Reclassify ranges in Google Earth Engine - google-earth-engine

Question

I would like to reclassify Global Forest Data values i.e like
0 - 20 % --> 1
21 - 49 % --> 0.5
50 - 100 % --> 0
However, I wasn`t able to find how to do this for ranges in GEE. Explanation for reclassifying individual numbers can be found here:
https://sites.google.com/site/globalsnowobservatory/home/Presentations-and-Tutorials/short-tutorial/remap
but simple procedure for ranges (without decision trees) is hard to find.
Could someone provide a simple solution for this?
// Example from https://developers.google.com/earth-engine/resample
// Load a MODIS EVI image.
var modis = ee.Image(ee.ImageCollection('MODIS/006/MOD13A1').first())
.select('EVI');
// Get information about the MODIS projection.
var modisProjection = modis.projection();
// Load and display forest cover data at 30 meters resolution.
var forest = ee.Image('UMD/hansen/global_forest_change_2015')
.select('treecover2000');
// Get the forest cover data at MODIS scale and projection.
var forestMean = forest
// Force the next reprojection to aggregate instead of resampling.
.reduceResolution({
reducer: ee.Reducer.mean(),
maxPixels: 1024,
bestEffort:true
})
// Request the data at the scale and projection of the MODIS image.
.reproject({
crs: modisProjection
});

Answer 1

If you want to make binary decisions about pixel values, you can use the ee.Image.where() algorithm. It takes an image of boolean values to specify where in an image to replace pixels with another image. The tidiest way to use it for this application is to use the ee.Image.expression() syntax (rather than specifying several boolean and constant images):
var reclassified = forestMean.expression('b(0) <= 20 ? 1 : b(0) < 50 ? 0.5 : 0');
b(0) refers to the value of the first band of the input image, and ? ... : is the ?: conditional operator which returns the part between ? and : if the condition to the left is true, and the part to the right of the : if the condition is false. So, you can use a series of ? ... : to write out several conditions concisely.
Runnable example with this line.