ITK "In place" filtering

a true "in-place" filter overwrites each pixel of the
input buffer with the resultant output pixel value and then passes the
input buffer as its output. All filters capable of this should inherit
from itk::InPlaceImageFilter (if you find one that doesn't, please let
us know). You to turn on/off this functionality eg.
filter->InPlaceOn(), filter->InPlaceOff().

The itk::MeanImageFilter does not inherit from
itk::InPlaceImageFilter, the reason being it is a neighbourhood
operation; neighbourhood operations *cannot* be run in-place (the
output for one pixel may be used as the input for another pixel!).

https://itk.org/pipermail/insight-users/2010-October/038501.html