I am developing a vision system based on ArUco visual markers. The ArUco library used is cv2.aruco.DICT_7X7_50 and the marker I have used is 2 cm on each side.

I have made the calibration using a 3.6 cm board for each square, obtaining results that I consider satisfactory.

The problem appeared when I started to validate the results provided by the vision system. To perform the validation, a UR10e collaborative robot has been used, in which the ArUco marker has been strategically placed on the tool connected to it. In this way, the robot tool is positioned in a known spatial position and, using a Logitech C525 camera, the marker is located and its position and orientation with respect to the camera is calculated (actually the position of the tool is calculated, for this the reference system is transferred from the center of the aruco to the center of the tool using homogeneous transformation matrices).

The problem I have is that when the tool and its marker are placed just in front of the camera, i.e. without any rotation in X or Y, I seem to have an ambiguity problem because the vision system returns a rotation value in Y. In the same way, when I try to correct the error manually, this numerical error is transferred to the X rotation.

I have posted this problem on GitHub and on the OpenCV forum, but they didn't do much help. Attached are the links to each of the topics:

• https://forum.opencv.org/t/ambiguity-problem-in-the-detection-of-the-aruco-marker-when-it-is-parallel-to-the-camera-plane/7783?u=danielbilbao12

• https://github.com/opencv/opencv_contrib/issues/3190#issue-1157441872

If you need more detailed information, please do not hesitate to ask for it.