
cube_vertex_indices[25] = 6;
cube_vertex_indices[26] = 5;
cube_vertex_indices[27] = 5;
cube_vertex_indices[28] = 1;
cube_vertex_indices[29] = 2;
cube_vertex_indices[30] = 0;
cube_vertex_indices[31] = 4;
cube_vertex_indices[32] = 7;
cube_vertex_indices[33] = 7;
cube_vertex_indices[34] = 3;
cube_vertex_indices[35] = 0;

btTriangleIndexVertexArray* cube_mesh = new btTriangleIndexVertexArray(cube_num_triangles, cube_vertex_indices.data(), 0, cube_num_vertices, (btScalar*)cube_vertex_positions.data(), 0);
btConvexTriangleMeshShape* cube_shape = new btConvexTriangleMeshShape(cube_mesh);
cube_shape->setLocalScaling(btVector3(1., 1., 1.));
cube_shape->setMargin(0.04f);
btVector3 cube_inertia(0, 0, 0);
btScalar cube_mass(1.0f);
cube_shape->calculateLocalInertia(cube_mass, cube_inertia);
btDefaultMotionState* cube_start_motion_state = new btDefaultMotionState(btTransform(btQuaternion(0, 0, 0, 1), btVector3(0, 3, 0)));
btRigidBody* cube = new btRigidBody(cube_mass, cube_start_motion_state, cube_shape, cube_inertia);
cube->setFriction(0.5);
cube->setRestitution(0.5);

float t = 0.00001f;

for (int i = 0; i < 10000000; i++)
{
dynamics_world->stepSimulation(t, 10, t / 10.0f);
btTransform cube_transform;
cube->getMotionState()->getWorldTransform(cube_transform);
std::cout << "Cube position = " << cube_transform.getOrigin().getY() << std::endl;
}
}


Using btConvexTriangleMeshShape as a dynamic collision shape isn't recommended. I would use a cube and a plane using the classes btBoxShape and btStaticPlaneShape respectively. They have distinct properties (boxes are solids, planes are one sided) that give better collision response. Colliding two triangle meshes together is unlikely to be supported well by Bullet.

Furthermore, btConvexTriangleMeshShape is for convex triangle hulls. A plane is not a convex shape.

You can read the author's response here: http://www.bulletphysics.org/Bullet/phpBB3/viewtopic.php?f=9&t=2927

Mesh versus mesh is not implemented for btBvhTriangleMeshShape, because btBvhTriangleMeshShape is supposed to be used only for 'static' world geometry without collision detection between static versus static.

While it's a btBvhTriangleMeshShape I expect the same (unsupported) results with btConvexTriangleMeshShape because the underlying colliders are very similar.

• Thanks very much for your answer. What I actually wanted to achieve originally, is a collision between two arbitrary (concave) triangles meshes; the example above is just a toy demo to show my problem. From what I've gathered, btGImpactMeshShape allows for this, although I was not able to get it working. Therefore, I tried btConvexTriangleMeshShape as an approximation to the shape, but this then gave me the problem I mentioned above. I have tried your suggestion, and using btBoxShape for both objects does not detect a collision, but I want to be able to extend this to triangle meshes... – Karnivaurus Dec 31 '15 at 3:41
• @Karnivaurus - with boxes it still does not detect collisions? Can you use one of the bullet samples and start from there? – Steven Dec 31 '15 at 3:46
• Oh sorry, that was a typo -- yes, using btBoxShape does detect a collision. But this is very simple, and for my work I need to do collisions between meshes... – Karnivaurus Dec 31 '15 at 3:48
• Right - so I think the issue is that mesh v mesh isn't going to work. Can you represent the mesh as a convex hull? In that case btConvexHullShapes can collide with one another. – Steven Dec 31 '15 at 3:56
• I've read in various places that Bullet uses GImpact for mesh vs mesh collisions, although I cannot find any full examples anywhere... I tried representing the mesh as a btConvexTriangleMeshShape which is what I have in my original post, but that gave the strange behaviour I discussed. Some of my objects are concave and I very much need to keep the concavity in the mesh. The project is actually simulating a robot hand picking up an object, where the object can be any shape, and most likely concave. I could probably use a btBoxShape for the fingers, but not for the object... – Karnivaurus Dec 31 '15 at 4:08