Users Manual

Table 7.3.2: Maximum detection distance examples for AprilTags with a
width of 𝑡 = 4 cm
AprilTag family Tag width Maximum distance
36h11 (recommended) 8 modules 1.1 m
16h5 6 modules 1.4 m
Table 7.3.3: Maximum detection distance examples for QR codes with a
width of 𝑡 = 8 cm
Tag width Maximum distance
29 modules 0.49 m
21 modules 0.70 m
7.3.3 Pose estimation
For each detected tag, the pose of this tag in the camera coordinate frame is estimated. A requirement for pose
estimation is that a tag is fully visible in the left and right camera image. The coordinate frame of the tag is aligned
as shown below.
Fig. 7.3.4: Coordinate frames of AprilTags (left) and QR codes (right)
The z-axis is pointing “into” the tag. Please note that for AprilTags, although having the white border included
in their definition, the coordinate system’s origin is placed exactly at the transition from the white to the black
border. Since AprilTags do not have an obvious orientation, the origin is defined as the upper left corner in the
orientation they are pre-generated in.
During pose estimation, the tag’s size is also estimated, while assuming the tag to be square. For QR codes, the
size covers the full tag. For AprilTags, the size covers only the black part of the tag, hence ignoring the outermost
white border.
The user can also specify the approximate size (±10%) of tags with a specific ID. All tags not matching this size
contraint are automatically filtered out. This information is further used to resolve ambiguities in pose estimation
that may arise if multiple tags with the same ID are visible in the left and right image and these tags are aligned in
parallel to the image rows.
Note: For best pose estimation results one should make sure to accurately print the tag and to attach it to a rigid
and as planar as possible surface. Any distortion of the tag or bump in the surface will degrade the estimated
pose.
Warning: It is highly recommended to set the approximate size of a tag. Otherwise, if multiple tags with
the same ID are visible in the left or right image, pose estimation may compute a wrong pose if these tags
7.3. TagDetect 78