Yo @nasimiasl …
UPDATE
I been thinking about the mip map levels and Unity.
Unity has built-in support for Texture2D with full mip chain. So re-wrote the exporter to just use Unity Internal Mipmapping (save me from having to scale image with bilinear filtering to generate the mip levels… Unity handles that internally if using Texture2D with mip chain enable).
I can then PACK the whole mip map chain:
Example 4096K Atlas with 1024K Textures and Full Mipmap Chain (11 total levels):
https://www.babylontoolkit.com/files/Terrains/FullChainTerrain_4096.zip
Example 2048K Atlas with 512K Textures and Full Mipmap Chain (10 total levels):
https://www.babylontoolkit.com/files/Terrains/FullChainTerrain_2048.zip
This should take care for that SHARPNESS when view lod past the original 5 levels… Ought to look great with supporting the whole mip map chain in the Texture Atlas… I would think 
ALSO
Would it help if i made a LOOKUP texture and encoded things like:
1… The rect of each splat in the atlas (vec4)
2… The tile/scale and offset of the main texture splats (not the mipmaps)
Because i cant see how you can reliably get the texture from a Unity Packed Texture Atlas and it seems to me that you would need to the know the tile and offset values for each splat (they can be different) for proper tile wrapping