In OpenCV 3 the function goodFeaturesToTrack returns an array of the following form
[[[1, 2]]
[[3, 4]]
[[5, 6]]
[[7, 8]]]
After converting that array into a Python list I get
[[[1, 2]], [[3, 4]], [[5, 6]], [[7, 8]]]
Although this is a list, if you see it has one more pair of brackets than it should and when I try to access an element via A[0][1] I get an error. Why the array and the list have that form? How should I fix it?
Because you have a 3d array with one element in second axis:
In [26]: A = [[[1, 2]], [[3, 4]], [[5, 6]], [[7, 8]]]
In [27]: A[0]
Out[27]: [[1, 2]]
And when you want to access the second item by A[0][1] it raises an IndexError:
In [28]: A[0][1]
---------------------------------------------------------------------------
IndexError Traceback (most recent call last)
<ipython-input-28-99c47bb3f368> in <module>()
----> 1 A[0][1]
IndexError: list index out of range
You can use np.squeeze() in order to reduce the dimension, and convert the array to a 2D array:
In [21]: import numpy as np
In [22]: A = np.array([[[1, 2]], [[3, 4]], [[5, 6]], [[7, 8]]])
In [33]: A = np.squeeze(A)
In [34]: A
Out[34]:
array([[1, 2],
[3, 4],
[5, 6],
[7, 8]])
goodFeaturesToTrack, you can find more details in documentation.