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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
:language: python
: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: 27
As usual the ENVISAT_ product is opened using the :func:`epr.open` function
that returns an :class:`epr.Product` instance.
In this case the :func:`epr.open` is used together with a ``with`` statement
so that the :class:`epr.Product` instance is closed automatically when the
program exits the ``with`` block.
.. raw:: latex
\fvset{fontsize=\footnotesize}
.. literalinclude:: examples/write_bitmask.py
:language: python
:lines: 51-52
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
:language: python
:lines: 55-56
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
:language: python
:lines: 63
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
:language: python
:lines: 48
*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
:language: python
:lines: 65-66
.. _Python: http://www.python.org
.. _ENVISAT: http://envisat.esa.int
|
