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.

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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 right clicking on an LPE name from the built-in LPEs list in RenderMan Controls' Add Channels/Outputs.

RenderMan for Katana

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

RIB

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

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DisplayChannel "color lpe:CD<L.>"

Built-in LPEs

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

LPE Outputs

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Custom LPE

RenderMan for Maya

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

RenderMan for Katana

Custom LPE is specified via PrmanOutputChannelDefine's Add Source:

LPE Macro

If the custom LPE is useful for other 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 LPE list after relaunching Maya.  For example:

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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.

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.

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.

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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.

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

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.

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

LPE Prefixes

• unoccluded – returns unoccluded 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.
• holdout – holdout LPE that directs the LPE output to the beauty channel.
• 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.
• 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

Examples

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