How to track a 3D point with always the same distance to the tvec and get the 2D image coordinates for that

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NVM I had a mistake in my Code.

There are 2 ways to do it. In my case all I had to do: Add my offset as object point in the function cv::projectPoints(...).

Other possibility: - Get rotation Matrix with cv::Rodrigues - tvec = rotMat * Offset + tvec - use cv::projectPoints where objectPoint is the origin (0,0,0) (Be aware you are changeing the tvec !!!)