Manually Label ROIs in Multispectral Image

This example uses:

This example shows how to manually select regions of interest (ROIs) from a multispectral image and save them in a shapefile.

This example 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. The Image Processing Toolbox Hyperspectral Imaging Library requires desktop MATLAB®, as MATLAB® Online™ and MATLAB® Mobile™ do not support the library.

Many supervised learning applications require labeled training data. This example shows how to manually label multispectral or hyperspectral images by selecting ROIs and saving them in a shapefile. You can use the shapefile to train deep learning networks.

In this example, you perform these steps:

Read a multispectral image and select multiple ROIs.
Convert the ROIs into geographic coordinates.
Save the geographic coordinates of the ROIs in a shapefile.
Read the shapefile and visualize the ROIs in a geographic axes.

Load Multispectral Data

Landsat 8 is an Earth observation satellite that carries the Operational Land Imager (OLI) and Thermal Infrared Sensor (TIRS) instruments.

The Landsat 8 data set has 8 spectral bands with wavelengths that range from 440 nm to 2200 nm. The data is 7721-by-7651 pixels in dimension with a spatial resolution of 30 meters.

Download the data set and unzip the file by using the downloadLandsat8Dataset helper function. The helper function is attached to this example as a supporting file.

zipfile = "LC08_L1TP_113082_20211206_20211206_01_RT.zip";
landsat8Data_url = "https://ssd.mathworks.com/supportfiles/image/data/" + zipfile;
downloadLandsat8Dataset(landsat8Data_url,pwd)

Downloading the Landsat 8 OLI dataset.
This can take several minutes to download...
Done.

Read the Landsat 8 multispectral data into the workspace as a hypercube object.

hCube = hypercube("LC08_L1TP_113082_20211206_20211206_01_RT_MTL.txt");

Estimate an RGB image from the data cube by using the colorize function. Apply contrast stretching to enhance the contrast of the output RGB image.

rgbImg = colorize(hCube,Method="rgb",ContrastStretching=true);

Adjust the intensity values of the image for better visualization using the imadjustn function.

rgbImg = imadjustn(rgbImg);

Read the spatial referencing information for the Landsat 8 data from the corresponding GeoTIFF image.

info = georasterinfo("LC08_L1TP_113082_20211206_20211206_01_RT_B1.TIF");

Calculate the data region using the corner coordinates of the GeoTIFF image.

R = info.RasterReference;
xlimits = R.XWorldLimits;
ylimits = R.YWorldLimits;
dataRegion = mappolyshape(xlimits([1 1 2 2 1]),ylimits([1 2 2 1 1]));
dataRegion.ProjectedCRS = R.ProjectedCRS;

Select ROIs and Save in Shapefile

Specify the number of ROIs to select. For this example, select three ROIs.

numOfAreas = 3;

Visualize the estimated RGB image. Use the pickPolyshape helper function, defined at the end of this example, to select rectangular ROIs and store the x- and y-coordinates of the ROIs in the cell arrays polyX and polyY, respectively.

figure
imshow(rgbImg)
polyX = cell(numOfAreas,1);
polyY = cell(numOfAreas,1);
for ch = 1:numOfAreas
    [x,y] = pickPolyshape(R);
    polyX{ch} = x;
    polyY{ch} = y;
end

Create ROI shapes from the ROI coordinates by using the mappolyshape (Mapping Toolbox) function.

shape = mappolyshape(polyX,polyY);
shape.ProjectedCRS = R.ProjectedCRS;

Create a geospatial table from the ROI shapes.

gt = table(shape,VariableNames="Shape");

Write the ROI shapes to the shapefile format. You can use this shapefile as labeled data.

shapewrite(gt,"Landsat8ROIs.shp")

Read Shapefile and Visualize ROIs in Geographic Axes

Read the shapefile as a geospatial table.

S = readgeotable("Landsat8ROIs.shp");
S.Shape.ProjectedCRS = R.ProjectedCRS;

Visualize the ROIs in a geographic axes along with the data region of the Landsat 8 multispectral image.

figure
geoplot(dataRegion)
hold on
geobasemap satellite
geoplot(S)

Supporting Functions

The pickPolyshape helper function performs these tasks:

Creates a customizable rectangular ROI.
Calculates the x- and y-coordinates of the corners of the ROI.
Transforms the intrinsic coordinates of the ROI to world coordinates.

function [xWorld,yWorld] = pickPolyshape(R)   
    roi = drawrectangle(Color="r");
    x1 = roi.Position(1);
    y1 = roi.Position(2);
    x2 = x1 + roi.Position(3);
    y2 = y1 + roi.Position(4);
    [xWorld,yWorld] = intrinsicToWorld(R,[x2 x1 x1 x2 x2],[y1 y1 y2 y2 y1]);
end