By default, the game uses one shared dirt map for all cars. Some mod authors also use custom dirt maps made for specific vehicles for better looking results.
Model: Mapping a dirt map
Dirt maps are UV-mapped like any other texture, however they commonly use the UV2 channel, or the UV1 channel for paint materials that use UV2 for the diffuse. More information about UV channels can be found in the Materials and Shaders guide.
A dirt map contains three textures. Each texture is a seperate greyscale texture saved in one RGB channel:
- Red: Applies dirt effects while roads are dry.
- Green: Decreases specularity while the car is dirty. By default it's a high contrast version of the red channel.
- Blue: Applies dirt effects while roads are wet.
The diffuse will be applied as a brown overlay on top of the surface. Lighter areas on each channel will have a stronger effect, darker areas a weaker effect. Black will have no effect at all.