Morphological Operations

  • Earth Engine uses focal operations like focalMax(), focalMin(), focalMedian(), and focalMode() to implement morphological operations for image processing.

  • Morphological operations, such as erosion, dilation, opening, and closing, can be performed using these focal operations to modify image structures.

  • Users can define kernels to specify the shape and size of the neighborhood used in the operations and control the number of times the operation is applied with the iterations argument.

  • An opening operation, for example, can be achieved by applying focalMin() followed by focalMax() with a defined kernel.

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():

Code Editor (JavaScript)

// 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.