I'm trying to use Bullet Physics for collision detection only. I don't need it to move any objects for me or handle rendering with callbacks. I just want to update object locations every frame and use it to tell me when I have collisions. To get the simplest example going, I'm trying to find collisions between objects with btBoxShape as their shape. Everything runs fine without crashes or apparent memory leaks, but I get no collisions so I must be making some mistakes somewhere. I'll try to keep this as brief as I can without leaving anything important out.
Here's my world setup function:
collisionConfig = new btDefaultCollisionConfiguration();
dispatcher = new btCollisionDispatcher(collisionConfig);
overlappingPairCache = new btDbvtBroadphase();
solver = new btSequentialImpulseConstraintSolver;
dynamicsWorld = new btDiscreteDynamicsWorld(dispatcher,
overlappingPairCache, solver, collisionConfig);
dynamicsWorld->setGravity(btVector3(0.0f, -9.8f, 0.0f));
Right now I have player and enemy objects of the type btCollisionObject*. I'm setting them up like this:
mPlayerBox = new btBoxShape(btVector3(1,3,1));
mPlayerObject = new btCollisionObject();
mPlayerObject->setCollisionShape(mPlayerBox);
btTransform playerWorld;
playerWorld.setIdentity();
//playerPos is a D3DXVECTOR3 that holds the camera position.
playerWorld.setOrigin(btVector3(playerPos.x, playerPos.y, playerPos.z));
mPlayerObject->setWorldTransform(playerWorld);
mPlayerObject->forceActivationState(DISABLE_DEACTIVATION);//maybe not needed
dynamicsWorld->addCollisionObject(mPlayerObject);
I do essentially the same thing with my enemy objects.
Then every frame I update all my objects with something like this:
btTransform updatedWorld;
updatedWorld.setIdentity();
updatedWorld.setOrigin(btVector3(position.x, position.y, position.z));
mPlayerObject->setWorldTransform(updatedWorld);
//do the same for my enemies, and then...
dynamicsWorld->performDiscreteCollisionDetection();
//Also tried doing this with stepSimulation(deltaTime, 7), but nothing changed.
//stepSimulation seems to only be for letting Bullet set world Transforms?
//check collisions with player
dynamicsWorld->contactTest(mPlayerObject, resultCallback);
int numManifolds = dynamicsWorld->getDispatcher()->getNumManifolds();
if(numManifolds > 0)
{
//there's a collision, execute blah blah blah
}
And finally, here's the structure that defines my result callback:
struct rCallBack : public btCollisionWorld::ContactResultCallback
{
btScalar rCallback::addSingleResult(btManifoldPoint& cp, const btCollisionObject*
colObj0, int partId0, int index0, const btCollisionObject* colObj1, int partId1,
int index1)
{
btVector3 ptA = cp.getPositionWorldOnA();
btVector3 ptB = cp.getPositionWorldOnB();
return 0;
}
}
I've looked at a lot of the demos, but they seem to mostly be leaving the movement up to Bullet, and since I'm moving characters at a set speed without any special physics when they collide, I had trouble adapting the examples into my application. The result callback actually came from this post on the forums: http://bulletphysics.org/Bullet/phpBB3/viewtopic.php?t=6816 It's about using triangle meshes, but it seemed closest to what I was trying to implement.
Anyway, if you read this far, thank you!! Any advice or links you could spare would be very much appreciated.
I am writing an IOS app with flighter shooting each other on 3D scene. I use bullet physics for collision detection I set the flighter as kinematic object, my logic move the flighter and then update the btMotionState worldTransform of the kinematic object. I also don't get any collision detections until i change the following two statements (set the masking and group to the same for both player and enemy)
dynamicsWorld->addRigidBody(mPlayerObject,1,1);
dynamicsWorld->addRigidBody(mEnemyObject,1,1);
...
dynamicsWorld->setInternalTickCallback(myTickCallback);
then i can see the
void myTickCallback(btDynamicsWorld *world, btScalar timeStep) {
int numManifolds = world->getDispatcher()->getNumManifolds();
printf("numManifolds = %d\n",numManifolds);
}
numManifolds value become 1 when object collides.
You can check the contact information as explained here:
Contact Information
The best way to determine if collisions happened between existing objects in the world, is to iterate over all contact manifolds. This should be done during a simulation tick (substep) callback, because contacts might be added and removed during several substeps of a single stepSimulation call. A contact manifold is a cache that contains all contact points between pairs of collision objects. A good way is to iterate over all pairs of objects in the entire collision/dynamics world:
//Assume world->stepSimulation or world->performDiscreteCollisionDetection has been called
int numManifolds = world->getDispatcher()->getNumManifolds();
for (int i=0;i<numManifolds;i++)
{
btPersistentManifold* contactManifold = world->getDispatcher()->getManifoldByIndexInternal(i);
btCollisionObject* obA = static_cast<btCollisionObject*>(contactManifold->getBody0());
btCollisionObject* obB = static_cast<btCollisionObject*>(contactManifold->getBody1());
int numContacts = contactManifold->getNumContacts();
for (int j=0;j<numContacts;j++)
{
btManifoldPoint& pt = contactManifold->getContactPoint(j);
if (pt.getDistance()<0.f)
{
const btVector3& ptA = pt.getPositionWorldOnA();
const btVector3& ptB = pt.getPositionWorldOnB();
const btVector3& normalOnB = pt.m_normalWorldOnB;
}
}
}
You may be interested in btGhostObject that keeps track of its own overlapping pairs.
Minimal runnable example
A sphere falling and hitting the ground.
Collisions are detected and printed to stdout.
Gnuplot visualization:
The "collision" line goes to 1 whenever the sphere touches the ground.
And for smaller restitution coefficients (0.5 and 0.5):
Here the ball stops jumping completely and touches the ground continuously.
Code:
#include <cstdio>
#include <cstdlib>
#include <vector>
#include <btBulletDynamicsCommon.h>
#define PRINTF_FLOAT "%7.3f"
constexpr float gravity = -10.0f;
constexpr float initialY = 10.0f;
constexpr float timeStep = 1.0f / 60.0f;
// TODO some combinations of coefficients smaller than 1.0
// make the ball go up higher / not lose height. Why?
constexpr float groundRestitution = 0.9f;
constexpr float sphereRestitution = 0.9f;
constexpr int maxNPoints = 500;
std::vector<btVector3> collisions;
void myTickCallback(btDynamicsWorld *dynamicsWorld, btScalar timeStep) {
collisions.clear();
int numManifolds = dynamicsWorld->getDispatcher()->getNumManifolds();
for (int i = 0; i < numManifolds; i++) {
btPersistentManifold *contactManifold = dynamicsWorld->getDispatcher()->getManifoldByIndexInternal(i);
// TODO those are unused. What can be done with them?
// I think they are the same objects as those in the main loop
// dynamicsWorld->getCollisionObjectArray() and we could compare
// the pointers to see which object collided with which.
{
const btCollisionObject *objA = contactManifold->getBody0();
const btCollisionObject *objB = contactManifold->getBody1();
}
int numContacts = contactManifold->getNumContacts();
for (int j = 0; j < numContacts; j++) {
btManifoldPoint& pt = contactManifold->getContactPoint(j);
const btVector3& ptA = pt.getPositionWorldOnA();
const btVector3& ptB = pt.getPositionWorldOnB();
const btVector3& normalOnB = pt.m_normalWorldOnB;
collisions.push_back(ptA);
collisions.push_back(ptB);
collisions.push_back(normalOnB);
}
}
}
int main() {
int i, j;
btDefaultCollisionConfiguration *collisionConfiguration
= new btDefaultCollisionConfiguration();
btCollisionDispatcher *dispatcher = new btCollisionDispatcher(collisionConfiguration);
btBroadphaseInterface *overlappingPairCache = new btDbvtBroadphase();
btSequentialImpulseConstraintSolver* solver = new btSequentialImpulseConstraintSolver;
btDiscreteDynamicsWorld *dynamicsWorld = new btDiscreteDynamicsWorld(
dispatcher, overlappingPairCache, solver, collisionConfiguration);
dynamicsWorld->setGravity(btVector3(0, gravity, 0));
dynamicsWorld->setInternalTickCallback(myTickCallback);
btAlignedObjectArray<btCollisionShape*> collisionShapes;
// Ground.
{
btTransform groundTransform;
groundTransform.setIdentity();
groundTransform.setOrigin(btVector3(0, 0, 0));
btCollisionShape* groundShape;
#if 1
// x / z plane at y = -1.
groundShape = new btStaticPlaneShape(btVector3(0, 1, 0), -1);
#else
// A cube of width 10 at y = -6.
// Does not fall because we won't call:
// colShape->calculateLocalInertia
// TODO: remove this from this example into a collision shape example.
groundTransform.setOrigin(btVector3(0, -6, 0));
groundShape = new btBoxShape(
btVector3(btScalar(5.0), btScalar(5.0), btScalar(5.0)));
#endif
collisionShapes.push_back(groundShape);
btDefaultMotionState* myMotionState = new btDefaultMotionState(groundTransform);
btRigidBody::btRigidBodyConstructionInfo rbInfo(0, myMotionState, groundShape, btVector3(0, 0, 0));
btRigidBody* body = new btRigidBody(rbInfo);
body->setRestitution(groundRestitution);
dynamicsWorld->addRigidBody(body);
}
// Sphere.
{
btCollisionShape* colShape = new btSphereShape(btScalar(1.0));
collisionShapes.push_back(colShape);
btTransform startTransform;
startTransform.setIdentity();
startTransform.setOrigin(btVector3(0, initialY, 0));
btVector3 localInertia(0, 0, 0);
btScalar mass(1.0f);
colShape->calculateLocalInertia(mass, localInertia);
btDefaultMotionState *myMotionState = new btDefaultMotionState(startTransform);
btRigidBody *body = new btRigidBody(btRigidBody::btRigidBodyConstructionInfo(
mass, myMotionState, colShape, localInertia));
body->setRestitution(sphereRestitution);
dynamicsWorld->addRigidBody(body);
}
// Main loop.
std::printf("step body x y z collision a b normal\n");
for (i = 0; i < maxNPoints; ++i) {
dynamicsWorld->stepSimulation(timeStep);
for (j = dynamicsWorld->getNumCollisionObjects() - 1; j >= 0; --j) {
btCollisionObject *obj = dynamicsWorld->getCollisionObjectArray()[j];
btRigidBody *body = btRigidBody::upcast(obj);
btTransform trans;
if (body && body->getMotionState()) {
body->getMotionState()->getWorldTransform(trans);
} else {
trans = obj->getWorldTransform();
}
btVector3 origin = trans.getOrigin();
std::printf("%d %d " PRINTF_FLOAT " " PRINTF_FLOAT " " PRINTF_FLOAT " ",
i,
j,
float(origin.getX()),
float(origin.getY()),
float(origin.getZ()));
if (collisions.empty()) {
std::printf("0 ");
} else {
std::printf("1 ");
// Yes, this is getting reprinted for all bodies when collisions happen.
// It's just a quick and dirty way to visualize it, should be outside
// of this loop normally.
for (auto& v : collisions) {
std::printf(
PRINTF_FLOAT " " PRINTF_FLOAT " " PRINTF_FLOAT " ",
v.getX(), v.getY(), v.getZ());
}
}
puts("");
}
}
// Cleanup.
for (i = dynamicsWorld->getNumCollisionObjects() - 1; i >= 0; --i) {
btCollisionObject* obj = dynamicsWorld->getCollisionObjectArray()[i];
btRigidBody* body = btRigidBody::upcast(obj);
if (body && body->getMotionState()) {
delete body->getMotionState();
}
dynamicsWorld->removeCollisionObject(obj);
delete obj;
}
for (i = 0; i < collisionShapes.size(); ++i) {
delete collisionShapes[i];
}
delete dynamicsWorld;
delete solver;
delete overlappingPairCache;
delete dispatcher;
delete collisionConfiguration;
collisionShapes.clear();
}
Based on: http://www.bulletphysics.org/mediawiki-1.5.8/index.php
Version of this focused on distinguishing which object touched which object: https://gamedev.stackexchange.com/a/120881/25171
GitHub upstream: https://github.com/cirosantilli/cpp-cheat/blob/503a3b6487ccb75334798839b5ed912270446d14/bullet/ground_ball.cpp
Tested on Bullet 2.83, Ubuntu 15.10.
来源：https://stackoverflow.com/questions/11175694/bullet-physics-simplest-collision-example