traits.trait_errors.TraitError

The code here is for analysing fMRI data, as follow:

```
def get_roi_surf(roi,vox_size,origin):
roi[roi==0]=-1
grid=tvtk.ImageData(spacing=vox_size,origin=origin)
grid.point_data.scalars=roi.T.ravel()
grid.point_data.scalars.name='scalars'
grid.dimensions=roi.shape
iso=tvtk.ImageMarchingCubes(input=grid) ## failed here
mass=tvtk.MassProperties(input=iso.output)
return mass.surface_area
```

```
Traceback (most recent call last):

File "", line 11, in
origSA[i]=get_roi_surf(rois[:,:,:,i].copy(),voxSize,(0,0,0))

File "", line 24, in get_roi_surf
iso=tvtk.ImageMarchingCubes(input=grid)

File "tvtk_classes/image_marching_cubes.py", line 58, in __init__
tvtk_base.TVTKBase.__init__(self, vtk.vtkImageMarchingCubes, obj, update, **traits)

File "/home/jack/anaconda3/lib/python3.7/site-packages/tvtk/tvtk_base.py", line 335, in __init__
super(TVTKBase, self).__init__(**traits)

File "/home/jack/anaconda3/lib/python3.7/site-packages/traits/trait_handlers.py", line 159, in _read_only
% (name, class_of(object))

TraitError: The 'input' trait of an ImageMarchingCubes instance is 'read only'.
```

And vtk version is 8.1.2
mayavi version is 4.6.2
traits version is 5.0.0
python version is 3.7.2