Create loop that create new imagecollection based on calculation of previous imagecollection

Question

I have imagecollection from sentinel which I would like to run few processes on it by using loop. I know what are the steps of the loop that I want and I have written draft that ia full of errores and I would like to get help in writing it correct.

The needd steps are:

1. apply the next function on each image:

//Apply Frost filter for speckle correction
function Frost(image, frostDamp, ksize)
// Frost is really a simplified version of GammaMap
// Make sure to pass the frost damp factor (frostDamp) as a NEGATIVE number
{
  // ~~(ksize/2) does integer division in JavaScript
  var distance_kernel = ee.Kernel.euclidean(~~(ksize/2));

  // Square kernel, ksize should be odd (typically 3, 5 or 7)
  var weights = ee.List.repeat(ee.List.repeat(1,ksize),ksize);

  var kernel = ee.Kernel.fixed(ksize, ksize, weights, ~~(ksize/2), ~~(ksize/2), false);

  // Get mean and variance
  var mean = image.reduceNeighborhood(ee.Reducer.mean(), kernel);
  var variance = image.reduceNeighborhood(ee.Reducer.variance(), kernel);

  var B = variance.multiply(frostDamp).divide(mean.multiply(mean));

  var W = B.exp().reduceNeighborhood(ee.Reducer.mean(), distance_kernel);

  return image.multiply(W).reduceNeighborhood(ee.Reducer.sum(), kernel).divide(W.reduceNeighborhood(ee.Reducer.sum(), kernel));
}

2. calculate new imagecollection , for each image ,each pixel will be calculted as folloing:

//here I would like to run calculation for each image in my imagecollection:
var calc=i.expression(
  '3.75-(-0.521*VH)+(0.34*VV)+(3.12*NRPB)', {
    'VH': i.select('VH_sum'),
    'VV': i.select('VV_sum'),
    'NRPB': VH.subtract(VV).divide(VH.add(VV)).rename('NRBP')
});

all of it will create new imagecollection.

I have tried to apply the first function using this:

var FrostDB=clippedSAR.map(function(im){ 
   return im.Frost(geometry);
});

but got this error:

im.Frost is not a function

My end goal is to create one loop that can run all the mentioned processes on an imagecollection, and also to get deeper understanding of loops in GEE

Answer 1

The error results because you are calling the Frost function on an ee.Image as if it were an Earth Engine method of the ee.Image object. However, Frost is a client-side function, so it can only be applied with the format as Sean describes (Frost(ee.Image)) or as a function to be mapped over an ee.ImageCollection like shown below. The later implementation requires that the function have only a single parameter - an ee.Image. Other client variables that you wish to include as parameters of the function must instead be defined as global variables to be accessed from within the function.

Pseudo script:

// Define global scope variables that will be accessible to Frost function.
var frostDamp = <some number>;
var ksize = <some number>;

// Define Frost filter for speckle correction.
function Frost(image) {
  <logic>;
}

// Apply the Frost function to image collection.
var frostFilterCol = myImgCol.map(Frost);

For further reference:

• Mapping a function over a collection
• Client vs server

Answer 2

I haven't been able to try it, as I don't have access to your different variables, but changing the applying of your function as follows should work.

var FrostDB=clippedSAR.map(function(im){ 
   im = Frost(ee.Image(im));
   return im;
});