Add NDVI calculation

Add a function that calculates NDVI for a given scene. It's better than
having to remember the equations all the time. The function can also
take care of adding some metadata and copying the metadata from the
original scene. The function allows customizing the names of the red and
NIR bands.

doc/api/index.rst

@@ -11,22 +11,37 @@ Input and output
 ----------------
 
 .. autosummary::
-   :toctree: generated/
+    :toctree: generated/
 
     load_scene
     save_scene
     load_panchromatic
 
-Processing
-----------
+Visualization
+-------------
 
 .. autosummary::
-   :toctree: generated/
+    :toctree: generated/
 
     composite
-    pansharpen
     equalize_histogram
     adjust_l1_colors
+
+Indices
+-------
+
+.. autosummary::
+    :toctree: generated/
+
+    ndvi
+
+Processing
+----------
+
+.. autosummary::
+    :toctree: generated/
+
+    pansharpen
     interpolate_missing
 
 Sample datasets

xlandsat/__init__.py

@@ -4,6 +4,7 @@
 from . import datasets
 from ._composite import composite
 from ._enhancement import adjust_l1_colors, equalize_histogram
+from ._indices import ndvi
 from ._interpolation import interpolate_missing
 from ._io import load_panchromatic, load_scene, save_scene
 from ._pansharpen import pansharpen

xlandsat/_indices.py

@@ -0,0 +1,41 @@
+# Copyright (c) 2022 The xlandsat developers.
+# Distributed under the terms of the MIT License.
+# SPDX-License-Identifier: MIT
+"""
+Calculate indices based on data in xarray.Datasets
+"""
+
+
+def ndvi(scene, red_band="red", nir_band="nir"):
+    """
+    Normalized difference vegetation index
+
+    Calculate the NDVI for the given scene, defined as:
+
+    .. math::
+
+        NDVI = \dfrac{NIR - Red}{NIR + Red}
+
+    Parameters
+    ----------
+    scene : :class:`xarray.Dataset`
+        A Landsat scene, as read with :func:`xlandsat.load_scene`.
+    red_band : str
+        The name of the variable in ``scene`` that corresponds to the red band.
+    nir_band : str
+        The name of the variable in ``scene`` that corresponds to the NIR band.
+
+    Returns
+    -------
+    ndvi : :class:`xarray.DataArray`
+        The calculated NDVI, with the metadata attributes from the original
+        scene.
+    """
+    red = scene[red_band]
+    nir = scene[nir_band]
+    result = (nir - red) / (nir + red)
+    result.name = "ndvi"
+    attrs = {"long_name": "normalized difference vegetation index"}
+    attrs.update(scene.attrs)
+    result = result.assign_attrs(attrs)
+    return result