.. module:: ezdxf.math
    :noindex:

.. _tut_coordinates:

.. _tut_ocs:

Tutorial for OCS/UCS Usage
==========================

For OCS/UCS usage is a basic understanding of vector math required, for a brush
up, watch the YouTube tutorials of `3Blue1Brown`_ about `Linear Algebra`_.

Second read the :ref:`Coordinate Systems` introduction please.

.. seealso::

    The free online book `3D Math Primer for Graphics and Game Development <https://gamemath.com/>`_
    is a very good resource for learning vector math and other graphic related topics,
    it is easy to read for beginners and especially targeted to programmers.


For :ref:`WCS` there is not much to say as, it is what it is: the main world
coordinate system, and a drawing unit can have any real world unit you want.
Autodesk added some mechanism to define a scale for dimension and text entities,
but because I am not an AutoCAD user, I am not familiar with it, and further
more I think this is more an AutoCAD topic than a DXF topic.

Object Coordinate System (OCS)
------------------------------

The :ref:`OCS` is used to place planar 2D entities in 3D space. **ALL** points
of a planar entity lay in the same plane, this is also true if the plane is
located in 3D space by an OCS. There are three basic DXF attributes that gives
a 2D entity its spatial form.

Extrusion
~~~~~~~~~

The extrusion vector defines the OCS, it is a normal vector to the base plane of
a planar entity. This `base plane` is always located in the origin of the
:ref:`WCS`.  But there are some entities like :class:`~ezdxf.entities.Ellipse`,
which have an extrusion vector, but do not establish an OCS.  For this entities
the extrusion vector defines only the extrusion direction and thickness defines
the extrusion distance, but all other points and directions in WCS.

Elevation
~~~~~~~~~

The elevation value defines the z-axis value for all points of a planar entity,
this is an OCS value, and defines the distance of the entity plane from the
`base plane`.

This value exists only in output from DXF versions prior to R11 as separated DXF
attribute (group code 38).  In DXF R12 and later, the elevation value is
supplied as z-axis value of each point. But as always in DXF, this simple rule
does not apply to all entities: :class:`~ezdxf.entities.LWPolyline` and
:class:`~ezdxf.entities.Hatch` have an DXF attribute :attr:`elevation` as a
3D point, where the z-values of this point is the elevation height and the
x-value and the y-value are 0.

Thickness
~~~~~~~~~

Defines the extrusion distance for an entity.

.. note::

    There is a new edition of this tutorial using UCS based transformation,
    which are available in `ezdxf` v0.11 and later: :ref:`tut_ucs_transform`

    This edition shows the **hard way** to accomplish the transformations by
    low level operations.

Placing 2D Circle in 3D Space
-----------------------------

The colors of the system axis follow the AutoCAD standard:

    - red is x-axis
    - green is y-axis
    - blue is z-axis

.. literalinclude:: src/ocs/circle.py
    :lines: 6-29

The following image shows the 2D circle in 3D space in AutoCAD `Left` and
`Front` view.  The blue line shows the OCS z-axis (extrusion direction),
elevation is the distance from the origin to the center of the circle in this
case 2.828, and you see that the x- and y-axis of the OCS and the WCS are not
aligned.

.. image:: gfx/ocs-circle-side-view.png
    :alt: circle in ocs as side view
.. image:: gfx/ocs-circle-front-view.png
    :alt: circle in ocs as front view

Placing LWPolyline in 3D Space
------------------------------

For simplicity of calculation I use the :class:`UCS` class in this example to
place a 2D pentagon in 3D space.


.. literalinclude:: src/ocs/lwpolyline.py
    :lines: 12-37

The following image shows the 2D pentagon in 3D space in AutoCAD `Left`, `Front`
and `Top` view.  The three lines from the center of the pentagon show the UCS,
the three colored lines in the origin show the OCS, the white lines in the
origin show the WCS.

The z-axis of the UCS and the OCS pointing in the same direction (extrusion
direction), and the x-axis of the UCS and the WCS pointing also in the same
direction.  The elevation is the distance from the origin to the center of the
pentagon and all points of the pentagon have the same elevation, and you see
that the y-axis of the UCS, the OCS and the WCS are not aligned.

.. image:: gfx/ocs-lwpolyline-left.png
    :alt: pentagon in ucs as side view
.. image:: gfx/ocs-lwpolyline-front.png
    :alt: pentagon in ucs as front view

Using UCS to Place 3D Polyline
------------------------------

It is much simpler to use a 3D :class:`~ezdxf.entities.Polyline` to create the
3D pentagon.  The :class:`UCS` class is handy for this example and all kind of
3D operations.

.. literalinclude:: src/ocs/polyline3d.py
    :lines: 13-43

.. image:: gfx/ucs-polyline3d.png
    :alt: 3d poyline with UCS


Placing 2D Text in 3D Space
---------------------------

The problem of placing text in 3D space is the text rotation, which is always
counter clockwise around the OCS z-axis, and ``0`` degree is the direction of
the positive OCS x-axis, and the OCS x-axis is calculated by the
:ref:`Arbitrary Axis Algorithm`.

Calculate the OCS rotation angle by converting the TEXT rotation angle (in UCS
or WCS) into a vector or begin with text direction as vector, transform this
direction vector into OCS and convert the OCS vector back into an angle in the
OCS xy-plane (see example), this procedure is available as
:meth:`UCS.to_ocs_angle_deg` or :meth:`UCS.to_ocs_angle_rad`.

AutoCAD supports thickness for the TEXT entity only for `.shx` fonts and not for
true type fonts.

.. literalinclude:: src/ocs/text.py
    :lines: 12-33

.. image:: gfx/ocs-text-top.png
    :alt: text in ucs as top view

.. image:: gfx/ocs-text-front.png
    :alt: text in ucs as front view

.. hint::

    For calculating OCS angles from an UCS, be aware that 2D entities, like TEXT
    or ARC, are placed parallel to the xy-plane of the UCS.

Placing 2D Arc in 3D Space
--------------------------

Here we have the same problem as for placing text, you need the start- and end
angle of the arc in degrees in the OCS, and this example also shows a shortcut
for calculating the OCS angles.

.. literalinclude:: src/ocs/arc.py
    :lines: 12-32

.. image:: gfx/ocs-arc-top.png
    :alt: arc in ucs as top view
.. image:: gfx/ocs-arc-front.png
    :alt: arc in ucs as front view

Placing Block References in 3D Space
------------------------------------

Despite the fact that block references (:class:`~ezdxf.entities.Insert`) can
contain true 3D entities like :class:`~ezdxf.entities.Line` or
:class:`~ezdxf.entities.Mesh`, the :class:`~ezdxf.entities.Insert` entity uses
the same placing principe as :class:`~ezdxf.entities.Text` or
:class:`~ezdxf.entities.Arc` shown in the previous chapters.

Placement by OCS coordinates and rotation about the OCS z-axis, can be achieved
the same way as for generic 2D entities.  The DXF attribute
:attr:`Insert.dxf.rotation` rotates a block reference around the block z-axis,
which is located in the :attr:`Block.dxf.base_point`. To rotate the block reference
around the WCS x-axis, a transformation of the block z-axis into the WCS x-axis
is required by rotating the block z-axis 90 degree counter-clockwise around
y-axis by using an UCS:

This is just an excerpt of the important parts, see the whole code of
`insert.py`_ at github.

.. literalinclude:: src/ocs/insert.py
    :lines: 47-69

.. image:: gfx/insert_1.png
.. image:: gfx/insert_2.png

To rotate a block reference around another axis than the block z-axis, you have
to find the rotated z-axis (extrusion vector) of the rotated block reference,
following example rotates the block reference around the block x-axis by
15 degrees:

.. literalinclude:: src/ocs/insert.py
    :lines: 74-88

.. image:: gfx/insert_3.png
.. image:: gfx/insert_4.png

The next example shows how to translate a block references with an
already established OCS:

.. literalinclude:: src/ocs/insert.py
    :lines: 90-99

Setting a new insert location is the same procedure without adding a translation
vector, just transform the new insert location into the OCS.

.. image:: gfx/insert_5.png
.. image:: gfx/insert_6.png

The next operation is to rotate a block reference with an established OCS,
rotation axis is the block y-axis, rotation angle is -90 degrees. First
transform block y-axis (rotation axis) and block z-axis (extrusion vector)
from OCS into WCS:

.. literalinclude:: src/ocs/insert.py
    :lines: 101-106

Build transformation matrix and transform extrusion vector and build new UCS:

.. literalinclude:: src/ocs/insert.py
    :lines: 107-114

Set new OCS attributes, we also have to set the rotation attribute even though
we do not rotate the block reference around the local z-axis, the new block
x-axis (0 deg) differs from OCS x-axis and has to be adjusted:

.. literalinclude:: src/ocs/insert.py
    :lines: 116-122

.. image:: gfx/insert_7.png
.. image:: gfx/insert_8.png

And here is the point, where my math knowledge ends, for more advanced CAD
operation you have to look elsewhere.

.. _Linear Algebra: https://www.youtube.com/watch?v=kjBOesZCoqc&list=PLZHQObOWTQDPD3MizzM2xVFitgF8hE_ab
.. _3Blue1Brown: https://www.youtube.com/channel/UCYO_jab_esuFRV4b17AJtAw
.. _insert.py: https://github.com/mozman/ezdxf/blob/master/docs/source/tutorials/src/ocs/insert.py
