Earth Engine - reduce image collection based on pixels with max in one band

Question

I'm trying to reduce an ImageCollection based on pixels with the highest value in a particular band (Sentinel 2, band 3, in case that matters). I also need to retain all bands in the output. So, I want to choose the greenest pixels from the image stack. The max() reducer doesn't appear to work this way? How would I go about this?

var s2 = s2_col.filterDate('2019','2022')
               .filterBounds(LGA)
               .reduce(ee.Reducer.max(s2_col.select('B3')))

Answer 1

You can use qualityMosaic to do this, or you can use ee.Reducer.max({numInputs:13}), however with the reducer, the band with the max you want to select on has to be the first band.

var s2 = s2_col.filterDate('2019','2022')
               .filterBounds(LGA)

// Move B3 to position 0 in the list of bands.
var bands = s2.first().bandNames()
bands = bands.remove('B3')
bands = bands.insert('B3', 0)

var result = s2.select(bands).reduce(ee.Reducer.max(13)).rename(bands)

Answer 2

Issues are produced because there is a wrong parameters inside max reducer (numInputs; Integer, default: 1). In this case, you can get the greenest pixels from the image stack in two ways; as it can be observed in following code.

var LGA = ee.Geometry.Polygon(
        [[[-118.491015625, 39.863116755771],
          [-118.491015625, 38.5006330236386],
          [-116.82109375, 38.5006330236386],
          [-116.82109375, 39.863116755771]]], null, false);

var imageVisParam = {"opacity":1,
                     "bands":["B3_max"],
                     "min":9326,
                     "max":16916,
                     "gamma":1};

var s2_col = ee.ImageCollection('COPERNICUS/S2_SR');

var s2 = s2_col.filterDate('2019','2022')
               .filterBounds(LGA)
               .select('B3')
               .reduce(ee.Reducer.max())
               .clip(LGA);

print(s2);

Map.centerObject(s2);
Map.addLayer(s2, imageVisParam, 's2');

var s2_new = s2_col.filterDate('2019','2022')
               .filterBounds(LGA)
               .select('B3');

var B3_max = s2_new.max().rename('B3_max')
                         .clip(LGA);

Map.addLayer(B3_max, imageVisParam, 'B3_max');

After running above code in GEE code editor, you can corroborate in Inspector tab that both loaded images are identical.