Quick Introduction

In RenderMan, in addition to the built-in AOVs, light path expressions (LPEs), which are adopted from Open Shading Language's Light Path Expression, specify what light transport paths to output to a display channel. In other words, we don't have to modify the shaders or plugins to output a custom AOV that uses the LPE.

The setup is the same as an AOV except we use a light path expression instead of an AOV channel name. Additionally, we can group the outputs by light groups and/or by geometry set up by a lpegroup.

LPE Tokens

To better understand how an LPE is constructed, the below tokens are used to define interactions. For most users, the pre-built LPE are easily selected and explained below.

C

<type event 'group'>

<L.'group'>

O

.

R

T

D

S

[^ ]

*

+

{n}

{min,}

{min,max}

In RIB

To specify an LPE in RIB, reference a DisplayChannel in the Display. This is the most basic way to view what's being output.

DisplayChannel "color directDiffuse" "string source" ["color lpe:CD[<L.>O]"]
Display "myRender.exr" "openexr" "directDiffuse"

Usage

LPE tokens are not necessarily the most artist-friendly things. For this reason, we provide a list of LPEs in our plugins that can be easily selected and output. We also provide options on how specific you want to be when it comes to output; from outputting all your direct or indirect interactions at once per type or outputting them per-lobe which provides greater control for advanced workflows.

Basic LPEs 

For basic workflows, the following LPE are recommended. Note that we are specifying the DisplayChannel and a name (which can be most anything). Note that these LPE will collect the light interaction from different lobes of materials and store them in the same AOV. For example: reflections from the Glass Lobe in PxrSurface will be combined with other specular reflections in the direct and indirectSpecular AOVs. This can simplify the outputs.

color emissive

color lpe:C[<L.>O]

color directDiffuse

color lpe:C<RD>[<L.>O]

color indirectDiffuse

color lpe:C<RD>.+[<L.>O]

color subsurface

color lpe:C<TD>.*[<L.>O]

color directSpecular

color lpe:C<RS>[<L.>O]

color indirectSpecular

color lpe:C<RS>.+[<L.>O]

color transmissive

color lpe:C<TS>.*[<L.>O]

RenderMan Still Life by Dylan Sisson

Per-Lobe LPEs

Bxdf materials may have more than one diffuse or specular lobe which are summed for D and S, respectively. However, in some situations, it may be desirable to output a specific lobe separately.

For example, some PxrSurface usage may involve the Clearcoat lobe. Normally this is summed under the S token. Routing this lobe to the S4 token will allow you to use S4 in your LPEs. These name-to-lobe mappings are controlled by render options. Up to 4 diffuse lobes and 8 specular lobes are available for LPEs. By default, S1 contains all unspecified specular lobes, so use any higher tokens such as S2 for explicit per-lobe LPEs. The standard lobe mappings are:

For advanced workflows that want to separate all scattering lobes into separate AOVs, the below LPE are provided. These capture the light interaction for each lobe, direct and indirect, and store them into separate AOVs. This is useful for tweaking individual effects at the cost of added AOVs and image output. The documentation for PxrMarschner and PxrSurface list these user lobes at the bottom of their page under an Advanced Rollout as it is for informational purposes and not actual operation.

color directDiffuseLobe

color lpe:CD2[<L.>O]

color indirectDiffuseLobe

color lpe:CD2.+[<L.>O]

color subsurfaceLobe

color lpe:CD3.*[<L.>O]

color directSpecularPrimaryLobe

color lpe:CS2[<L.>O]

color indirectSpecularPrimaryLobe

color lpe:CS2.+[<L.>O]

color directSpecularRoughLobe

color lpe:CS3[<L.>O]

color indirectSpecularRoughLobe

color lpe:CS3.+[<L.>O]

color directSpecularClearcoatLobe

color lpe:CS4[<L.>O]

color indirectSpecularClearcoatLobe

color lpe:CS4.+[<L.>O]

color directSpecularIridescenceLobe

color lpe:CS5[<L.>O]

color indirectSpecularIridescenceLobe

color lpe:CS5.+[<L.>O]

color directSpecularFuzzLobe

color lpe:CS6[<L.>O]

color indirectSpecularFuzzLobe

color lpe:CS6.+[<L.>O]

color transmissiveSingleScatterLobe

color lpe:CS7.*[<L.>O]

color directSpecularGlassLobe

color lpe:C<RS8>[<L.>O]

color indirectSpecularGlassLobe

color lpe:C<RS8>.+[<L.>O]

color transmissiveGlassLobe

color lpe:C<TS8>.*[<L.>O]

color emissive

color lpe:C[<L.>O]

Caustics are not included in the built-in list because they overlap with indirectdiffuse lpe:C<RD>[DS]+[<L.>O] However, you can select caustics with the expression lpe:CDS+[<L.>O]  Below you can find an example of the caustics LPE output, Indirect Diffuse, and the difference between them. Note that the Caustics LPE output doesn't capture indirect lighting from object to object, in this case the red cube. You can create the image on the far right with the indirect diffuse minus the caustic result using the following source: lpe:C<RD>D[DS]*[<L.>O]

LPE Macro

If a custom LPE is useful for your shots, you can add your custom LPE by creating a macro in rendermn.ini file. The custom LPE will appear in the RenderMan for Maya (RfM) LPE list after relaunching Maya.  For example:

/prman/lpe/macro/myDiffuse    CD<L.>

LPE Groups

An LPE group (lpegroup) allows us to specify which objects we want to use for an LPE channel. The attributes can be added in RenderMan plugins using the software's menu system such as PrmanObjectSettings in Katana.

LPE Group Example RIB

Attribute "identifier" "string lpegroup" ["ground"]
DisplayChannel "varying color plane2Shadow" "string source" ["color lpe:shadows;C<..'ground'><L.>"]

Predefined LPE Groups

For the built-in LPE such as shadowcollector, it assumes a predefined lpegroup named "collector". So for the objects that we want to collect shadow, we can simply name its lpegroup to "collector". Specifying lpe:shadowcollector will collect the shadow for these objects.

This may be useful for creating holdouts using a manual workflow.

Advanced LPE Group Logic

You can use the usual regular expression character classes syntax to define more complex LPEs by considering each lpegroup as a single token.

For instance, provided you are using lpegroups "foo" and "bar", you can define the following LPEs:

C<.D'foo'>L
C<.D[^'foo']>L
C<.D['foo''bar']>L
C<.D[^'foo''bar']>L

Light Groups

By default, LPEs retrieve the response to all lights in the scene. To limit the LPE to the contribution due to a single light or set of lights, assign each light to the group by setting the __group parameter on each light to the same name.

For the short LPE name, suffix the name with '_' followed by the light group name. For the long expression, place the light group name in single quotes inside the LPE.

For instance, say we have a light group named "key".  The following are both valid LPEs:

DisplayChannel "varying color lpe:diffuse_key"
DisplayChannel "varying color lpe:CD<L.'key'>"

LPE Prefixes

• unoccluded – returns unoccluded or unshadowed result.
• noclamp – returns unclamped result.
• nothruput – does not apply thruput (thruput is the accumulative albedo of the objects hit by rays).
• shadows – returns collected shadows.
• holdouts – returns only holdout light paths (light paths with one or more holdout events)
• overwrite – instead of outputting the accumulated result, overwrite it. One example of using this is for the albedo output where we do not want an accumulated result.
• noinfinitecheck – do not do any infinite check.

User Defined Signals

• U tokens specify a user defined signal. A Bxdf can output any user defined signal via the RixBXLobeWeights class. There are twelve of them, from U1 to U12. By default, all user defined signals are set to U1. More on this can be found in the Developer's Guide.
• If you have more than one user defined signal, it is important to direct each user defined signal to a different U token by setting them in rendermn.ini. Otherwise, all user defined signals that are assigned to the same U token will override each others!
• By default, U2 is set to the albedo output in the installed rendermn.ini:

/prman/lpe/user2                 Albedo,DiffuseAlbedo,SubsurfaceAlbedo,HairAlbedo

Examples

• In the documentation examples we often use <L.> in place of L. There's no functional difference.
• lpe:CD<L.> means camera with diffuse scattering event for light scattering type.
• lpe:CO means camera with emissive objection (since emissive does not need any light, there is no L).
• Using . in place of [DS] is shorthand, if you are using a User Lobe, you must specify U and cannot use the . shorthand
• lpe:CS*D*[LO] means all light paths minus caustic light paths. While caustic paths (defined here as diffuse followed by specular) may contribtue significantly to the final image, they are a common source of fireflies and flickering. 
• Similarly, lpe:CD+[LO] means all diffuse light paths minus caustics and lpe:CS+D*[LO] means all specular light paths minus caustics.