---
title: "PxrDirt"
canonical: "https://rmanwiki-26.pixar.com/space/REN26/19661607/PxrDirt"
format: markdown
---
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*PxrDirt* renders local geometric occlusion. This is useful for applying shading effects to nooks and crevices.

> ⚠️ A traditional ambient occlusion effect can be accomplished by setting the occluded color to black and unoccluded color to white.

### Parameters

#### Occluded and Unoccluded

Resulting colors for occluded and unoccluded shading.

#### Num Samples

Controls the number of occlusion samples per camera hit point. More samples reduces can the noise at the cost of increased render time.

#### Sample Distribution

Possible values are *uniform* and *cosine* (Lambertian).

|  | **Description** |
| --- | --- |
| **Uniform** | Rays are not weighted in any particular direction in the hemisphere above the shading point. |
| **Cosine** | Cosine distribution is commonly used to render ambient occlusion. |

#### Cosine Spread

Sample spread of rays when using *cosine* distribution. A value of 1.0 gives a perfect Lambertian distribution.

#### Falloff

Falloff can be used to control the softness of the occlusion effect.

#### Max Distance

The max distance at which objects may occlude. Zero is infinite (all objects occlude).

#### Direction

Trace rays in the direction of the surface normal (*outside*), inverse to the normal (*inside*), or in *both* directions.

#### Bias Direction

Add a directional bias to the normal vector. Values must be specified in world space.

#### Bias Co-ordinate System

Name of bias direction co-ordinate system transform such as object, world or camera

#### Trace Set

Restrict ray tracing to a subset of objects defined by one or more trace groups  
  
  
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