Users Manual

have the same orientation and are approximately aligned in parallel to the image rows. However, even if the
approximate size is not given, the TagDetect components try to detect such situations and filter out affected
tags.
Below tables give approximate precisions of the estimated poses of AprilTags and QR codes. We distinguish
between lateral precision (i.e., in x and y direction) and precision in z direction. It is assumed that quality is set
to “H” and that the rc_visard’s viewing direction is roughly parallel to the tag’s normal. The size of a tag does
not have a significant effect on the lateral or z precision; however, in general, larger tags improve precision. With
respect to precision of the orientation especially around the x and y axes, larger tags clearly outperform smaller
ones.
Table 7.3.4: Approximate pose precision for AprilTags
Distance rc_visard 65 - lateral rc_visard 65 - z rc_visard 160 - lateral rc_visard 160 - z
0.3 m 0.4 mm 0.9 mm 0.4 mm 0.8 mm
1.0 m 0.7 mm 3.3 mm 0.7 mm 3.3 mm
Table 7.3.5: Approximate pose precision for QR codes
Distance rc_visard 65 - lateral rc_visard 65 - z rc_visard 160 - lateral rc_visard 160 - z
0.3 m 0.6 mm 2.0 mm 0.6 mm 1.3 mm
1.0 m 2.6 mm 15 mm 2.6 mm 7.9 mm
7.3.4 Tag re-identification
Each tag has an ID; for AprilTags it is the family plus tag ID, for QR codes it is the contained data. However, these
IDs are not unique, since the same tag may appear multiple times in a scene.
For distinction of these tags, the TagDetect components also assign each detected tag a unique identifier. To help
the user identifying an identical tag over multiple detections, tag detection tries to re-identify tags; if successful,
a tag is assigned the same unique identifier again. Tag re-identification compares the positions of the corners
of the tags in a static coordinate frame to find identical tags. Tags are assumed identical if they did not or only
slightly move in that static coordinate frame. For that static coordinate frame to be available, dynamic-state
estimation (Section 6.3) must be switched on. If it is not, the sensor is assumed to be static; tag re-identification
will then not work across sensor movements.
By setting the max
_
corner
_
distance threshold, the user can specify how much a tag is allowed move in the static
coordinate frame between two detections to be considered identical. This parameter defines the maximum distance
between the corners of two tags, which is shown in Fig. 7.3.5. The Euclidean distances of all four corresponding
tag corners are computed in 3D. If none of these distances exceeds the threshold, the tags are considered identical.
Fig. 7.3.5: Simplified visualization of tag re-identification. Euclidean distances between associated tag corners in
3D are compared (red arrows).
7.3. TagDetect 79