# Bullet Physics - Casting a ray straight down from a rigid body (first person camera)

I've implemented a first person camera using Bullet--it's a rigid body with a capsule shape. I've only been using Bullet for a few days and physics engines are new to me. I use btRigidBody::setLinearVelocity() to move it and it collides perfectly with the world. The only problem is the Y-value moves freely, which I temporarily solved by setting the Y-value of the translation vector to zero before the body is moved. This works for all cases except when falling from a height.

When the body drops off a tall object, you can still glide around since the translate vector's Y-value is being set to zero, until you stop moving and fall to the ground (the velocity is only set when moving). So to solve this I would like to try casting a ray down from the body to determine the Y-value of the world, and checking the difference between that value and the Y-value of the camera body, and disable or slow down movement if the difference is large enough.

I'm a bit stuck on simply casting a ray and determining the Y-value of the world where it struck. I've implemented this callback:

struct AllRayResultCallback : public btCollisionWorld::RayResultCallback{
AllRayResultCallback(const btVector3& rayFromWorld, const btVector3& rayToWorld)
: m_rayFromWorld(rayFromWorld), m_rayToWorld(rayToWorld), m_closestHitFraction(1.0){}

btVector3   m_rayFromWorld;
btVector3   m_rayToWorld;
btVector3   m_hitNormalWorld;
btVector3   m_hitPointWorld;
float       m_closestHitFraction;

virtual btScalar addSingleResult(btCollisionWorld::LocalRayResult& rayResult, bool normalInWorldSpace)
{
if(rayResult.m_hitFraction < m_closestHitFraction)
m_closestHitFraction = rayResult.m_hitFraction;

m_collisionObject = rayResult.m_collisionObject;
if(normalInWorldSpace){
m_hitNormalWorld = rayResult.m_hitNormalLocal;
}
else{
m_hitNormalWorld = m_collisionObject->getWorldTransform().getBasis() * rayResult.m_hitNormalLocal;
}

m_hitPointWorld.setInterpolate3(m_rayFromWorld, m_rayToWorld, m_closestHitFraction);
return 1.0f;
}
};


And in the movement function, I have this code:

btVector3 from(pos.x, pos.y + 1000, pos.z); // pos is the camera's rigid body position
btVector3 to(pos.x, 0, pos.z); // not sure if 0 is correct for Y

AllRayResultCallback callback(from, to);
Base::getSingletonPtr()->m_btWorld->rayTest(from, to, callback);


So I have the callback.m_hitPointWorld vector, which seems to just show the position of the camera each frame. I've searched Google for examples of casting rays, as well as the Bullet documentation, and it's been hard to just find an example. An example is really all I need.

Or perhaps there is some method in Bullet to keep the rigid body on the ground?

I'm using Ogre3D as a rendering engine, and casting a ray down is quite straightforward with that, however I want to keep all the ray casting within Bullet for simplicity.

Could anyone point me in the right direction? Thanks.

It turns out the solution is much simpler than the original attempt, and there's no need for subclassing if you're just testing one ray collision.

Here's all you must do to retrieve the collision vector:

btVector3 btFrom(camPos.x, camPos.y, camPos.z);
btVector3 btTo(camPos.x, -5000.0f, camPos.z);
btCollisionWorld::ClosestRayResultCallback res(btFrom, btTo);

Base::getSingletonPtr()->m_btWorld->rayTest(btFrom, btTo, res); // m_btWorld is btDiscreteDynamicsWorld

if(res.hasHit()){
printf("Collision at: <%.2f, %.2f, %.2f>\n", res.m_hitPointWorld.getX(), res.m_hitPointWorld.getY(), res.m_hitPointWorld.getZ());
}