CODEWITHSUNDEEP
pythonopencvcomputer-visioncamera-calibrationaruco
Gytis Bernotas
Gytis Bernotas

Reputation: 49

Problems with OpenCV drawFrameAxes() function

I noticed an issue with the cv::drawFrameAxes() function. When the triad lines reach far outside of the image, the lines are distorted, no longer coincident with the axes as they should appear.

I was working on detecting charuco boards using OpenCV+Contrib 4.10.0. Once I have detected and estimated its pose, I am trying to plot the axis; however, I can see different axis if I change their lengths even if everything remains the same. I am providing a sample image, camera params and code to reproduce. Take a look a the attached images as well. The axis even 'break', or keeps only the outline without the fill, at longer axis length, e.g. 10.

I get the same results if I do this on simulated images as well as OpenCV (opencv-python) 4.7.0.72.

Different axis length plotted on one image

Different axis lengths

Example picture

import os
import numpy as np
import matplotlib.pyplot as plt
import cv2

# camera params after calibration
mtx = np.array([[3341.0930071 ,    0.        , 2020.04757797],
   [   0.        , 3332.54238818, 1108.10670945],
   [   0.        ,    0.        ,    1.        ]])
   
dist = np.array([-0.10474984,  0.04565679,  0.00948721,  0.        ,  0.        ])

img_rgb = cv2.imread("some img")
img_rgb = cv2.cvtColor(img_rgb, cv2.COLOR_RGB2BGR)
img = cv2.cvtColor(img_rgb, cv2.COLOR_RGB2GRAY)

# board info
dictionary = cv2.aruco.getPredefinedDictionary(cv2.aruco.DICT_4X4_100)
board = cv2.aruco.CharucoBoard((7, 5), 0.055, 0.041, dictionary)

params = cv2.aruco.DetectorParameters()
detector = cv2.aruco.ArucoDetector(dictionary, params)

# detect markers and charuco corners
marker_corners, marker_ids, rejectedCandidates = detector.detectMarkers(img)

ret, charucoCorners, charucoIds = cv2.aruco.interpolateCornersCharuco(marker_corners, marker_ids, img, board)

ret, rvec, tvec = cv2.aruco.estimatePoseCharucoBoard(charucoCorners, charucoIds, board, mtx, dist, None, None)

# start plotting
axis_size = 0.1
result_short = cv2.drawFrameAxes(np.copy(img_rgb), mtx, dist, rvec, tvec, axis_size)

axis_size = 5
result_long = cv2.drawFrameAxes(np.copy(img_rgb), mtx, dist, rvec, tvec, axis_size)

# plot
plt.subplot(1,2,1)
plt.imshow(result_short)
plt.subplot(1,2,2)
plt.imshow(result_long)

plt.show()

result_both = cv2.drawFrameAxes(np.copy(img_rgb), mtx, dist, rvec, tvec, 0.2)
result_both = cv2.drawFrameAxes(result_both, mtx, dist, rvec, tvec, 2)

Edit: Changed the example picture to an uncropped version, changed variable name mtx2 to mtx, added more relevant info.

Upvotes: -1

Views: 276

Answers (1)

Gytis Bernotas
Gytis Bernotas

Reputation: 49

This has been reproduced by others and have been reported as a bug on OpenCV GitHub.

Upvotes: 0

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