In production it often become necessary to create non-physical effects and art-directed changes to your shot. Light Path Expressions (LPE) are a powerful way to collect and output specific light paths for alteration later.

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LPE, Cryptomatte, and Trace Sets should improve the flexibility of your pipeline and when managed correctly, avoid expensive re-rendering for artistic tweaks.Hand-authoring LPE can be a tedious process and for artists it's not always desired to do these manually. This document aims to improve the artist's understanding and usage of LPE.

The Basics

To understand what is collected in an LPE, we will talk about the parts below and some context. Firstly, we always begin at the camera, so we begin an expression using C for Camera as the starting point for light collection.

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