Add NDSI, NDBI, BAI, MSAVI2, and OSAVI spectral indices (#1044)

* Add NDSI, NDBI, BAI, MSAVI2, and OSAVI spectral indices (#1037)

Five new indices in xrspatial/multispectral.py with all four backends
(numpy, cupy, dask+numpy, dask+cupy):

- NDSI: normalized difference snow index (green vs SWIR1)
- NDBI: normalized difference built-up index (SWIR1 vs NIR)
- BAI: burn area index (spectral distance to charcoal point)
- MSAVI2: modified SAVI with self-adjusting soil line
- OSAVI: optimized SAVI with fixed L=0.16

NDSI and NDBI reuse existing _normalized_ratio kernels.
BAI, MSAVI2, and OSAVI have dedicated kernels.

* Add tests for NDSI, NDBI, BAI, MSAVI2, and OSAVI (#1037)

Tests cover CPU (numpy), Dask+numpy, CuPy, and Dask+CuPy backends
for all five indices. Includes uint dtype tests and cross-index
consistency checks (NDBI = -NDMI, OSAVI ≈ NDVI for large DN values).

* Add docs and README entries for new spectral indices (#1037)

Add BAI, MSAVI2, NDBI, NDSI, OSAVI to multispectral.rst reference
docs and README feature matrix (18 indices total, up from 13).

* Add user guide notebook for NDSI, NDBI, BAI, MSAVI2, OSAVI (#1037)

Notebook 35_Spectral_Indices.ipynb with synthetic 5-class scene
(snow, urban, burned, vegetation, bare soil) demonstrating all
five new spectral indices. Self-contained, no external data.

Author: brendancol

README.md

@@ -299,15 +299,20 @@ write_vrt('mosaic.vrt', ['tile1.tif', 'tile2.tif'])  # generate VRT
 | Name | Description | Source | NumPy xr.DataArray | Dask xr.DataArray | CuPy GPU xr.DataArray | Dask GPU xr.DataArray |
 |:----------:|:------------|:------:|:----------------------:|:--------------------:|:-------------------:|:------:|
 | [Atmospherically Resistant Vegetation Index (ARVI)](xrspatial/multispectral.py) | Vegetation index resistant to atmospheric effects using blue band correction | Kaufman & Tanre 1992 | ✅️ |✅️ | ✅️ |✅️ |
+| [Burn Area Index (BAI)](xrspatial/multispectral.py) | Spectral distance to charcoal reflectance point for burn scar detection | Chuvieco et al. 2002 | ✅️ |✅️ | ✅️ |✅️ |
 | [Enhanced Built-Up and Bareness Index (EBBI)](xrspatial/multispectral.py) | Highlights built-up areas and barren land from thermal and SWIR bands | As-syakur et al. 2012 | ✅️ |✅️  | ✅️ |✅️ |
 | [Enhanced Vegetation Index (EVI)](xrspatial/multispectral.py) | Enhanced vegetation index reducing soil and atmospheric noise | Huete et al. 2002 | ✅️ |✅️ | ✅️ |✅️ |
 | [Green Chlorophyll Index (GCI)](xrspatial/multispectral.py) | Estimates leaf chlorophyll content from green and NIR reflectance | Gitelson et al. 2003 | ✅️ |✅️ | ✅️ |✅️ |
+| [Modified Soil Adjusted Vegetation Index (MSAVI2)](xrspatial/multispectral.py) | Self-adjusting soil line vegetation index, no L parameter needed | Qi et al. 1994 | ✅️ |✅️ | ✅️ |✅️ |
 | [Normalized Burn Ratio (NBR)](xrspatial/multispectral.py) | Measures burn severity using NIR and SWIR band difference | USGS Landsat | ✅️ |✅️ | ✅️ |✅️ |
 | [Normalized Burn Ratio 2 (NBR2)](xrspatial/multispectral.py) | Refines burn severity mapping using two SWIR bands | USGS Landsat | ✅️ |✅️ | ✅️ |✅️ |
+| [Normalized Difference Built-up Index (NDBI)](xrspatial/multispectral.py) | Picks out built-up and urban areas from SWIR and NIR bands | Zha et al. 2003 | ✅️ |✅️ | ✅️ |✅️ |
 | [Normalized Difference Moisture Index (NDMI)](xrspatial/multispectral.py) | Detects vegetation moisture stress from NIR and SWIR reflectance | USGS Landsat | ✅️ |✅️ | ✅️ |✅️ |
+| [Normalized Difference Snow Index (NDSI)](xrspatial/multispectral.py) | Separates snow and ice from clouds using green and SWIR bands | Hall et al. 1995 | ✅️ |✅️ | ✅️ |✅️ |
 | [Normalized Difference Water Index (NDWI)](xrspatial/multispectral.py) | Maps open water bodies using green and NIR band difference | McFeeters 1996 | ✅️ |✅️ | ✅️ |✅️ |
 | [Modified Normalized Difference Water Index (MNDWI)](xrspatial/multispectral.py) | Detects water in urban areas using green and SWIR bands | Xu 2006 | ✅️ |✅️ | ✅️ |✅️ |
 | [Normalized Difference Vegetation Index (NDVI)](xrspatial/multispectral.py) | Quantifies vegetation density from red and NIR band difference | Rouse et al. 1973 | ✅️ |✅️ | ✅️ |✅️ |
+| [Optimized Soil Adjusted Vegetation Index (OSAVI)](xrspatial/multispectral.py) | SAVI with fixed L=0.16, tuned for sparse vegetation | Rondeaux et al. 1996 | ✅️ |✅️ | ✅️ |✅️ |
 | [Soil Adjusted Vegetation Index (SAVI)](xrspatial/multispectral.py) | Vegetation index with soil brightness correction factor | Huete 1988 | ✅️ |✅️ | ✅️ |✅️ |
 | [Structure Insensitive Pigment Index (SIPI)](xrspatial/multispectral.py) | Estimates carotenoid-to-chlorophyll ratio for plant stress detection | Penuelas et al. 1995 | ✅️ |✅️ | ✅️ |✅️ |
 | [True Color](xrspatial/multispectral.py) | Composites red, green, and blue bands into a natural color image | Standard | ✅️ | ✅️ | ✅️ | ✅️ |

docs/source/reference/multispectral.rst

@@ -11,6 +11,13 @@ Atmospherically Resistant Vegetation Index (ARVI)
 
     xrspatial.multispectral.arvi
 
+Burn Area Index (BAI)
+=====================
+.. autosummary::
+    :toctree: _autosummary
+
+    xrspatial.multispectral.bai
+
 Enhanced Built-Up and Bareness Index (EBBI)
 ============================================
 .. autosummary::
@@ -32,6 +39,13 @@ Green Chlorophyll Index (GCI)
 
     xrspatial.multispectral.gci
 
+Modified Soil Adjusted Vegetation Index (MSAVI2)
+=================================================
+.. autosummary::
+    :toctree: _autosummary
+
+    xrspatial.multispectral.msavi2
+
 Normalized Burn Ratio (NBR)
 ===========================
 .. autosummary::
@@ -46,13 +60,27 @@ Normalized Burn Ratio 2 (NBR2)
 
     xrspatial.multispectral.nbr2
 
+Normalized Difference Built-up Index (NDBI)
+===========================================
+.. autosummary::
+    :toctree: _autosummary
+
+    xrspatial.multispectral.ndbi
+
 Normalized Difference Moisture Index (NDMI)
 ===========================================
 .. autosummary::
     :toctree: _autosummary
 
     xrspatial.multispectral.ndmi
 
+Normalized Difference Snow Index (NDSI)
+=======================================
+.. autosummary::
+    :toctree: _autosummary
+
+    xrspatial.multispectral.ndsi
+
 Normalized Difference Water Index (NDWI)
 ========================================
 .. autosummary::
@@ -74,6 +102,13 @@ Normalized Difference Vegetation Index (NDVI)
 
     xrspatial.multispectral.ndvi
 
+Optimized Soil Adjusted Vegetation Index (OSAVI)
+=================================================
+.. autosummary::
+    :toctree: _autosummary
+
+    xrspatial.multispectral.osavi
+
 Soil Adjusted Vegetation Index (SAVI)
 =====================================
 .. autosummary::

examples/user_guide/35_Spectral_Indices.ipynb

@@ -66,11 +66,16 @@
 from xrspatial.hillshade import hillshade  # noqa
 from xrspatial.mahalanobis import mahalanobis  # noqa
 from xrspatial.multispectral import arvi  # noqa
+from xrspatial.multispectral import bai  # noqa
 from xrspatial.multispectral import evi  # noqa
 from xrspatial.multispectral import mndwi  # noqa
+from xrspatial.multispectral import msavi2  # noqa
 from xrspatial.multispectral import nbr  # noqa
+from xrspatial.multispectral import ndbi  # noqa
+from xrspatial.multispectral import ndsi  # noqa
 from xrspatial.multispectral import ndvi  # noqa
 from xrspatial.multispectral import ndwi  # noqa
+from xrspatial.multispectral import osavi  # noqa
 from xrspatial.multispectral import savi  # noqa
 from xrspatial.multispectral import sipi  # noqa
 from xrspatial.pathfinding import a_star_search  # noqa