getOptimalNewCameraMatrix() alpha value and camera matrix

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While reading image_pipeline/CameraInfo documentation, in the section "4.1 Simple monocular rectification" it says:

But since ROS Electric, the camera_calibration package does monocular calibration to get K' using OpenCV's getOptimalNewCameraMatrix() function with argument 'alpha'=0.0 that cause K' ≠ K.

But this is not true right? K' = K if alpha=0. The camera matrix, i.e. the focal lenghts and principal points change only if alpha > 0. I think this can also be seen in the code of getOptimalNewCameraMatrix(), assuming image size doesn't change.

Or did I miss something?

Thanks!!