// Script provided by Dr. Andreas Braun within the SNAP forum on 2021-03-10


// Load basic variables (explained in previous sections)
var start = ee.Date('2020-01-01');
var end = ee.Date('2020-12-31');
var point = ee.Geometry.Point([107.58, 16.47]);
var colors = {min: 0, max: 4000, bands: ['B4', 'B3', 'B2']};
var S2 = ee.ImageCollection('COPERNICUS/S2');
var S2_filtered = S2
.filterDate(start,end)
.filterBounds(point)
.filter(ee.Filter.lt('CLOUDY_PIXEL_PERCENTAGE', 10));
Map.centerObject(point, 14);


// Create a function which calculates the NDVI and adds it to a product
var calculate_NDVI = function(func_image) {
  func_image = ee.Image(func_image);
  var ndvi_band = func_image.normalizedDifference(['B8', 'B4']).rename('NDVI_band');
  return func_image.addBands(ndvi_band);
};

// Calculate the NDVI for all images in the filtered collection
var S2_NDVI = S2_filtered.map(calculate_NDVI);
print('Image collection after mapping NDVI function', S2_NDVI);


// Calculate the variation of the NDVI over the entire year 2020
var NDVI_2020 = S2_NDVI.select('NDVI_band').reduce(ee.Reducer.stdDev());
Map.addLayer(NDVI_2020, {min: 0.01, max: 0.28, palette: ['black', 'blue', 'yellow']}, "Standard deviation NDVI")


// Create a temporal plot of the NDVI values of this point
print(ui.Chart.image.series(S2_NDVI.select('NDVI_band'), point));