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This page provides information about Light Path Expressions.

Overview

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Column width 45% Light Path Expressions, or LPEs are a very powerful and advanced tool for extracting specific lighting events from the scene to a separate channel. This allows for a very fine control of the image in compositing. For example, LPEs allow breaking the GI down to different types of light sources like lights, the environment, and self-illuminating objects, or breaking down the GI to its separate bounces, or capturing only self-reflections, or the SSS that's only seen behind glass and similar for compositing control of only this aspect of the image. UI Text Box type tip Read our Introduction to Light Path Expressions article. The right-click menu offers a list of all symbols in LPEs, as well as presets to get you started. There are preset equivalents to already available render elements (like Beauty, Lighting, GI, Light Select types, etc). There are also Misc and masking presets for the GI coming from the environment or for the GI coming from a specific object. UI Text Box type note Light Path Expressions that capture specific events after the second GI bounce will work correctly only with Brute Force as a secondary GI engine Also available is a list of boolean operations that allow more complex expressions to be rendered in a single channel. For example, in this way, some compositing operations like union or subtraction can be done in the renderer , instead of in the compositing application. UI Text Box type tip Тhere is a dedicated LPE Builder available to help you compose expressions and track exactly which ray paths they capture. Column width 10%   Column width 33%

Scene Setup

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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 captures contributions of the light(s) added to the Light Select set. If the Light Select is empty, the expression will capture captures contributions for all scene lights.

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.

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For example, if you need to capture GI that bounced off of one object onto another, the 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.

LPE Symbols

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

A valid expression starts from the camera (C), captures ray spawn and scattering events, and ends at light (L), emissive object (O), or the environment (B). 

Presets

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A number of presets are available for Light Path Expressions.

There are preset equivalents to already available render elements (like Beauty, Lighting, GI, Light Select types, etc).

The Misc and masking presets are useful in common-use cases.

Any preset can be used as a starting point for building other expressions, and like any LPE, they can be combined with boolean operations.

Boolean operations

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Boolean operations allow us to combine expressions. We can complement (^), intersect (&), or unite (|) expressions, or we can subtract (-) one expression from another.

Boolean operations can also be very useful in a number of scenarios to simplify the compositing workflow.