How to format xy points for undistortPoints with the python cv2 api?

Question

I'm having trouble formatting the x,y list of points to pass to undistortPoints (opencv 2.4.1).

The error message is c++ specific and complains about the array of points not being of type CV_32FC2. Shouldn't I be able to pass in an Nx2 numpy array?

import cv2

camera_matrix = array(mat('1.3e+03, 0., 6.0e+02; 0., 1.3e+03, 4.8e+02; 0., 0., 1.'), dtype=float32)
dist_coeffs = array(mat('-2.4-01, 9.5e-02, -4.0e-04, 8.9e-05, 0.'), dtype=float32)

test = zeros((10,2), dtype=float32)

print test.shape, type(test)

xy_undistorted = cv2.undistortPoints(test, camera_matrix, dist_coeffs)

results in:

opencv/modules/imgproc/src/undistort.cpp:279: error: (-215) CV_IS_MAT(_src) && CV_IS_MAT(_dst) && (_src->rows == 1 || _src->cols == 1) && (_dst->rows == 1 || _dst->cols == 1) && _src->cols + _src->rows - 1 == _dst->rows + _dst->cols - 1 && (CV_MAT_TYPE(_src->type) == CV_32FC2 || CV_MAT_TYPE(_src->type) == CV_64FC2) && (CV_MAT_TYPE(_dst->type) == CV_32FC2 || CV_MAT_TYPE(_dst->type) == CV_64FC2) in function cvUndistortPoints

In samples/python2/video.py there is usage of projectPoints which takes an array and reshapes it (-1,3) resulting in an Nx3 array for that function, it seems like the same format should work here.

Answer 1

I have tested Abid Rahman's method and it works well, but there are some issues to note.

calib = np.reshape(calib, [3, 3])
dist = np.array(kp_mat, dtype=np.float32)
newcameramtx, _ = cv2.getOptimalNewCameraMatrix(calib, dist, (w, h), 1, (w, h))
# undistort
dst = cv2.undistortPoints(pt, calib, dist, None, newcameramtx)
dst = np.squeeze(dst)

Note

1. points should be shape [1,N,2] or [N,1,2], with coordinate [x,y]
2. new camera matrix should be specified. Or the output will be normalized coordinate.

Answer 2

I don't know much about camera calibration. But seeing your code and the error, I changed it as follows:

import cv2
import numpy as np
camera_matrix = np.array([[1.3e+03, 0., 6.0e+02], [0., 1.3e+03, 4.8e+02], [0., 0., 1.]], dtype=np.float32)
dist_coeffs = np.array([-2.4-01, 9.5e-02, -4.0e-04, 8.9e-05, 0.], dtype=np.float32)

test = np.zeros((10,1,2), dtype=np.float32)
xy_undistorted = cv2.undistortPoints(test, camera_matrix, dist_coeffs)

print xy_undistorted

Below is the result I got, Check if it is correct:

[[[ 0.0187303   0.01477836]]

 [[ 0.0187303   0.01477836]]

 [[ 0.0187303   0.01477836]]

 [[ 0.0187303   0.01477836]]

 [[ 0.0187303   0.01477836]]

 [[ 0.0187303   0.01477836]]

 [[ 0.0187303   0.01477836]]

 [[ 0.0187303   0.01477836]]

 [[ 0.0187303   0.01477836]]

 [[ 0.0187303   0.01477836]]]

What is the problem :

The error says, source should be having EITHER one row OR one column. And it should be of CV_32FC2 or CV_64FC2, means two channels and floating point. So make your src of shape (10,1,2) or (1,10,2). Both methods work and give same result ( I checked it myself ) . Only problem is, I don't know if it is correct, so check it yourself.