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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.

 

You will find a useful set of video tutorials and discussion of these practices on the RenderMan Community site.

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.

LPE EventsDescriptionExample
C
Camera 
<type event 'group'>
Scattering event specified by scattering type, scattering event, or lpe groupC<.[DS]'floor'>[LO] matches all light paths from the floor lpe group
<L.'group'>
Light, i.e. analytic and geometric lightsC[DS]*<L.'key'> matches all light paths from the key light group
O
Emissive object, i.e. camera visible lights and material glowC[DS]*O matches all emissive object light paths
Wildcard  
.
A period will match any relevant event<.D> where the period represents R or T
Scattering Types  
R
ReflectionC<R[DS]>[DS]*[LO] matches all reflected light paths
T
Transmission or refractionC<T[DS]>[DS]*[LO] matches all transmitted light paths
Scattering Events  
D
Diffuse lobes (D1 to D4)CD[DS]*[LO] matches all diffuse light paths
S
Specular/Glossy lobes (S1 to S8)CS[DS]*[LO] matches all specular light paths
U
User lobes used to output material properties (U1 to U12)CU2L matches the camera visible user2 lobe, typically albedo
Grouping  
( )Parentheses allow grouping of a sequence of LPE eventsCDSL, C(DS)L, and CD(SL) are all functionally the same
Alternatives  
|A vertical bar separates alternativesCD|(SD)L matches direct diffuse or specular-diffuse light paths
[ ]Square brackets that match any one of the enclosed eventsC[D2D3]L matches direct diffuse2 or diffuse3 light paths
[^ ]
Square brackets that match any one event not contained within the bracketsC[^D2D3]L matches all light paths besides diffuse2 and diffuse3 (including specular and user)
Qualifiers  
*
An asterisk will match the preceding event zero or more timesC[DS]*[LO] matches all light paths, including emissive
+
A plus sign will match the preceding event one or more timesC[DS]+[LO] matches direct and indirect light paths, but not emissive
{n}
Curly brackets that match the preceding event exactly n timesC<T[DS]>{2}[LO] matches double transmission light paths
{min,}
Curly brackets that match the preceding event min or more timesC[DS]{2,}[LO] matches all indirect light paths
{min,max}
Curly brackets that match the preceding event at least min but not more than max timesC[DS]{0,1}[LO] matches direct and emissive light paths

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.

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.

 

Understand that emissive LPE results are a special case. In the materials provided it's referred to as "Glow" or emission. Light sources directly visible to the camera are also output into this LPE. In the below example, the candle flames are using the glow lobe in PxrSurface.

DisplayChannelExpression
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]

 

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 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.

 

Note that the emissive LPE is repeated below. For materials in RenderMan it's typically called Glow.

DisplayChannelExpression
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]

 

 

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:

  • It is important to use <L.> instead of only L so that it will be compatible with light groups. Otherwise, the renderer will output a warning.
  • The lpe: prefix will be automatically added to your macro's name when the file is parsed.

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

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:

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:


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:

 

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).