Pixel wise NDVI calculation in GEE

Question

I have a collection of Landsat images filtered over forest regions of India over a time interval of 6 years(2013 - 2018). I have again filtered this collection such that I have one image per 4 months, that is I have filtered on a quarterly basis so that I will have 3 images per year.

Now, for a particular point, there will be 18 pixels representing it over 6 years. I have to find, out of these 18 pixels, how many satisfy the condition - NDVI < 0.25. Suppose, 10 pixels satisfy this condition, then in my output image, the pixel representing that particular point should have its value = 10. Like this I have to do for all the points in my selected region.

I have no idea as to how to do this pixel wise computation. Can somebody help me on this?

In my code, I have done till filtering quarterly wise.

// FILTERING OUT FOREST COVER IN INDIA

var treeCanopyCover = hansen.clip(india).select('treecover2000');
var greater25 = treeCanopyCover.gte(25);

// NDVI FUNCTION

function addNDVI(image) {
var ndvi = image.normalizedDifference(['B5', 'B4']).rename('NDVI');
return image.addBands(ndvi);
}

// FILTERING THE COLLECTION

var startDate = ee.Date('2013-01-01');
var endDate = ee.Date('2019-06-30');
var filtered = l8.filterBounds(india)
      .filterDate(startDate, endDate)
      .filterMetadata('CLOUD_COVER','less_than',25)
      .map(addNDVI);  

var maskedCollection = filtered.map(function(image){
  var x = image.updateMask(greater25);      // filtering over forest regions
  return x;
});

// FILTERING QUARTERLY WISE

var map_m = function(y)
{
  var months = [1, 5, 9];
  var filtered_col = months.map(function(m) 
{
var start = ee.Date.fromYMD(y, m, 1)
var end = start.advance(3, 'month');
var filt = maskedCollection.filterDate(start,end).mean();


return filt.set('year', y)
          .set('month', m)
          .set('system:time_start',ee.Date.fromYMD(y,m,1)) ;
  });
  return filtered_col;
};

var years = ee.List.sequence(2013, 2019);
var quarterlyimages = years.map(map_m).flatten();
var z = ee.ImageCollection(quarterlyimages);

Answer 1

I think count method of ImageCollection could help with what you're doing.

Assume col is the ImageCollection that you want to deal with. Every image in this col has NDVI band that holds NDVI value. The code below will result in an image called NDVI_count in which value of every pixel represents the number of pixels at the corresponding location in col that have NDVI < 0.25.

var NDVI_count = col.select('NDVI')
  .map(function(img) { return img.updateMask(img.select('NDVI').lt(0.25)) })
  .count()

The map function in the above code just does one thing: mask out any pixels that have NDVI >= 0.25. The count method will count the number of valid pixels (i.e. not-masked pixel) throughout the collection at each specific location.

Hope this helps.