How to throw object in the direction of camera?

[patrikpatrik]

@daniel Chappuis, Hello I hate to bother you but can you point me in the right direction with this question?

If lets say my camera is pointed in a direction that I moved with my mouse, and I want to throw an object with a left click in that same direction, I cant seem to make it throw in that direction.

Currently, if I leave it at default the object does throw in that direction with this:

rp3d::Vector3 m_ObjectPosition(CameraPos.x, CameraPos.y, CameraPos.z);
rp3d::Quaternion m_ObjectOrientation = rp3d::Quaternion::identity();
rp3d::Transform m_ObjectTransform(m_ObjectPosition, m_ObjectOrientation);

Would I need to apply my sin and cos rotation to the updated position or would I need to update my quaternion somehow and not use identity? I apologize in advance for a trivial question but I haven't slept in days and reactphysics3d has been my obsession!

[patrikpatrik]

I always do this but I think I've managed to find a solution although I'm not sure if it's the correct way. There were 2 steps to this:

1. To get the direction of the object in the heading or rotation of the camera. This was achieved by using ray casting the inverse projection matrix and view matrix to get the ray direction.
2. This ray direction is then multiplied by a speed factor to setLinearVelocity(...), which is independent from setting m_ObjectionPosition(...)

[DanielChappuis]

If I understand correctly, you are looking for the direction of your camera in world-space. Since you view matrix V transforms points from world-space to camera space. Therefore, the inverse view matrix $V^{-1}$ will transform a point from camera-space to world-space. This means that if you want to have the direction $d_w$ of the camera in world-space, you need to multiply the direction $d_c$ of the camera in camera-space (for instance the negative z-axis if the camera is looking in this direction) by the inverse of the view matrix $V^{-1}$ to get the camera direction in world-space.

$$ d_w = V^{-1} d_c $$

Also note that the view matrix $V$ is a rotation matrix and therefore the inverse of V is simply it's transpose $V^{-1} = V^T$.

I hope this helps.

[patrikpatrik]

Thank you Daniel. I might try this method as well!