Normalized difference vegetation index
computes the normalized difference vegetation index (NDVI) value for each pixel in the data
cube and returns an NDVI image. The NDVI image displays the vegetation cover regions of the
input hyperspectral data. The function computes the NDVI value using the red (R) band and
the near-infra red (NIR) band images in the data cube. The
output = ndvi(
function uses the 670 nm and 800 nm band reflectance values for the red and NIR band images
The function divides the input image into distinct blocks,
processes each block, and then concatenates the processed output of each block to form the
output matrix. Hyperspectral images are multi-dimensional data sets that can be too large to fit
in system memory in their entirety. This can cause the system to run out of memory while running
ndvi function. If you encounter such an issue, perform block
processing by using this syntax.
ndvi(hcube,'BlockSize',[50 50]) divides the input image
into non-overlapping blocks of size 50-by-50 and then computes the NDVI values for pixels in
To perform block processing by specifying the
pair argument, you must have MATLAB® R2021a or a
This function requires the Image Processing Toolbox™ Hyperspectral Imaging Library. You can install the Image Processing Toolbox Hyperspectral Imaging Library from Add-On Explorer. For more information about installing add-ons, see Get and Manage Add-Ons.
Measure Vegetation Cover in Hyperspectral Data Using NDVI Image
Read hyperspectral data into the workspace.
hcube = hypercube('indian_pines.dat');
Compute the NDVI value for each pixel in the data cube.
ndviImg = ndvi(hcube);
Estimate a contrast-stretched RGB image from the original data cube by using the
rgbImg = colorize(hcube,'Method','RGB','ContrastStretching',true);
Display the original and the NDVI image.
fig = figure('Position',[0 0 1200 600]); axes1 = axes('Parent',fig,'Position',[0 0.1 0.4 0.8]); imshow(rgbImg,'Parent',axes1) title('RGB Image of Data Cube') axes2 = axes('Parent',fig,'Position',[0.45 0.1 0.4 0.8]); imagesc(ndviImg,'Parent',axes2) colorbar title('NDVI Image')
Vegetation regions typically have NDVI values from 0.2 and 0.8. NDVI values less than or equal to 0.2 indicate the absence of vegetation. Perform thresholding of NDVI image to segment the vegetation regions. Specify the threshold value.
threshold = 0.2;
Generate a binary image with a intensity value
1 for pixels with a score greater than or equal to the specified threshold. All other pixels have a value 0. The regions in the binary image with a value of
1 correspond to the vegetation regions in the data cube with NDVI values greater than the threshold.
bw = ndviImg > threshold;
Overlay the binary image on to the RGB image and display the overlaid image.
overlayImg = imoverlay(rgbImg,bw,[0 1 0]); figure imagesc(overlayImg) title('Vegetation Region Overlaid on RGB Image')
Compute the vegetation cover based on the total number of pixels in a spectral band and the number of pixels with an NDVI value greater than 0.2.
numVeg = find(bw == 1); imgSize = size(hcube.DataCube,1)*size(hcube.DataCube,2); vegetationCover = length(numVeg)/imgSize
vegetationCover = 0.5696
Identify Vegetation and Non-Vegetation Spectra
Load 2-D spectral data containing 20 endmembers of the Indian Pines data set into the workspace.
Load the wavelength values for each band of the Indian Pines data set into the workspace.
Reshape the 2-D spectral data into a 3-D volume data using the
[numSpectra,spectralDim] = size(endmembers); dataCube = reshape(endmembers,[numSpectra 1 spectralDim]);
Create a 3-D
hypercube object, with a singleton dimension, by specifying the 3-D volume data
dataCube and wavelength information
wavelength to the
hCube = hypercube(dataCube,wavelength);
Compute the NDVI value for each spectrum in the hypercube object.
ndviVal = ndvi(hCube);
Vegetation spectra typically have NDVI values greater than zero and non-vegetation spectra typically have NDVI values less than zero. Perform thresholding to separate the vegetation and non-vegetation spectra.
index = ndviVal > 0;
Plot the vegetation and non-vegetation endmembers.
subplot(2,1,1) plot(endmembers(index,:)') title("Vegetation endmembers") xlabel("Bands") ylabel("Reflectance Values") axis tight subplot(2,1,2) plot(endmembers(~index,:)') title("Non-vegetation endmembers") xlabel("Bands") ylabel("Reflectance Values") axis tight
hcube — Input hyperspectral data
Input hyperspectral data, specified as a
object. The functions reads the hyperspectral data cube from the
DataCube property of the object and then computes the NDVI value
of each pixel.
blocksize — Size of data blocks
2-element vector of positive integers
Size of the data blocks, specified as a 2-element vector of positive integers. The elements of the vector correspond to the number of rows and columns in each block, respectively. The size of the data blocks must be less than the size of the input image. Dividing the hyperspectral images into smaller blocks enables you process large data sets without running out of memory.
blocksizevalue is too small, the memory usage of the function reduces at the cost of increased execution time.
blocksizevalue is large or equal to the input image size, the execution time reduces at the cost of increased memory usage.
'BlockSize',[20 20] specifies the size of each data block
output — Output NDVI image
Output NDVI image, returned as a matrix of size
M-by-N. M and
N are spatial dimensions of the input data cube. If the data type
of the input data cube is
double, the output data type is also
double. Otherwise, the output data type is
The function computes the NDVI value for each pixel as
The values are in the range [-1, 1]. A value close to 1 indicates healthy vegetation, 0 indicates unhealthy vegetation, and -1 indicates no vegetation.
 Haboudane, D. “Hyperspectral Vegetation Indices and Novel Algorithms for Predicting Green LAI of Crop Canopies: Modeling and Validation in the Context of Precision Agriculture.” Remote Sensing of Environment 90, no. 3 (April 15, 2004): 337–52. https://doi.org/10.1016/j.rse.2003.12.013.