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.. _creating-objects:
Creating Data Objects
=====================
The three-dimensional datatypes in yt follow a fairly simple protocol. The
basic principle is that if you want to define a region in space, that region
must be identifiable from some sort of cut applied against the cells --
typically, in yt, this is done by examining the geometry.
Creating a new data object requires modifications to two different files, one
of which is in Python and the other in Cython. First, a subclass of
:class:`~yt.data_objects.data_containers.YTDataContainer` must be defined;
typically you actually want to subclass one of:
:class:`~yt.data_objects.data_containers.YTSelectionContainer0D`
:class:`~yt.data_objects.data_containers.YTSelectionContainer1D`
:class:`~yt.data_objects.data_containers.YTSelectionContainer2D`
:class:`~yt.data_objects.data_containers.YTSelectionContainer3D`.
The following attributes must be defined:
* ``_type_name`` - this is the short name by which the object type will be
known as. Remember this for later, as we will have to use it when defining
the underlying selector.
* ``_con_args`` - this is the set of arguments passed to the object, and their
names as attributes on the data object.
* ``_container_fields`` - any fields that are generated by the object, rather
than by another derived field in yt.
The rest of the object can be defined in Cython, in the file
``yt/geometry/selection_routines.pyx``. You must define a subclass of
``SelectorObject``, which will require implementation of the following methods:
* ``fill_mask`` - this takes a grid object and fills a mask of which zones
should be included. It must take into account the child mask of the grid.
* ``select_cell`` - this routine accepts a position and a width, and returns
either zero or one for whether or not that cell is included in the selector.
* ``select_sphere`` - this routine returns zero or one whether a sphere (point
and radius) is included in the selector.
* ``select_point`` - this identifies whether or not a point is included in the
selector. It should be identical to selecting a cell or a sphere with
zero extent.
* ``select_bbox`` - this returns whether or not a bounding box (i.e., grid) is
included in the selector.
* ``_hash_vals`` - this must return some combination of parameters that
semi-uniquely identifies the selector.
Once the object has been defined, it must then be aliased within
``selection_routines.pyx`` as ``typename_selector``. For instance,
``ray_selector`` or ``sphere_selector`` for ``_type_name`` values of ``ray``
and ``sphere``, respectively.
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