Sample points randomly on an image using Google Earth Engine

Question

I have an image ('biome') and a feature collection ('grid'). I want to sample points on the image, precisely 10 points per feature(a grid cell). Also, I want to drop points that intersect the masked pixel (oceans) of an image.

When I try with the ee.Image.sample function, I can drop points in the ocean but I can't specify the number of points per grid cell.

When I try with ee.FeatureCollection.randomPoints function, I can specify the number of points per grid cell but I can't drop the points in the ocean.

Does anyone have any ideas?

Link to the script : https://code.earthengine.google.com/91366ac39038271ed66115eb9bb87e7f

var biome = ee.Image("OpenLandMap/PNV/PNV_BIOME-TYPE_BIOME00K_C/v01");
var grid = american.coveringGrid('EPSG:4326', 500000); 

Map.addLayer(biome, null, "biome");
Map.addLayer(grid, null, "grid");

// sample function
var points = biome.sample({
  region : grid.geometry(),
  scale : 30,
  numPixels : 10
});

print(points);
Map.addLayer(points, null, 'points');

//randomPoints function

var points1 = grid.map(function(feature){
  return ee.FeatureCollection.randomPoints({
    region:feature.geometry(), points:10})
});
var points1 = points1.flatten();

print(points1);
Map.addLayer(points1);

// How to sample 10 points randomly per grid cell while dropping all sampled points that intersect masked pixels of biome image? 

Answer 1

You were close; you want a combination of the two. Use a mapped stratifiedSample on the biome's mask band.

var points1 = grid.map(function(feature){
  return biome.mask().gt(0).selfMask().stratifiedSample({
    region : feature.geometry(),
    numPoints: 10,
  })
})

Specifying a scale of 30 is extreme overkill since the biome map seems to be 1km.

https://code.earthengine.google.com/29d9fc6cca666a49788a3047e7bb4f9a