--- title: "Introduction to Light Path Expressions" canonical: "https://documentation.chaos.com/space/VMAYA/111739610/Introduction%20to%20Light%20Path%20Expressions" format: markdown --- This page provides details about what the Light Path Expressions are.   ## **Overview** --- If you are not familiar with using Light Path Expressions (LPEs), this page is a good place to start. The sections below will try to explain what LPEs are, how they work and what are the possible use cases. > ✅ The [Advanced Light Path Expressions article](https://docs-chaos.atlassian.net/wiki/spaces/VMAYA/pages/111739227) goes into greater detail about complex LPE usage and concepts. ## **What are LPEs?** --- Light Path Expressions are a special type of render element with advanced capabilities. They use regular expression patterns to record selected events of light contribution along the path of a ray. This can be used for very fine control in compositing by allowing you to tweak, for example, the GI coming from lights, the environment or self-illuminating objects separately, or each separate GI bounce, or the self-reflections of an object or the way SSS is seen behind refraction without re-rendering. The possible applications are practically endless. ## **Light Paths** --- ## **Ray Spawn Events and Scattering Types** --- When a ray hits a surface, it can reflect, refract (transmit), or pass through a volume. These are the ray spawn events which we can record with LPE symbols. We use the **R** symbol for reflection and **T** for refraction and sub-surface scattering. They can be used directly, e.g. **R** captures reflections of any scattering type and is also equivalent to ****. We can further specify the kind of hit we want to trace by combining R and T with one or more scattering types - diffuse (**D**), glossy (**G**), or singular (**S**). If we don’t specify that, all scattering types are assumed for our R or T events. This allows us to filter the results only from diffuse reflection, for example with ** **or combine glossy and singular reflection ** **.   By using ray spawn events and scattering types, we can recreate some familiar render elements or create custom single or multiple event expressions. ---