V-Ray’s Dirt map enables you to quickly add detail and depth without generating large bitmap textures. Have a look at the documentation for examples of what each property does. For this tutorials I’ll be using a simple road section as example model to demonstrate how to set up and control the Dirt map.
Download sample SketchUp Model (SketchUp 8, V-Ray for SketchUp 1.49.01)
The starting point is this very simple scene. At first we will look at just the road material and use use plain colours in order to see the effects of the Dirt map.
The concept is simple, use a Dirt map in the transparency slot so a second underlying diffuse layer will bleed through.
In order to control the effect just for the road surface it has been grouped and the Dirt map is set to “Ignore other objects”.
The other key property here is “Invert Normal”. When you inspect the SketchUp model you will see that some of the edges of the road geometry is extruded downwards a little bit. This is done in order for the Dirt map to generate the dirt where you want it.
To clearly illustrate how the dirt effect looks I made the underlying diffuse layer red.
Observe how the Dirt map in the transparency map let the underlying red diffuse show through around the areas where I had extruded the edges. When we use a dark grey colour for the second diffuse it appear like this:
If you wonder why I didn’t just use the Dirt map in the main diffuse layer it is because I want to have the opportunity to use the Dirt map with textures which I can scale and position within SketchUp. Lets expand the example by adding grass to the model.
Using the same setup as with the road material we create a second diffuse layer with a Dirt map in the transparency of the top diffuse layer. When creating these setups I usually use a bright colour such a red to visually ensure I’ve set them up right.
Again I grouped the grass by itself and extruded the edges where I wanted to see the Dirt effect. The result is a scene with more subtle details that enhance the bordering edge between materials and objects. The diffuse underneath was a dark green colour. I could also have used a texture for the second diffuse in order to create even more variation.
In the final scene I added some extruded edges along the length of the roads to break up the flat surface even further.
Note that at the T intersection the centre lines does not meet – I added a slight gap in order for the dirt mapping to look softer.
This simple method let you quickly flat boring surfaces with subtle shading without creating huge texture maps specificity for that geometry. You can easily reuse the V-Ray materials as long as you create and organize the geometry in similar fashion.
Observe how the quality of the subdivisions affect the appearance of the dirt. Some times I prefer to use less subdivs because the noise fits well.