Texture coordinates, also called UVs, are pairs of numbers stored in the vertices of a mesh. These numbers are often used to stretch a 2D texture onto a 3D mesh. Game engines commonly use two texture coordinates, U and V, for mapping the width and height of a texture. Texture coordinates are measured in a scale of 0.0 to 1.0, with 0.0 and 1.0 at opposite sides of the texture. When a model has a UV distance greater than 1 (for example, UV goes from -1 to 2) then the texture will tile across the model.
Trackmania needs your model to have two UV maps; aptly named BaseMaterialand Lightmap.
¶ BaseMaterial (UV1)
This is the base layer of the material, typically mapping your Diffuse/Roughness/Normal. You can download all the base game texture files and use these textures to get an accurate representation of the texture on your mesh. As mentioned before; most textures can tile horizontally and vertically. There are some textures such as ItemCactus_D, ItemBarrier_D that do not tile.
UVs in BaseMaterial UV1 are allowed to overlap, mirror and go beyond the 0-1 UV space to get the results you’re looking for.
¶ Lightmap (UV2)
Lightmap is a texture map that stores pre-rendered lighting. Lighting in Trackmania is static (i.e. not dynamic), hence the use of light maps in the game. Because the light map lighting is baked, this allows the surfaces to receive bounce lighting, color bleeding and other effects for a realistic-looking result. The representation of this is tied to your in-game settings, however. Using lower graphics settings reduces the amount of bounce lighting and color bleed. However, you do not need to run the game at the highest settings to bake out a better light map; that is done in the Map Editor’s Shadow Calculation.
Traditionally, Lightmaps store the color and brightness of pre-baked lighting. The BaseMaterial is applied to the mesh with no lighting, and the Lightmap is then multiplied against it, darkening the diffuse where the light map has shadows. Because the light map is multiplicative, this can only darken the surface. Typically a Lightmap is laid out so that every surface needs to have its own unique space within the 0-1 UV square.
You can create the Lightmap UVs with an automatic unwrap & pack algorithm. Automatic unwrapping is faster and easier to create but it tends to waste more texture space, so the Lightmap pixels end up larger on the model, meaning the lighting is of lesser quality.
Time-permitting, for the best results it is best to start with a copy of the BaseMaterial UV1 (if they’re laid out well), un-mirror any mirrored parts, un-overlap any overlaps, and scale up the UVs for areas that need greater lighting detail. Then pack them all into the 0-1 UV square with appropriate edge padding space between the UVs.