.. _bress-segments:

Segments
--------

.. index::
    single: segments

This tool segments the seafloor based on elevation and reflectivity values.
It extends the functionalities provided by the **Landforms** tool (see :ref:`bress-landforms`)
by analyzing the reflectivity values co-located with the elevation nodes that belong to each area kernel.

How To Use?
^^^^^^^^^^^    
    
* Select the **Segments** tab (:numref:`fig_segments`) on the bottom of the BRESS interface.

.. index::
    single: segments

* In **Setting**, check the settings of your choice.

* To change the **Settings** for **Segments v2**:

    * Click the **Unlock** button, and click **OK** to the dialogue.

    * The parameters in common with the **Landforms** tool are described in :ref:`bress-landforms`.

    * The reflectivity **Histogram** (see :ref:`reflectivity_histo`) can be customized by modifying:
        * The **min** and the **max** fields that represent the minimum and the maximum values for the histogram's bins.
        * The **nr.bins** field sets the number of bins in which the range between the minimum and the maximum values on the histogram are split.

    * The **range percentage** and the **delta bins** fields are used in the **Splitting** processing step (see :ref:`splitting`) to identify peaks in the reflectivity histogram of each area kernel.

    * The **intersection** and the **min.samples** fields are used during the **Merging** processing step (see :ref:`merging`) to identify the area kernels that can be merged together due to similarity in the reflectivity texture.

    * In addition to the **Landforms** products, the tool can provide the following three kinds of outputs (and they can be saved as ASCII grids or plotted):
        * **Segments** (see :ref:`segments`).
        * **Segments Statistics** (see :ref:`segments_stats`).
        * **Mosaic mask** (see :ref:`mosaic_mask`).
        * **Cropped DTM** (see :ref:`cropped_dtm`).

* To reset the **Parameters** to the default, initial values, click the **Reset** button.

* In **Execution**, click **Segments v2**.

.. _fig_segments:
.. figure:: _static/segments.png
    :width: 640px
    :align: center
    :alt: segments tab
    :figclass: align-center

    The **Segments** tab.


|

-----------------------------------------------------------

|

How Does It Work?
^^^^^^^^^^^^^^^^^

.. _reflectivity_histo:

Reflectivity Histogram
======================

.. index::
    single: reflectivity histogram

The **Reflectivity Histogram** is used to characterize the texture in each *area kernel*.


.. _splitting:

Area Kernels Splitting
======================

.. index::
    single: area kernels, splitting

The **splitting** step evaluates the *reflectivity histogram* of each *area kernel* to identify multi-modality (that is, multiple peaks) (see :numref:`fig_splitting`).

The **range percentage** value is used to identify a threshold density (as a percentage of the dynamic range of each histogram) for classifying potential peaks (and, thus, multi-modality) in an area kernel.

The **delta bins** value represents a filter that removes peaks that have an higher peak in the surrounding histogram bins.

.. _fig_splitting:
.. figure:: _static/splitting.png
    :width: 400px
    :align: center
    :alt: splitting
    :figclass: align-center

    An example of reflectivity histogram with two detected peaks.



.. _merging:

Area Kernels Merging
====================

.. index::
    single: area kernels, merging

The **merging** step compares pairs of *reflectivity histograms* to detect similarity in reflectivity textures (see :numref:`fig_merging`).

The **intersection** value provides the criterion used to evaluate if a pair of reflectivity histograms are similar enough to be classified as part of the same segment.

The **min.samples** value is used to ignore the area kernels that are too small in size to have a reliable reflectivity histogram.

.. _fig_merging:
.. figure:: _static/merging.png
    :width: 400px
    :align: center
    :alt: merging
    :figclass: align-center

    An example of histogram comparison. The pair of reflectivity histograms has an intersection value of 71.56%.


.. _segments:

Segments
========

.. index::
    single: segments

A **segment** represents an area that has been classified with the same landform type and similar reflectivity texture.


.. _segments_stats:

Segments Statistics
===================

.. index::
    single: segments, stats

The **statistics** for each segment are optionally calculated and stored in a .csv file.

The statistical values (i.e., median, mean, and standard deviation) are based on the mosaic values and
the :ref:`stats_layers` of all the valid nodes in a segment.


.. _mosaic_mask:

Mosaic Mask
===========

.. index::
    single: mosaic, mask

The **mosaic mask** has the same information as *segments*, but resampled at the resolution of the input reflectivity grid.

It is usually used for theme-based seafloor characterization.


.. _cropped_dtm:

Cropped DTM
===========

.. index::
    single: DTM, cropped

The **cropped DTM** is obtained by removing all the unclassified nodes from the input elevation grid.