VRayDirt is a texture map that can be used to simulate a variety of effects: for example, dirt around the crevices of an object, or to produce an ambient occlusion pass.

UI Path: Texture > V-Ray > VRayDirt

Occluded Color – Specifies the color that will be returned by the texture for occluded areas. You can also use a texture map for this parameter.

Unoccluded Color – Specifies the color that will be returned by the texture for unoccluded areas. You can also use a texture map for this parameter.

Radius – 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.

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.

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. Low Noise threshold values in the Image Sampler clean the result further.

Ambient Streaks – Draws streaks across dirt effect respective to ambient occlusion.

Inner Streaks – Draws streaks across dirt effect respective to inner occlusion.

Streaks Size – Controls the thickness of all streaks.

Ignore for GI – When enabled, the dirt effect is ignored in GI calculations. 

Consider Same Object Only – When enabled, the dirt affects 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. 

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

Ignore Invisible Objects – When enabled, VRayDirt 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 disabled (the 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, the object does not occlude itself. It is occluded only by other objects in the scene.

Environment Occlusion – When enabled, VRayDirt includes environment contribution to the shading of unoccluded areas.

Occlusion Mode – Specifies the mode in which the dirt map is going to be calculated.

Ambient occlusion – Normal ambient occlusion is calculated.
Reflection occlusion (Phong, Blinn, Ward)  – Reflection occlusion is 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.
Inner occlusion – Allows the user to invert the effect with respect to surface normals - e.g. instead of crevices, open corners are shaded with the occluded color. This mode changes the direction of tracing the rays. When this mode is selected, the rays are traced inside the surface, otherwise, they are traced outside the surface. 
Ambient + inner occlusion – A combination of Ambient occlusion and Inner occlusion modes used for enhanced weathering effect.

Affect Reflection Elements – When enabled, the ambient reflection affects the reflection render elements. This can be used to create reflection mask.

Glossiness – 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 cause multiple rays to be traced in the approximate reflection direction.

Include/Exclude – Allows you to disable/enable the calculation of the VRayDirt map for specific objects. Excluded object will not be shaded by the VRayDirt map.

Affected By – Specifies a list of objects which will affect the calculation of the VRayDirt map. Excluded objects will be considered "invisible" for the calculations of the VRayDirt map - they don't produce the effect over other objects.