Use TerrainLighting.j3md with TerrainGrid

How I can create alphamap for use in TerrainLighting.j3md material with TerrainGrid? My created alphamap repeated on each tile.

TerrainGridAlphaMapTest.java is an example of this. It only covers a couple of grid tiles with the alphamap, but it is easy to expand it to load more.

Everything works. Thanks for the quick response.