Dirt

White Color – This is the color that is returned by the texture for unoccluded areas. You can also use a texture map for this parameter.

Black Color – This is the color that is returned by the texture for occluded areas. You can also use a texture map for this parameter.

Radius – This parameter determines the amount of area (in scene units) where the VRayDirt effect is produced. You can also use a texture to 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.

Render Nodes – Allows you to disable the calculation of the VRayDirt map for specific objects. Excluded object is not be shaded by the VRayDirt map.

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

Mode - Allows you to specify the mode in which the dirt map is going to be calculated. 

Ambient occlusion - Normal ambient occlusion will be calculated.
Reflection occlusion (Phong, Blinn, Ward) - Reflection occlusion will be used. The difference between ambient and reflection occlusion is basically in the direction in which rays are traced. With ambient occlusion rays are traced in all directions uniformly while 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 used. 

Distribution - This parameter 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.

Glossiness - This parameter controls the spread of the rays traced for reflection occlusion. A value of 1 means that just a single ray is traced (just like when tracing clear reflections), smaller values make the spread of the traced rays.

Subdivs - Controls the number of samples that V-Ray takes to calculate the dirt effect. Lower values render faster but produce a more noisy result.

X/Y/Z - These parameters will bias the normals to the (X, Y, Z) axes, so that the dirt effect is forced to those directions. These parameter can also take negative values for inverting the direction of the effect. 

Invert Normal - This option allows the user to revert the effect with respect to surface normals - e.g. instead of crevices, open corners will be shaded with the occluded color. This parameter will change the direction of tracing the rays. When it is off, the rays are traced outside the surface; when on, they are traced inside the surface. 

Affect Reflection Elements - When enabled, the ambient reflection will affect the reflection render elements. This can be used to create reflection mask. 

Ignore for GI - Determines whether the dirt effect will be taken into consideration for GI calculations or not. 

Work With Transparency - When on, VRayDirt will take 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, VRayDirt must examine and evaluate the material on the occluding objects. 

Ignore Self Occlusion - When enabled, objects will not be able to occlude themselves. 

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

Consider Same Object Only - When on, the dirt will affect only the objects themselves, without including contact surfaces and edges. If off, the entire scene geometry is participating for the final result.

Double Sided - When enabled, rays will be traced both in the direction of the normal and in the opposite direction. This creates an effect where both crevices and edges are occluded. 

Render Nodes as Inclusive List - When enabled, the Render Nodes becomes an inclusive list. 

Result Nodes as Inclusive List - When enabled, the Affects Result Nodes list becomes an inclusive list.