def get_roilist_from_atlas(atlas_img):
return get_unique_nonzeros(check_img(atlas_img).get_data())<EOL>
Extract unique values from the atlas and returns them as an ordered list.

Parameters
----------
atlas_img: img-like object or str
    Volume defining different ROIs.
    Can either be:
    - a file path to a Nifti image
    - any object with get_data() and get_affine() methods, e.g., nibabel.Nifti1Image.
    If niimg is a string, consider it as a path to Nifti image and
    call nibabel.load on it. If it is an object, check if get_data()
    and get_affine() methods are present, raise TypeError otherwise.

Returns
-------
np.ndarray
    An 1D array of roi values from atlas volume.

Note
----
The roi with value 0 will be considered background so will be removed.