Exporting bitmasks
-------------------

This tutorial shows how to generate bit masks from ENVISAT_ flags information
as "raw" image using PyEPR_.

The example code (:download:`examples/write_bitmask.py`) is a direct
translation of the C sample program `write_bitmask.c`_ bundled with the
EPR API distribution.

The program is invoked as follows:

.. code-block:: sh

    $ python write_bitmask.py <envisat-product> <bitmask-expression> \
    <output-file>

.. _ENVISAT: http://envisat.esa.int
.. _PyEPR: https://github.com/avalentino/pyepr
.. _`write_bitmask.c`: https://github.com/bcdev/epr-api/blob/master/src/examples/write_bitmask.c


The :download:`examples/write_bitmask.py` code consists in a single function
that also includes command line arguments handling:

.. raw:: latex

    \fvset{fontsize=\footnotesize}

.. literalinclude:: examples/write_bitmask.py
   :linenos:

In order to use the Python_ EPR API the :mod:`epr` module is imported:

.. raw:: latex

    \fvset{fontsize=\footnotesize}

.. literalinclude:: examples/write_bitmask.py
   :lines: 25

As usual the ENVISAT_ product is opened using the :func:`epr.open` function
that returns an :class:`epr.Product` instance.

.. raw:: latex

    \fvset{fontsize=\footnotesize}

.. literalinclude:: examples/write_bitmask.py
   :lines: 49-50

Scene size parameters are retrieved form the :class:`epr.Product` object
using the :meth:`epr.Product.get_scene_width` and
:meth:`epr.Product.get_scene_height` methods:

.. raw:: latex

    \fvset{fontsize=\footnotesize}

.. literalinclude:: examples/write_bitmask.py
   :lines: 54-55

The EPR API allows to manage data by means of :class:`epr.Raster` objects, so
the function :func:`epr.create_bitmask_raster`, specific for bitmasks, is used
to create a :class:`epr.Raster` instance.

.. seealso:: :func:`epr.create_raster`

Data are actually read using the :meth:`epr.Product.read_bitmask_raster` method
of the :class:`epr.Product` class:

.. raw:: latex

    \fvset{fontsize=\footnotesize}

.. literalinclude:: examples/write_bitmask.py
   :lines: 62

The :meth:`epr.Product.read_bitmask_raster` method receives in input the
*bm_expr* parameter that is set via command line:

.. raw:: latex

    \fvset{fontsize=\footnotesize}

.. literalinclude:: examples/write_bitmask.py
   :lines: 46

*bm_expr* is a string that define the logical expression for the definition
of the bit-mask. In a bit-mask expression, any number of the flag-names (found
in the DDDB) can be composed with “(”, ”)”, “NOT”, “AND”, “OR”.

Valid bit-mask expression are for example::

    flags.LAND OR flags.CLOUD

or::

    NOT flags.WATER AND flags.TURBID_S

Finally data are written to disk as a flat binary file using the
:meth:`numpy.ndarray.tofile` method of the :data:`epr.Raster.data` attribute
of the :class:`epr.Raster` objects that exposes data via the
:class:`numpy.ndarray` interface:

.. raw:: latex

    \fvset{fontsize=\footnotesize}

.. literalinclude:: examples/write_bitmask.py
   :lines: 64-65


.. _Python: http://www.python.org
.. _ENVISAT: http://envisat.esa.int
