Extracting pixel values in sample polygons in Google Earth Engine

Question

I am trying to generate histogram of pixel values for my sample polygons. I used the code suggested on Histogram Matching in Google Earth Engine. Its working but I am not able to control the bin size so I want to extract the raw data (that the code is using to create the histogram) in csv so that I can plot the histogram in excel with more control of histogram display. Here is what I have tried. Is it possible to get the pixel data that GEE is using to plot histogram.

var s2 = ee.ImageCollection("COPERNICUS/S2");
var admin1 = ee.FeatureCollection("FAO/GAUL_SIMPLIFIED_500m/2015/level1");
  
var MP = admin1.filter(ee.Filter.eq('ADM1_NAME', 'Madhya Pradesh'));
var geometry = MP.geometry();
var rgbVis = {min: 0.0, max: 3000, bands: ['B4', 'B3', 'B2']};
Map.centerObject(geometry,9.5);
var filtered1 = s2.filter(ee.Filter.lt('CLOUDY_PIXEL_PERCENTAGE', 10))
  .filter(ee.Filter.date('2020-03-15', '2020-03-21'))
  .filter(ee.Filter.bounds(geometry));
  function addIndices(image) {
  var ndvi = image.normalizedDifference(['B8', 'B4']).rename('ndvi');
  var ndwi = image.normalizedDifference(['B3', 'B8']).rename('ndwi');
  return image.addBands(ndvi).addBands(ndwi);
}
var withIndices1 = filtered1.map(addIndices);
var compositeA = withIndices1.median().clip(geometry);
var palette = [
  'FFFFFF', 'CE7E45', 'DF923D', 'F1B555', 'FCD163', '99B718',
  '74A901', '66A000', '529400', '3E8601', '207401', '056201',
  '004C00', '023B01', '012E01', '011D01', '011301'];
  var ndviVis = {min:0, max:0.5, palette: palette };
  var NDVI = compositeA.select('ndvi');
  Map.addLayer(compositeA, rgbVis, 'Karnataka ');
  Map.addLayer(NDVI,ndviVis,'NDVI');
  
  
  ////////// Define a function to display histogram of a band////////
//////////NDVI:Unburned
var showHistogram = function(image, name, band) {
  var options = {
  title: 'Histogram of ' + name,
  fontSize: 20,
  hAxis: {title: 'DN'},
  vAxis: {title: 'count of DN'},
  };
    
  var histogram = ui.Chart.image.histogram({
  image: image.select(band),
  region: Unburned,
  maxBuckets: Math.pow(2, 8),
  scale: 20,
  });
  
  var histogram = histogram.setSeriesNames([band])
    .setOptions(options);
  print(histogram);
};

// Show histograms
showHistogram(compositeA, 'NDVI Unburned', 'ndvi');

//////////NDVI:Burned
var showHistogram = function(image, name, band) {
  var options = {
  title: 'Histogram of ' + name,
  fontSize: 20,
  hAxis: {title: 'DN'},
  vAxis: {title: 'count of DN'},
  };
    
  var histogram = ui.Chart.image.histogram({
  image: image.select(band),
  region: Burned,
  maxBuckets: Math.pow(2, 8),
  scale: 20,
  });
  
  var histogram = histogram.setSeriesNames([band])
    .setOptions(options);
  print(histogram);
};

// Show histograms
showHistogram(compositeA, 'NDVI Burned', 'ndvi');

Answer 1

You can use ee.Image.sampleRegions() (see here) on your created composite image to extract all pixel values within your sample polygons, and then export it as a CSV using Export.table.toDrive() (see here).

Example code:

var sampled_pixels = compositeA.sampleRegions(MP)
Export.table.toDrive(sampled_pixels) // CSV by default