A terrain is split into one or more tiles. The terrain tiles defines the geometry (height, normals, holes) of the terrain. Each tile has a set of material layers (dirt, grass, decals, roads) that define the appearance. The material layers are applied (blended) one after the other, which means that a layer can override previous layers.

Geometry textures:
The HeightTexture defines height values (absolute heights in world space) of the terrain.

NormalTexture contains the terrain normal vectors. This texture encodes the normal vectors like a standard "green-up" normal map, i.e. the world space +x component of the normal is stored in the red channel. The world space up (+y) component is stored in the blue channel. The negative world space z component is stored in the green channel.

HoleTexture defines holes in the terrain. The texture is used like an alpha mask texture. If the alpha channel contains 0, then there is a hole in the terrain.

Tile dimensions:
OriginX and OriginZ define the tile origin in world space - which corresponds to center of the first texel of the textures. CellSize defines the horizontal distance between two height values. The texture coordinate u is aligned with the positive x-axis. The texture coordinate v is aligned with the positive z-axis. This means, if the cell/texel size is 1 world space unit and the texture is 1025 x 513 texels large, then the terrain tile covers the area between (OriginX, *, OriginZ) and (OriginX + 1024, *, OriginZ + 512).

Mipmaps:
All textures should contain mipmaps. Ideally, the mipmaps are generated using 3 x 3 downsampling instead of the usual 2 x 2 downsampling. - Suitable textures can be created using the TerrainHelper class.

Miscellaneous:
Please notes that the normal vectors depend on the scale of the terrain. That means, if height values in the HeightTexture are scaled, then the normal map has to be updated too. The normal map also needs to be updated if the CellSize is changed! (This is not done automatically. Normal textures can be created using the TerrainHelper.)

OriginX and OriginZ should always be an integer multiple of the CellSize. For example, if the cell size is 0.5, valid origin values are -0.5, 0, 0.5, 1, etc.

Cache invalidation:
When the Terrain is used with the TerrainNode, then the terrain data is cached in clipmaps. Therefore, it is important to notify the terrain system when a tile or layer has changed and the cached data is invalid. When tiles or layers are added to or removed from the terrain, this happens automatically. But when the properties or the contents of tiles/layers are changed, the affected region needs to be invalidated explicitly by calling the appropriate Invalidate method of the Terrain or the TerrainTile. For example, when the contents of a height map is changed, the affected region on the terrain needs to be invalidated by calling Invalidate(Aabb) or Invalidate(TerrainTile).

See Terrain and TerrainLayer for more information.

Reference