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Exporting band data
-------------------
This tutorial shows how to convert ENVISAT_ raster information from dataset
and generate flat binary rasters using PyEPR_.
The program generates as many raster as the dataset specified in input.
The example code (:download:`examples/write_bands.py`) is a direct
translation of the C sample program `write_bands.c`_ bundled with the
EPR API distribution.
The program is invoked as follows:
.. code-block:: sh
$ python write_bands.py <envisat-product> \
<output directory for the raster file> <dataset name 1> \
[<dataset name 2> ... <dataset name N>]
.. _ENVISAT: http://envisat.esa.int
.. _PyEPR: https://github.com/avalentino/pyepr
.. _`write_bands.c`: https://github.com/bcdev/epr-api/blob/master/src/examples/write_bands.c
Import section
~~~~~~~~~~~~~~
To use the Python_ EPR API one have to import :mod:`epr` module.
At first import time the underlaying C library is opportunely initialized.
.. raw:: latex
\fvset{fontsize=\footnotesize}
.. literalinclude:: examples/write_bands.py
:language: python
:lines: 1-15
The main program
~~~~~~~~~~~~~~~~
The main program in quite simple (this is just an example).
.. raw:: latex
\fvset{fontsize=\footnotesize}
.. literalinclude:: examples/write_bands.py
:language: python
:lines: 55-
It performs some basic command line arguments handling and then open the
input product.
.. raw:: latex
\fvset{fontsize=\footnotesize}
.. literalinclude:: examples/write_bands.py
:language: python
:lines: 97-99
Finally the core function (:func:`write_raw_image`) is called on each band
specified on the command:
.. raw:: latex
\fvset{fontsize=\footnotesize}
.. literalinclude:: examples/write_bands.py
:language: python
:lines: 100-101
The :func:`write_raw_image` core function
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The core function is :func:`write_raw_image`.
.. raw:: latex
\fvset{fontsize=\footnotesize}
.. literalinclude:: examples/write_bands.py
:language: python
:pyobject: write_raw_image
It generates a flat binary file with data of a single band whose name is
specified as input parameter.
First the output file name is computed using the :mod:`os` module.
.. raw:: latex
\fvset{fontsize=\footnotesize}
.. literalinclude:: examples/write_bands.py
:language: python
:lines: 23-25
Then the desired band is retrieved using the :meth:`epr.Product.get_band`
method and some of its parameters are loaded in to local variables:
.. raw:: latex
\fvset{fontsize=\footnotesize}
.. literalinclude:: examples/write_bands.py
:language: python
:lines: 26-27
Band data are accessed by means of a :class:`epr.Raster` object.
.. seealso:: :func:`epr.Band.read_as_array`
The :meth:`epr.Band.create_compatible_raster` is a facility method that
allows to instantiate a :class:`epr.Raster` object with a data type compatible
with the band data:
.. raw:: latex
\fvset{fontsize=\footnotesize}
.. literalinclude:: examples/write_bands.py
:language: python
:lines: 32-34
Then data are read using the :meth:`epr.Band.read_raster` method:
.. raw:: latex
\fvset{fontsize=\footnotesize}
.. literalinclude:: examples/write_bands.py
:language: python
:lines: 36-37
Then the output file object is created (in binary mode of course)
.. raw:: latex
\fvset{fontsize=\footnotesize}
.. literalinclude:: examples/write_bands.py
:language: python
:lines: 39
and data are copied to the output file one line at time
.. raw:: latex
\fvset{fontsize=\footnotesize}
.. literalinclude:: examples/write_bands.py
:language: python
:lines: 41-42
Please note that it has been used :data:`epr.Raster.data` attribute of the
:class:`epr.Raster` objects that exposes :class:`epr.Raster` data with the
powerful :class:`numpy.ndarray` interface.
.. note::
copying one line at time is not the best way to perform the task
in Python_. It has been done just to mimic the original C code:
.. code-block:: c
out_stream = fopen(image_file_path, "wb");
if (out_stream == NULL) {
printf("Error: can't open '%s'\n", image_file_path);
return 3;
}
for (y = 0; y < (uint)raster->raster_height; ++y) {
numwritten = fwrite(epr_get_raster_line_addr(raster, y),
raster->elem_size,
raster->raster_width,
out_stream);
if (numwritten != raster->raster_width) {
printf("Error: can't write to %s\n", image_file_path);
return 4;
}
}
fclose(out_stream);
A by far more pythonic_ solution would be::
raster.data.tofile(out_stream)
.. _Python: http://www.python.org
.. _pythonic: http://www.cafepy.com/article/be_pythonic
|
