This page provides information about Light Path Expressions.
Overview
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.
Read our Introduction to Light Path Expressions article.
The drop-down menu available in the LPE option offers a list of all symbols in LPEs, as well as presets to get you started.
Light Path Expressions that capture specific events after the second GI bounce will work correctly only with Brute Force as a secondary GI engine
Scene Setup
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.
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.
Syntax
The LPE syntax is based on regular expressions.
Expressions describe a light path that starts from the camera and traces all light bounces to a light source.
Essentially, the syntax follows the direction in which a ray travels in the scene and captures information along the ray path.
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.
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.
LPE Symbols and Presets
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).
Ray spawn events other than C or V 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.
See a full list of LPE Symbols below:
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 | C<[RT][SG]>*<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.
