How to know the camera intrinsic parameter of Unity camera using XRSettings ?

Unity Engine
1

I’m trying superimposing the 3D Object in Unity on the streaming camera image.
To do this, I need the intrinsic parameter of camera which renders for HMD.
I’m using the StreamVR Plugin on Unity and VIVE Pro HMD.
I calculated the intrinsic parameter from FOV and renderred image resolution, and superimposed using it.
But the result position was not match the object position on streaming image.
Therefore I think the calculated intrinsic parameters are wrong, but why?
I also noticed the image center of renderrd image for HMD was not match the camera center in Unity space,
which means that the object placed in front of camera in Unity was not renderrd on the center of image.
On left eye image, that is shifted right side from the center position.
Finally we conducted the camera calibration for the virtual camera in Unity. But I believe the theoretical value should be calculated.
If you know how to gain the correct intrinsic parameter of Unity camera when using VR, would you tell me about that.

Best regards.

2

It turns out that Unity does something funky. during render it will change the camera parameters, render, and then change them back. So if you look at for example the FOV, it’s not accurate. I was able to work around this by reading them during OnRenderImage