Table of Contents

Quick Introduction

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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. We can also use a short descriptive name (e. g. lpe:diffuse) as well as the long LPE expression (e.g. lpe:CD<L.>). Additionally, we can group the outputs by light groups and/or by geometry set up by a lpegroup.

Usage

An LPE is made up of tokens which are not the most artist-friendly things. For this reason, we provide a list of built-in LPEs with short descriptive names and hide the non-artist friendly expressions.

RenderMan for Maya

In RfM, an LPE can be easily added by clicking on an LPE name from the built-in LPEs list in Render Settings. Note in the UI that the Source field is where the LPE Expression is required. The LPE listed below are already built into Maya.

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RenderMan for Katana

In RfK, an LPE is specified via PrmanOutputChannelDefine. For a list of built-in LPE names, please see the LPE tables below.

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RIB

For DisplayChannel, we can use either the short name or long expression, e.g., lpe:diffuse or lpe:CD<L.>

DisplayChannel "color lpe:diffuse"

or

DisplayChannel "color lpe:CD<L.>"

Built-in LPEs

In RenderMan, these names are enough to output the supplied LPEs below without inputting an Expression.

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

U

[^ ]

*

+

{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 

Anchor
commonLPE commonLPE

...

For basic workflows, the following LPE are recommended. Note that below we 're are specifying the DisplayChannel and a name (which can be most anything) as opposed to the examples above using a built-in naming scheme. 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 for rendering.

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

color lpe:C[<L.>O]

color directDiffuse

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

color indirectDiffuse

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

color subsurface

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

color directSpecular

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

color indirectSpecular

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

color transmissive

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

...

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 S2 token will allow you to use S2 in your LPEs. Up to 4 diffuse lobes and 8 specular lobes are available for LPEs. By default, S1 contains all the specular lobes, so use any higher tokens such as S2 for your per-lobe LPE's.

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.

color directDiffuseLobe

color lpe:CD2[<L.>O]

color indirectDiffuseLobe

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

color subsurfaceLobe

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

color directSpecularPrimaryLobe

color lpe:CS2[<L.>O]

color indirectSpecularPrimaryLobe

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

color directSpecularRoughLobe

color lpe:CS3[<L.>O]

color indirectSpecularRoughLobe

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

color directSpecularClearcoatLobe

color lpe:CS4[<L.>O]

color indirectSpecularClearcoatLobe

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

color directSpecularIridescenceLobe

color lpe:CS5[<L.>O]

color indirectSpecularIridescenceLobe

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

color directSpecularFuzzLobe

color lpe:CS6[<L.>O]

color indirectSpecularFuzzLobe

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

color transmissiveSingleScatterLobe

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

color directSpecularGlassLobe

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

color indirectSpecularGlassLobe

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

color transmissiveGlassLobe

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

color emissive

color lpe:C[<L.>O]

For objects that have an LPE group called "collector", lpe:shadowcollector collects shadows. This may be useful for creating holdouts.

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Caustics are not included in the built-in list because they overlap with lpe:indirectdiffuse. with indirectdiffuse lpe:C<RD>[DS]+[<L.>O] However, you can select caustics with the expression lpe:CD[S]+<L.>

...

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]

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Custom LPE

RenderMan for Maya

A custom LPE can be added in RenderMan Controls' Add Channels/Outputs:

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RenderMan for Katana

Custom LPE is specified via PrmanOutputChannelDefine's Add Source:

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LPE LPE Macro

If the a custom LPE is useful for other 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:

...

LPE Groups

An LPE group (lpegroup) allows us to specify which objects we want to be use for an LPE channel.

RenderMan for Maya

In RfM, we add an lpegroup by selecting the objects and add the lpegroup RenderMan attribute.

RenderMan for Katana

In RfK, we add an lpegroup via PrmanObjectSettings' attributes/identifier/lpegroup.

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<.[DS]'ground'><L.>"]

Predefined LPE Groups

For the built-in LPE such as shadowcollector or reflectioncollector, it assumes a predefined lpegroup named "collector". So for the objects that we want to collect shadow or reflection, 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.

...

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

...

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.

...

LPE

...

Option "lpe" "string specular2" ["Clearcoat"]
DisplayChannel "color lpe:CS2<L.>"

This will cause a slight performance penalty but may be desirable to see lobes independently.

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 – holdout LPE that directs the LPE output to the beauty channel.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.

LPE Tokens

• C – camera
• D – diffuse
• S – any specular (includes S1, S2, ...)
• D – any diffuse (includes D1, D2, ...)
• R – reflection
• O – emissive object
• L – geometric area light
• T – transmission (refraction/subsurface)
• <> – specifies <ScatteringType ScatteringEvent lpegroup>
  • Scattering types: L (light), T (transmission), R (relection), or O (emissive object)
  • Scattering events: D (diffuse), S (specular)
  • lpegroup: This is optional; if specified, the LPE only applies to that lpegroup
• () – for grouping
• [] – for regular expression. []* means 0 or more. []+ means 1 or more
• U – user defined signals (selected by number, U1 through U12)

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.> means Light scattering type with all scattering events. For light group AOVs, using <L.> is required instead of Lin 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).