Morphological Operations

AI-generated Key Takeaways

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• Earth Engine uses focal operations like focalMax() and focalMin() for morphological operations such as erosion, dilation, opening, and closing.

• These morphological operators are instance methods of the Image class and are shortcuts for the more general reduceNeighborhood() method.

• An opening operation can be performed by applying focalMin() followed by focalMax().

• The kernel argument defines the neighborhood of pixels used in the computation, and the iterations argument specifies how many times the operator is applied.

Earth Engine implements morphological operations as focal operations, specifically focalMax(), focalMin(), focalMedian(), and focalMode() instance methods in the Image class. (These are shortcuts for the more general reduceNeighborhood(), which can input the pixels in a kernel to any reducer with a numeric output. See this page for more information on reducing neighborhoods). The morphological operators are useful for performing operations such as erosion, dilation, opening and closing. For example, to perform an opening operation, use focalMin() followed by focalMax():

// Load a Landsat 8 image, select the NIR band, threshold, display.
var image = ee.Image('LANDSAT/LC08/C02/T1_TOA/LC08_044034_20140318')
            .select(4).gt(0.2);
Map.setCenter(-122.1899, 37.5010, 13);
Map.addLayer(image, {}, 'NIR threshold');

// Define a kernel.
var kernel = ee.Kernel.circle({radius: 1});

// Perform an erosion followed by a dilation, display.
var opened = image
             .focalMin({kernel: kernel, iterations: 2})
             .focalMax({kernel: kernel, iterations: 2});
Map.addLayer(opened, {}, 'opened');

Note that in the previous example, a kernel argument is provided to the morphological operator. The pixels covered by non-zero elements of the kernel are used in the computation. The iterations argument indicates how many times to apply the operator.