---
title: "Light Path Expressions"
canonical: "https://documentation.chaos.com/space/VRAYHOUDINI/113279441/Light%20Path%20Expressions"
format: markdown
---
This page provides information about Light Path Expressions.


## <span style="color: #333333">**Overview**</span>

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## <span style="color: #333333">**Scene Setup**</span>

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<span style="color: #333333">The scene setup is similar to the Light Select workflow. As LPEs are a mode of the Light Select render element, the expression will capture contributions of the light(s) added to the Light Select set. If the Light Select is empty, the expression will capture contributions for all scene lights.</span>

<span style="color: #333333">LPEs also allow extracting information from the environment, for example allow capturing the GI that comes from the environment only. This does not require adding lights to the Light Select set.</span>


## <span style="color: #333333">**Syntax**</span>

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<span style="color: #333333">The LPE syntax is based on regular expressions.</span>

<span style="color: #333333">Expressions describe a light path that starts from the camera and traces all light bounces to a light source.</span>

<span style="color: #333333">Essentially, the syntax follows the direction in which a ray travels in the scene and captures information along the ray path.</span>

<span style="color: #333333">Keep this information in mind, as light in the scene travels from the light source to the camera, but the expression is in reverse, following the path of the rays shot from the camera.</span>

<span style="color: #333333">For example, if you need to capture GI bounced off of one object onto another, light will travel from the light, onto the object that casts GI, then onto the object that receives GI and end at the camera. But in LPEs, just like in ray tracing, the expression will be in reverse, starting from the camera, asking for what object receives GI, then what object casts the GI and end at the light source.</span>


## <span style="color: #333333">**LPE Symbols and Presets**</span>

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<span style="color: #333333">The LPE alphabet consists of the symbols representing a ray event/type and additional regular expression symbols (“.”, “+”, “*”, “<”, “>”, “[“, “]”, “|”, “?”, “^” and additional label strings).</span>

<span style="color: #333333">A valid expression starts from the camera (</span><span style="color: #333333">**C**</span><span style="color: #333333">), captures ray spawn and scattering events and ends at light (</span><span style="color: #333333">**L**</span><span style="color: #333333">), emissive object (</span><span style="color: #333333">**O**</span><span style="color: #333333">) or the environment (</span><span style="color: #333333">**B**</span><span style="color: #333333">).</span>

<span style="color: #333333">Ray spawn events other than </span><span style="color: #333333">**C**</span><span style="color: #333333"> or </span><span style="color: #333333">**V**</span><span style="color: #333333"> can be combined with scattering types (e.g. </span><span style="color: #333333">**<RD>**</span><span style="color: #333333"> - capture only diffuse reflection) to get a more specific ray event or they can be used directly (e.g. </span><span style="color: #333333">**R**</span><span style="color: #333333"> - capture reflections of any scattering type. This is also equivalent to </span><span style="color: #333333">**<R.>**</span><span style="color: #333333">)</span>

<span style="color: #333333">Ray termination events can also be combined with each other when you want to capture the contribution of a light together with emissive objects, e.g. </span><span style="color: #333333">**C<RD>.*[LO]**</span><span style="color: #333333"> will capture the GI coming from lights or emissive objects.</span>

<span style="color: #333333">See a full list of LPE Symbols below:</span>

<details>
<summary>LPE Symbols</summary>

![image](media://e408db81-ba54-41bc-a40a-2094b2540919)

|  |  |  |
| --- | --- | --- |
| **Symbol** | **Description** | **Type** |
| **  Initial ray spawn event  ** |
| C | Camera ray | Initial ray spawn event |
| **Ray spawn events** |
| R | Reflection ray | Ray spawn event |
| T | Transmission ray | Ray spawn event |
| V | Volume ray | Ray spawn event |
| **  Scattering types  ** |
| D | Diffuse scattering | Scattering type |
| G | Glossy scattering | Scattering type |
| S | Singular scattering | Scattering type |
| s | Straight (no) scattering | Scattering type |
| **  Ray termination (final) events  ** |
| L | Light hit | Ray intersection event |
| O | Emitting object hit | Ray intersection event |
| B | Background hit | Ray intersection event |


|  |  |
| --- | --- |
| **Preset** | **Light path expression** |
| **Render Elements** |
| RGB | C.* |
| Self-Illumination | C[OL] |
| Background | CB |
| Lighting | C<RD>L |
| Specular | C<R[GS]>L |
| GI | C<RD>(.+L|.*[OB]) |
| SSS | C<TD>.+L |
| Reflection | C<R[GS]>.+L |
| Refraction | C<T[GS]>.+L |
| Atmosphere | CV.* |
| **Light Selects** |
| Direct Illumination | CRL |
| Direct Diffuse | C<RD>L |
| Direct Specular | C<R[GS]>L |
| Full | C.*L |
| Indirect | CR.+L |
| Indirect Diffuse | C<RD>.+L |
| Indirect Specular | C<R[GS]>.+L |
| **Misc** |
| Self-illumination, no lights | CO |
| GI from Self-Illumination | C<RD>.*O |
| GI from Environment | C<RD>.*B |
| **Object/Material Select** |
| Indirect for Object ID 1 | CR'1'.+L |
| Indirect for all Object IDs but 1 and 2 | CR[^'1''2'].+L |
| RGB for Material ID 1 | C.'m1'.* |
| Self-Reflections by Material ID 1 | C<R[GS]'m1'>.'m1'+L |
| **SSS** |
| Scattering from Objects directly visible to the Camera | C<TD>.+L |
| Scattering from Objects occluded behind reflections and refractions | C<[RT][SG]>+<TD>.+L |
| Scattering from Objects both directly visible or occluded behind reflections and refractions | *<TD>.+L |

> ✅ It will be faster and more efficient to use the already available render elements instead of their LPE equivalents in the presets, of course - the examples are there as a point of reference.
</details>