The V-Ray Dirt texture map can be used to simulate a variety of effects. For example, dirt can be used around the crevices of an object or to produce an ambient occlusion pass.

||mat Network|| > V-Ray> Texture: Procedural > V-Ray Dirt

The TexDirt node provides inputs for controlling various texture properties. Most correspond to parameters in the section below.

Mode – Specifies the mode in which the dirt map is calculated. The difference between ambient and reflection occlusion is in the direction in which rays are traced.

Ambient Occlusion – Normal ambient occlusion is calculated. Rays are traced in all directions uniformly. 
Phong/Blinn/Ward Reflection Occlusion – Reflection occlusion is used. With reflection occlusion, the direction depends on the viewing direction (just as when calculating reflections) and the spread of the rays depends on the Reflection glossiness and BRDF type use.

Unoccluded – The color that is returned by the texture for unoccluded areas. A texture map can be used for this parameter.

Occluded – The color that is returned by the texture for occluded areas. A texture map can be used for this parameter.

Radius – Determines the amount of area (in scene units) where the Dirt effect is produced. A texture map can control the radius. The texture intensity is multiplied by the radius to calculate the final radius at a given surface point. If the texture is white at a given surface point, the full radius value is used. If the texture is black, a radius of  0.0  is used.

Distribution – Forces the rays to gather closer to the surface normal. The effect is that the dirt area is being narrowed closer to the contact edges. For ambient occlusion, set this parameter to  1.0  to get distribution similar to the ambient lighting on a diffuse surface.

Falloff – Controls the speed of the transition between occluded and unoccluded areas.

Use Texture For Excluded – When enabled, returns the unoccluded color texture if the object is occluded. Otherwise, returns black (0, 0, 0).

Render Nodes – Disables the calculation of the Dirt map for specific objects. Excluded object is not shaded by the Dirt map.

Affect Result Nodes – Specifies a list of objects which affect the calculation of the Dirt map. Excluded objects are considered "invisible" for the calculations of the Dirt map.

As Inclusive List – When enabled, treats the render_nodes list as inclusive.

Double Sided – When enabled, the occlusion on both sides of the surface is calculated.

Invert Normal – Reverts the effect with respect to surface normals - e.g. instead of crevices, open corners are shaded with the occluded color. This parameter changes the direction of tracing the rays. When disabled , the rays are traced outside the surface; when enabled, they are traced inside the surface.

Work With Transparency – When enabled, V-Ray Dirt takes into account the opacity of the occluding objects. This can be used, for example, if you want to calculate ambient occlusion from opacity-mapped trees etc. When off (by default), occluding objects are always assumed to be opaque. Note that working with correct opacity is slower, since, in that case, V-Ray Dirt must examine and evaluate the material on the occluding objects.

Ignore in GI Calculation – Determines whether the dirt effect is taken into consideration for GI calculations or not.

Environment Occlusion – When enabled, V-Ray uses the environment when calculating the occlusion of areas that are not occluded by other objects.

Consider Same Object Only – When enabled, the dirt affects only the objects themselves, without including contact surfaces and edges. When disabled, the entire scene geometry is participating for the final result.

Ignore Self Occlusion – When enabled, objects are not able to occlude themselves.

Subdivs As Samples – When enabled, the Subdivs become independent of the image sampler quality

Subdivs – Controls the number of samples that V-Ray takes to calculate the dirt effect. Lower values render faster but produce noisier results.

Bias X/Y/Z – Bias the normals to the (X, Y, Z) axes, so that the dirt effect is forced to those directions. Consider that these parameter can also take negative values for inverting the direction of the effect.