This page provides information about the Light Path Expressions RE mode.
Overview
Light Path Expressions or LPEs are a very powerful and advanced tool for extracting specific lighting contribution events from the scene to a separate channel. This allows for very fine control of the image in compositing. For example, LPEs allow capturing only self-reflections, or the 1st bounce of GI, or the SSS that's only seen behind glass and similar for compositing control of only this aspect of the image.
The LPE syntax is based on regular expressions. Expressions describe a light path that starts from the camera and bounces around the scene until it reaches a light source.
The scene setup is similar to the Light Select workflow. As LPEs are a mode of the Light Select, the expression will capture contributions of the light that is added to the Light Select set. If the Light Select is empty, the expression will extract contributions from all scene lights. LPEs also allow extracting information from the environment, for example allow capturing the GI that comes from the environment only.
<note>we recommend using BF+BF for GI with LPEs</note>
LPE Symbols
The LPE alphabet consists of the symbols representing a ray event/type and additional regular expression symbols (“.”, “+”, “*”, “<”, “>”, “[“, “]”, “|”, “?”, “^” and additional label strings).
Ray spawn events other than C can be combined with scattering types (e.g. <RD> - capture only diffuse reflection) to get a more specific ray event or they can be used directly (e.g. R - capture reflections of any scattering type. This is also equivalent to <R.>)
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. C<RD>.*[LO] will capture the GI coming from lights or emissive objects.
Symbol | Description | Type | Final |
Initial ray spawn event | |||
C | Camera ray | Initial ray spawn event | No |
Ray spawn events | |||
R | Reflection ray | Ray spawn event | No |
T | Transmission ray | Ray spawn event | No |
V | Volume ray | Ray spawn event | No |
Scattering types | |||
D | Diffuse scattering | Scattering type | No |
G | Glossy scattering | Scattering type | No |
S | Singular scattering | Scattering type | No |
s | Straight (no) scattering | Scattering type | No |
Ray termination events | |||
L | Light hit | Ray intersection event | Yes |
O | Emitting object hit | Ray intersection event | Yes |
B | Background hit | Ray intersection event | Yes |