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SolvePNP_P3P doesn't work for 4 points. Please help.

asked 2019-09-27 13:08:18 -0500

updated 2019-09-27 20:37:40 -0500

supra56 gravatar image

Hello, I'm using the solvePnP() function to get the extrinsics for a camera. I'm calibrating wrt the ground. I have taken 4 points on the ground. I tried to use the solvePnP_P3P flag because I was using exactly 4 points but I get an error when I run the function.

System details: Windows 10. OpenCV 4.1

Code snippet:

import numpy as np
from numpy.linalg import inv
import cv2
import sys


# with open('./calib/calibration.yaml') as f:
#     loadeddict = yaml.load(f)
# camera_matrix = loadeddict.get('camera_matrix')
# dist_coeffs = loadeddict.get('dist_coeff')


camera_matrix = [[1029.61564, 0.00000000, 916.314412],
 [0.00000000 ,1026.37518 ,526.909999],
 [0.00000000, 0.00000000, 1.00000000]]

dist_coeffs = [[-0.3725, 0.21018, -0.001127, 0.00013219, -0.09244]] 

#tnsPoints = [(0.0, 1.0, 0.0), (3.0, 1.0, 0.0), (2.0, 6.0, 0.0), (0.0, 6.0, 0.0)]
tnsPoints = [(0.0, 0.0, 0.0), (1.0, 0.0, 0.0), (0.0, 1.0, 0.0), (0.0, 0.0, 1.0)]

imPoints = [(809, 915), (1181, 893), (1020, 362), (811, 372)]

retval, rvec, tvec = cv2.solvePnP(np.array(tnsPoints),np.array(imPoints),np.asarray(camera_matrix),np.asarray(dist_coeffs), flags=cv2.SOLVEPNP_P3P)

Error that I get:

Traceback (most recent call last):
  File "C:\Users\pavithra\Pictures\Camera Roll\project_3D_to_2D.py", line 2016, in <module>
    retval, rvec, tvec = cv2.solvePnP(np.array(tnsPoints),np.array(imPoints),np.asarray(camera_matrix),np.asarray(dist_coeffs), flags=cv2.SOLVEPNP_P3P)
cv2.error: OpenCV(4.1.0) C:\projects\opencv-python\opencv\modules\calib3d\src\solvepnp.cpp:92: error: (-215:Assertion failed) ( (npoints >= 4) || (npoints == 3 && flags == SOLVEPNP_ITERATIVE && useExtrinsicGuess) ) && npoints == std::max(ipoints.checkVector(2, CV_32F), ipoints.checkVector(2, CV_64F)) in function 'cv::solvePnP'

Please let me know if anybody has any idea of what is going on.

Thank you.

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answered 2020-04-08 15:28:04 -0500

noshky gravatar image

Object points array must be of shape (4, 3, 1) (your's is (4, 3)), and image points must be of of shape (4, 2, 1) (your's is (4, 2)).

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Asked: 2019-09-27 13:08:18 -0500

Seen: 167 times

Last updated: Apr 08