Google Earth Engine: how to negate the .and() operator on an image with NULL values

Question

How does one negate a ".and()" operation on an image in GEE if this image does not contain 0/1 values but NULL/1 values?

From the documentation:

not() - Returns 0 if the input is non-zero, and 1 otherwise. Arguments: this:value (Image): The image to which the operation is applied. Returns: Image

For example if I want to retreive all loss in the Hansen tree loss dataset that does not overlap with a tree cover density >= 30. I use this code:

var gfw = ee.Image('UMD/hansen/global_forest_change_2018_v1_6');
var tc = gfw.select('treecover2000');
var loss = gfw.select('loss');
var lossyear = gfw.select('lossyear');
var frst = tc.gte(30);
var not_overlapping_loss = lossyear.and(frst.not())

which works fine. But when I try the same code replacing "frst" by an "asset" I uploaded that contain only "NULL/1" values, it seems the resulting image contains only NULL values as the (not existing) 0's are never converted to 1's...

Answer 1

Without knowing how your actual asset image looks like, I think you mean that all 'NULL' values are actually masked pixels? If your first operation on those asset image will be:

var newImage = ee.Image('ASSSET_ID').unmask(0);

All the masked pixels will have a value and thus will be used in the "and()" operation.

Answer 2

Thanks. I found a solution. One can use expression to transform the null values to 1 and 1 values to 0 so that the .and() operator will filter correctly the null values... Not sure how efficient it is or if there are better solutions but it works...

var inv_frst = frst.gte(1).expression('s ? 0 : 1', { 's': frst.select(0) });
var not_overlapping_loss = lossyear.gte(1).and(inv_frst );