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Inconsistent orientation of detected/generated ArUco and AprilTags #1195
Description
ArUco and AprilTag detection both uses a clockwise orientation, however, AprilTag corners start with bottom right while ArUco starts with upper left. See the example below:
Code:
import cv2
def draw_markers_with_corners(img, corners):
for i, marker_corners in enumerate(corners):
# Get the corners of the marker
corner_points = marker_corners.reshape((4, 2))
corner_points = corner_points.astype(int)
# Draw each corner with its ordinal position
for j, corner in enumerate(corner_points):
# Draw corner point
cv2.circle(img, tuple(corner), 5, (0, 0, 255), -1)
# Label the corner with its ordinal position (0, 1, 2, 3)
cv2.putText(
img,
str(j),
(corner[0] + 10, corner[1] + 10),
cv2.FONT_HERSHEY_SIMPLEX,
0.5,
(255, 0, 0),
2,
)
return img
img = cv2.imread("markers.jpeg")
parameters = cv2.aruco.DetectorParameters()
april_dict = cv2.aruco.getPredefinedDictionary(cv2.aruco.DICT_APRILTAG_36h11)
april_detector = cv2.aruco.ArucoDetector(april_dict, parameters)
aruco_dict = cv2.aruco.getPredefinedDictionary(cv2.aruco.DICT_4X4_250)
aruco_detector = cv2.aruco.ArucoDetector(aruco_dict, parameters)
img_marked = img.copy()
april_corners, _, _ = april_detector.detectMarkers(img)
img_marked = draw_markers_with_corners(img_marked, april_corners)
aruco_corners, _, _ = aruco_detector.detectMarkers(img)
img_marked = draw_markers_with_corners(img_marked, aruco_corners)
cv2.imwrite("markers_detected.jpeg", img_marked)The same issue persists (likely due to pre-defined dictionary configurations) for generation:
The generated AprilTag 36h11 - 0 (wrong orientation, see above)
The generated ArUco 4x4_250 - 0 (correct orientation, see above)
Code:
import cv2
april_dict = cv2.aruco.getPredefinedDictionary(cv2.aruco.DICT_APRILTAG_36h11)
aruco_dict = cv2.aruco.getPredefinedDictionary(cv2.aruco.DICT_4X4_250)
img = cv2.aruco.generateImageMarker(aruco_dict, 0, 200, borderBits=1)
cv2.imwrite("aruco.png", img)
img = cv2.aruco.generateImageMarker(april_dict, 0, 200, borderBits=1)
cv2.imwrite("april.png", img)
Activity
qian-chu commentedon Jan 28, 2026
Also reported in https://stackoverflow.com/questions/79044142/why-is-the-order-of-the-incoming-corners-different-between-apriltag-and-aruco-ma
s-trinh commentedon Jan 28, 2026
The generated image uses: https://github.com/opencv/opencv/blob/55a8996ffe660444b0a3c268c967f67b133ab332/modules/objdetect/src/aruco/apriltag/predefined_dictionaries_apriltag.hpp#L11961-L11966
33in decimal gives in binary:0010 0001161in decimal gives in binary:1010 0001In my opinion, the following should be tested/verified:
s-trinh commentedon Jan 29, 2026
AprilTag official repository
Using the opencv_demo.cc in the AprilTag repository, it gives the following corners order:
You can use this tool to generate the marker images:
Regeneration of the AprilTag codeword
I have regenerated the AprilTag codeword in order to try to match the ArUco convention:
It seems to work.
25h9
36h11
Tag image generation
Using your Python script and with the following parameters, it gives for:
qian-chu commentedon Feb 13, 2026
Thanks so much for trying out! This looks great to me. I would personally love to see your branch merged to fix the issue for good. For me personally, I have a workaround to simply re-order the detected corners for AprilTags for now. Up to you how you would like to proceed.
Fix the AprilTag markers dictionaries: generating the AprilTag marker…
Merge pull request #28823 from s-trinh:fix_apriltag_corners_order_upd…
Merge pull request opencv#28823 from s-trinh:fix_apriltag_corners_ord…