要实现Bullet Player Controller的侧向重力，你可以使用Bullet物理引擎提供的力和约束来模拟侧向的重力效果。以下是一个使用Bullet物理引擎实现侧向重力的示例代码：

#include 
#include 

int main() {
    // 创建碰撞检测配置和碰撞检测世界
    btDefaultCollisionConfiguration* collisionConfiguration = new btDefaultCollisionConfiguration();
    btCollisionDispatcher* dispatcher = new btCollisionDispatcher(collisionConfiguration);
    btDbvtBroadphase* overlappingPairCache = new btDbvtBroadphase();
    btSequentialImpulseConstraintSolver* solver = new btSequentialImpulseConstraintSolver;
    btDiscreteDynamicsWorld* dynamicsWorld = new btDiscreteDynamicsWorld(dispatcher, overlappingPairCache, solver, collisionConfiguration);
    dynamicsWorld->setGravity(btVector3(0, -9.8f, 0)); // 设置全局重力

    // 创建地面碰撞形状
    btCollisionShape* groundShape = new btBoxShape(btVector3(btScalar(50.), btScalar(50.), btScalar(50.)));

    // 创建地面刚体
    btTransform groundTransform;
    groundTransform.setIdentity();
    groundTransform.setOrigin(btVector3(0, -50, 0));
    btScalar mass(0.);
    btVector3 localInertia(0, 0, 0);
    btDefaultMotionState* groundMotionState = new btDefaultMotionState(groundTransform);
    btRigidBody::btRigidBodyConstructionInfo groundRigidBodyCI(mass, groundMotionState, groundShape, localInertia);
    btRigidBody* groundRigidBody = new btRigidBody(groundRigidBodyCI);
    dynamicsWorld->addRigidBody(groundRigidBody);

    // 创建玩家碰撞形状
    btCollisionShape* playerShape = new btCapsuleShape(1.0, 2.0);

    // 创建玩家刚体
    btTransform playerTransform;
    playerTransform.setIdentity();
    playerTransform.setOrigin(btVector3(0, 10, 0));
    btScalar playerMass(1.0);
    btVector3 playerInertia(0, 0, 0);
    playerShape->calculateLocalInertia(playerMass, playerInertia);
    btDefaultMotionState* playerMotionState = new btDefaultMotionState(playerTransform);
    btRigidBody::btRigidBodyConstructionInfo playerRigidBodyCI(playerMass, playerMotionState, playerShape, playerInertia);
    btRigidBody* playerRigidBody = new btRigidBody(playerRigidBodyCI);
    dynamicsWorld->addRigidBody(playerRigidBody);

    // 模拟10帧
    for (int i = 0; i < 10; i++) {
        dynamicsWorld->stepSimulation(1 / 60.f, 10);
        
        // 应用侧向重力
        btVector3 playerVelocity = playerRigidBody->getLinearVelocity();
        playerVelocity.setY(0); // 确保只应用侧向重力
        playerRigidBody->applyCentralForce(playerVelocity * -1);

        // 打印玩家位置
        btTransform playerTransform;
        playerRigidBody->getMotionState()->getWorldTransform(playerTransform);
        std::cout << "Player position: " << playerTransform.getOrigin().getX() << ", " << playerTransform.getOrigin().getY() << ", " << playerTransform.getOrigin().getZ() << std::endl;
    }

    // 清理内存
    dynamicsWorld->removeRigidBody(playerRigidBody);
    delete playerRigidBody->getMotionState();
    delete playerRigidBody;
    delete playerShape;
    dynamicsWorld->removeRigidBody(groundRigidBody);
    delete groundRigidBody->getMotionState();
    delete groundRigidBody;
    delete groundShape;
    delete dynamicsWorld;
    delete solver;
    delete overlappingPairCache;
    delete dispatcher;
    delete collisionConfiguration;

    return 0;
}

这个示例代码使用Bullet物理引擎创建了一个简单的场景，包括一个地面和一个玩家角色。在模拟时，每一帧都会应用侧向重力来模拟玩家的侧向运动。你可以根据实际情况进行修改和扩展。