TerrainDetail
TerrainDetail has to be a child of a MaterialVariant object. The MaterialVariant.BaseMaterial property of the parent MaterialVariant object need to be one of the supported terrain Material, for example, it can be Basalt but not Plastic.
Terrain renders with different textures for it's top(+y), bottom(-y), and side(horizontal to y axis) faces. If a MaterialVariant has no TerrainDetail children, all faces are rendered as MaterialVariant specified. At most 3 TerrainDetail objects can be added to MaterialVariant, one for each face. TerrainDetail overrides the terrain appearance of a certain face.
For example, it can achieve this kind of effect: the top surface of Grass has lots of grass. On side surfaces, there are less grass. On bottom surfaces there are no grass.
Summary
Properties
Determines the color of the surface.
The face this TerrainDetail overrides.
Determines texture tiling method.
Determines which parts of the surface are metal and are non-metal.
Modifies the lighting of the surface by adding bumps, dents, cracks, and curves without adding more polygons.
Determines the apparent roughness across the surface.
Determines the scale of textures.
Properties
ColorMap
This property determines the color of the surface. This texture is sometimes called the albedo texture. The alpha channel is not used.
Face
Face that this TerrainDetail overrides. When more than one TerrainDetail objects with the same face exist under a MaterialVariant, only one of them works.
MetalnessMap
This property determines which parts of the surface are metal and are non-metal. A metalness map is a grayscale image where black pixels correspond to non-metals and white pixels correspond to metals.
Metals only reflect light the same color as the metal, and they reflect much more light than non-metals. Most materials in the real world can be categorized either metals or non-metals. For this reason, most pixels in a metalness map will be either pure black or pure white. Values in between are typically used to simulate dirt or grunge on top of an underlying metal area.
When Lighting.EnvironmentSpecularScale is 0, metalness has no effect. For the most realistic reflections, setting EnvironmentSpecularScale and Lighting.EnvironmentDiffuseScale to 1, and Lighting.Ambient and Lighting.OutdoorAmbient to (0,0,0) is recommended.
NormalMap
This property modifies the lighting of the surface by adding bumps, dents, cracks, and curves without adding more polygons.
Normal maps are RGB images that modify the surface's normal vector used for lighting calculations. The R, G, and B channels of the NormalMap correspond to the X, Y, and Z components of the local surface vector respectively, and byte values of 0 and 255 for each channel correspond linearly to normal vector components of -1 and 1.016 respectively. This range is stretched slightly from -1 to 1 so that a byte value of 127 maps to exactly 0. The normal vector's Z axis is always defined as the direction of the underlying mesh's normal. A uniform (127,127,255) image translates to a completely flat normal map where the normal is everywhere perpendicular to the mesh surface. This format is called "tangent space" normal maps. Roblox does not support world space or object space normal maps.
Incorrectly flipped normal components can make bumps appear like indents. If you import a normal map and notice the lighting looks off, you may need to invert the G channel of the image. The X and Y axes of the tangent space frame correspond to the X and Y directions in the image after it's transformed by the mesh UVs. If you view your normal map in an image editor as if it were displayed on a surface, normals pointing towards the right side of the screen should appear more red, and normals pointing towards the top side of your screen should appear more green.
The terms "DirectX format" and "OpenGL format" are sometimes used to describe whether the G channel of the normal map is inverted or not. Roblox expects the OpenGL format.
Roblox expects imported meshes to include tangents. Modeling software may also refer to this as "tangent space" information. If you apply a normal map and it does not seem to make any visual difference, you may need to re-export your mesh along with its tangent information from modeling software.
RoughnessMap
This property determines the apparent roughness across the surface. A roughness map is a grayscale image where black pixels correspond to a maximally smooth surface, and white pixels correspond to a maximally rough surface.
Roughness refers to how much variation the surface has on a very small scale. Reflections on smooth surfaces are sharp and concentrated. Reflections on rough surfaces are more blurry and dispersed.
StudsPerTile
Determines the scale of textures. Larger values for this property will lead to the textures appearing larger, and repeating less frequently.