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.

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.
Inner Occlusion – Inverts 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.

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. See the Radius example below.

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. See the Distribution example below.

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.

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

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 objects are 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.

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

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

Streaks – When enabled, gives ability to blur the streaks. Bare in mind that a new algorithm is calculating this effect, and the visual representation of the streaks is different.

Blur – Controls the amount of the blur of the streaks. See the Streak Blur example below.

Size – Controls the thickness of all streaks. See the Streaks Size example below.

Density – Controls the density of the streaks. See the Streak Density example below.

Affect Alpha – When enabled, the texture automatically receives an alpha component from the generated dirt distribution and this way makes the unoccluded area of the V-Ray Dirt map transparent. When disabled, the alpha values from the maps used in Occluded and Unoccluded color slots are applied. 

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

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. See the Ignore in GI Calculation example below.

Ignore Bump – When enabled, bump maps are excluded from V-Ray Dirt computations. The option is useful, when the bump effect is too strong and may produce undesirable effects such as holes. See the Ignore Bump example below.

Thin Mode – When enabled, V-Ray Dirt enhances the look of thin surfaces by casting less dirt on them. It is available only for Inner occlusion mode. See the Thin Mode example below.

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. See the Consider Same Object Only example below.

Same Object Mode – Specifies how the same objects to be considered. You can choose from By Render ID, By Object ID, By User Attributed, and Default.

Same Object Attribute – Specifies the user attribute name when By User Attribute mode is selected.

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

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

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 parameters can also take negative values for inverting the direction of the effect.

Rays distribution (X,Y,Z) axis
Equal angle between rays distribution
Distribution parameter = 0.0

Rays distribution (X,Y,Z) axis
Different angle between rays distribution
Distribution parameter > 0.0

Distribution: 1.0

Distribution: 3.0

Distribution: 10.0

Example: Ignore in GI Calculation

This parameter determines whether the dirt effect is visible during the GI calculation.

Example: Consider Same Object Only

This example shows the effect of the Consider Same Object Only parameter.

Example: Ignore Bump

When enabled, this option excludes bump effects from the V-Ray Dirt computations.

Example: Thin Mode

Enhances the look of thin surfaces by casting less dirt on them. Available only for Inner Occlusion mode.

Example: Streaks Blur

For this example, Streaks Blur is enabled and higher values increase the blur effect.

Example: Streaks Density

For this example, Density controls the density of the streaks.