So I am working in C# using XNA and am trying to create a 6DOF camera for an airplane game using quaternions. I have a FPS quaternion camera made and working fine. The rotation of the aircraft camera is perfectly fine, but when I compute the forward vector of the aircraft to figure out the vector in which to apply forward movement, it doesn't always go in the correct direction. So in short, how can I find the current forward vector of a camera using the rotation quaternion in XNA?

I know there are tons of quaternion questions on here, but I haven't found any C# ones that have worked for me.

Multiply the quaternion by whatever vector "forward" is in your coordinate system - most likely (0,0,1). You have to have a definition of forward - what direction do you want to get if the rotation is 0.

Edit: use this or this function

The problem with XNA is that there is no operator overload for multiplying a Vector3 and a quaternion. This is my current line of code to try to calculate the forward vector:

Vector3 forwardVector = Vector3.Transform(Vector3.Forward, camera.RotationQuaternion);

It only works in about half of the octants of the 3D space though.

So I appreciate the help, but I was able to figure it out on my own. So for anyone who encounters this problem with quaternions in XNA, the solution follows:

Quaternion temporaryQuaternion = camera.RotationQuaternion;
temporaryQuaternion.Conjugate();
Vector3 forwardVector = Vector3.Transform(Vector3.Forward, temporaryQuaternion);

If anyone can explain the math behind why you use the invese of the rotation quaternion to apply the transform to the world forward, I'd love to hear it.