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Dirt is a texture map that can be used to simulate a variety of effects such as dirt around the crevices of an object. It can also be used to produce an ambient occlusion pass.

Mode – Specifies the procedural mode that is used to generate texture:

Ambient Occlusion – – Normal ambient occlusion is calculated.
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.   See the Inner Occlusion example below.
Ambient + Inner Occlusion – A combination of Ambient Occlusion and Inner Occlusion modes used for enhanced weathering effect.
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, whereas 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.

Unoccluded Color – Specifies the color that is returned by the texture for unoccluded areas. If a texture is selected, it overrides the color as long as the texture checkbox is enabled.

Occluded Color – Specifies the color that is returned by the texture for occluded areas. If a texture is selected, it overrides the color as long as the texture checkbox is enabled.

Radius – Specifies the amount of area (in project units) where the Dirt effect is produced. If a texture is also used, the Radius value determines the amount of area where the texture blends.

Falloff – Controls the speed of the transition between occluded and unoccluded areas. For more information, please see the Falloff Parameter 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.

Glossiness – Controls the spread of the rays traced for reflection occlusion. This is only used in Phong, Blinn, or Ward ambient occlusion mode.

Streaks Ambient – Draws streaks across dirt effect respective to ambient occlusion. Note that a bias direction and value have to be specified for the streaks to appear. See the Streaks Ambient example below.

Streaks Inner – Draws streaks across dirt effect respective to inner occlusion. Note that a bias direction and value have to be specified for the streaks to appear.

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

Self-occlusion only – When enabled, the dirt affects only the objects, without including contact surfaces and edges.

Ignore self-occlusion – When enabled, the object does not occlude itself. It is still occluded by other objects in the project.

Double Sided – When enabled, the rays needed to generate the dirt map are shot from both sides of the faces (both in the direction of the normals and in the opposite direction).

Mode – Specifies the multiplication mode of the colors. 

Multiplier Multiplier – The color used for blending is black.
Blend Amount –The color used for blending is the one specified in the color slot.

Unoccluded Color – Controls the intensity of the Unoccluded color. A texture map can be assigned to this channel. 

Occluded Color – Controls the intensity of the Occluded color. A texture map can be assigned to this channel. 

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