.. include:: ../links.inc

.. _marker:

Marker
======

Description
-----------

The ``Marker`` is an object that is responsible for feature extraction. It
primarily operates on data loaded into memory by :ref:`Data Reader <datareader>`
and stored in the ``data`` key of each :ref:`data type <data_types>` in the
:ref:`Data object <data_object>`. In some cases, it can also operate on
pre-processed data as obtained from the :ref:`Preprocess <preprocess>` step of
the pipeline.

.. important::

   This pre-process is not similar to pre-processing done by tools like FSL,
   SPM, AFNI, etc., . For example, one can perform confound removal on loaded
   data and then perform feature extraction.

Markers are meant to be used inside the DataGrabber context but you can operate
on them outside the context as long as the actual data is in the memory and the
Python runtime has not garbage-collected it.

If you are interested in using already provided Markers, please go to
:doc:`../builtin`. And, if you want to implement your own Marker, please check
out :doc:`../extending/marker`.