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This paper introduces the quARtet marker, a 3D-printable fiducial composed of four tilted AprilTags within a compact footprint, designed to improve near-frontal pose estimation for robotic manipulation of transparent or reflective labware. By presenting each tag at a non-frontal angle even when the marker is viewed directly, the quARtet marker restores crucial perspective cues that are lost with single planar tags, significantly reducing orientation and position errors. Experimental results demonstrate a substantial improvement in pose estimation accuracy and a reduction in grasp slip for specific layouts, offering a trade-off between estimation robustness and graspability.
Four tilted AprilTags in a single footprint deliver near-perfect frontal pose estimation for labware, outperforming single planar tags by an order of magnitude.
Robotic manipulation of labware is difficult when transparent or reflective objects must be identified and localized. Coded planar fiducials are a practical retrofit: easy to print, they leave the marked face flat and graspable. Yet a single planar tag is least reliable in near-frontal views, where perspective cues fade. Non-planar geometries restore those cues but intrude on the flat face that a parallel-jaw gripper must contact. Our idea is to tilt multiple tags within one compact footprint, so that each tag is seen at a non-frontal angle even when the marker faces the camera. We propose the quARtet marker, a 3D-printable fiducial embodying this idea: all detected corners of its four tilted AprilTags enter one Perspective-n-Point solve, and a shared configuration defines the fabricated geometry and the detector model. Because tilting consumes flat area, its three layouts trade pose-estimation consistency against graspability. In robot-referenced, same-setup fixed-camera experiments, all three layouts reduced the mean frontal orientation error from 2.18 degree for a single planar tag to 0.24-0.47 degree and the root-mean-square position error from 1.50 to 0.17-0.20 mm. A robot-mounted-camera pose-hold test confirmed this separation under closed-loop visual feedback. In swing-down trials under identical conditions, the two layouts with flat contact strips retained the object with about 2 mm of in-grasp slip, whereas the layout without flat strips slipped by roughly 100 mm. For the tested conditions, the results support a rule: the layout without flat strips when pose-estimation consistency dominates, a layout with flat strips when the marked face must remain graspable.