if-then-else condition in Google Earth Engine

Question

I have Sample RGB combination from 3 different Bands

// Input RGB imagery
var image = ee.Image("users/k2yulianto/Exercise/Ras/Imagery");
// Use Tree bands from RGB Imagery
var bands = ['b1', 'b2', 'b3'];
var b1 = ['b1'];
var b2 = ['b2'];
var b3 = ['b3'];
// Display the inputs imagery
Map.addLayer (image, {bands: ['b1', 'b2', 'b3'], max: 1, gamma:1, opacity:0.80} , 'RGB imagery',true);
Map.setCenter (101.4476, 0.4925, 12);

And I have also sampling point distribution on top of RGB Imagery.

// Sampling point on top of RGB imagery to identify 4 diferent class
var Sampling = ee.FeatureCollection("users/k2yulianto/Exercise/Sampling/Sampling");
Map.addLayer (Sampling, {opacity:0.80},'Sampling point', true);

Based on the sampling point, I am to produce decision tree using WEKA to distinguish the type of class on RGB imagery.

//var exercise = ee.Algorithms.if(condition,truecase,falsecase)
var exercise = ee.Algorithms.If(b2 <= (0),(ee.Algorithms.If(b3 <= (0),"Sample 1","Sample 2")),(ee.Algorithms.If(b1 <= (0),"Sample 3","Sample 4")));
print('Imagery output: ',exercise);

I want to display if-then-else condition into an image classification.

// Display the 4 Class exercise output imagery
var RGBPalette = 
['#5f932c', // Class 01
'#bdff09', // Class 02 
'#fdf613', // Class 03 
'#abfffb', // Class 04
];
Map.addLayer(exercise,{min: 0, max: 4, palette: RGBPalette}, 'Exercise imagery');

After I run the scripts, I got an error warning:

Cannot add an object of type to the map.

Here is my full code:

https://code.earthengine.google.com/77b2238d1a628c59dfa440a27adfa378

How can I solve this?

Answer 1

The function you are using to solve the problem (ee.Algorithms.If) is for using an if statement inside a GEE loop (map or iterate), but not for this case.

For this, there are a couple of ways to do it, but I think the easier would be using the expression function:

var exercise = image.expression('b(1)<=0 ? (b(2)<=0 ? 1 : 2) : (b(0)<=0 ? 3 : 4)').rename('class')

Inside parenthesis you can use the number of the band, like in this example b(1) is band b2, or you could use the name b("b2") (https://developers.google.com/earth-engine/guides/image_math)

https://code.earthengine.google.com/708bfe3d64299bc0982390f0047be9ad