Monocular camera undistorted but unknown intrinsics

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Hi,

What's the convention for filling in K, D, R and P matrices for camera_info if you have a monocular camera that is undistorted but the intrinsics are unknown?

Since it's a monocular camera, R is identity, and the fourth column of P is 0 but what do I fill in for K and D?

D: The images are already undistorted so I suppose all elements could be 0? K: This is unknown and obviously cannot be used for algorithms that needs intrinsics but just for completeness what does one fill in it? Leave it all 0s?

I'm looking publish the image and camera_info as close to the ROS convention as possible. Also, do I publish this as image_rect?