Counting number of unmasked pixels per image in collection using Google Earth Engine

Question

I'm trying to count the number of pixels that are masked by the cloud mask of the image collection given by the Earth Engine dataset. I have clipped my collection with a ROI and now I want to keep the images with 10% or less of clouds (masked pixels). By now, I am only able to count them for a single image but not for the entire collection.

I am now using this code for one image but I am not able to reproduce this for the entire collection.

var Peyto_pixelscount = Peytoclip.first().select('ALBEDO').reduceRegion({
  reducer: ee.Reducer.count(),
  geometry: Peyto_glacier.geometry(),
  scale: 30,
  maxPixels: 1e9
});

print(Peyto_pixelscount)

Answer 1

To get to your end goal of filtering your clipped image collection by 10% or less cloud cover, I would write a function that:

1. counts the number of unmasked pixels in the image
2. counts the number of pixels in the unmasked image
3. calculates the percentage of masked pixels and sets this as a metadata property

I would then map this onto your image collection and filter it by the added metadata property.

Note that I'm assuming that your images are only masked by a cloud mask.

function get_cloud_cover_roi(image){ 

  var Peyto_pixelscount = image.select('ALBEDO').reduceRegion({
    reducer: ee.Reducer.count(),
    geometry: image.geometry(),
    scale: 30,
    maxPixels: 1e9
  }).get('ALBEDO')

  var npix = image.select('ALBEDO').unmask().reduceRegion({
    reducer: ee.Reducer.count(),
    geometry: image.geometry(),
    scale: 30,
    maxPixels: 1e9
  }).get('ALBEDO')

  var cloud_cover_roi = ee.Number(1)
      .subtract(ee.Number(Peyto_pixelscount).divide(npix))
      .multiply(100)

  return image.set('cloud_cover_roi', cloud_cover_roi)

}

var Peytoclip_filtered = Peytoclip
    .map(get_cloud_cover_roi)
    .filterMetadata('cloud_cover_roi', 'less_than', 10)

Answer 2

Similar logic to the previous answer but in a function call.

geom could be any region in the image, or image.geometry() for the whole image.

/** 
 * return the percentage of masked pixels in an image region. 
 */
function calculateMaskedPercent(image, band, geom) {
   var maskedPixelCount = image.select(band).reduceRegion({
    reducer: ee.Reducer.count(),
    geometry: geom,
    scale: 30,
    maxPixels: 1e9
  }).get(band);

  var totalPixelCount  = image.unmask().select(band).reduceRegion({
    reducer: ee.Reducer.count(),
    geometry: geom,
    scale: 30,
    maxPixels: 1e9
  }).get(band);
 
  var cloud_cover_roi = ee.Number(1)
      .subtract(ee.Number(maskedPixelCount).divide(totalPixelCount))
      .multiply(100)
  return cloud_cover_roi;
}