...

The caret ^ is used to exclude something. Sometimes your light path may have an event you want to avoid. In many cases this could even be an LPE group (explained later) you wish to omit. Let's say you want all the light paths exceptf or a couch except for a couch in a diffuse reflection pass, C<RD[^'couch']>*[<L.>O] will return everything except for the couch.

Shorthand

Above we are explicitly stating what we want using the tokens. We can use shorthand and some following examples will use it. I can substitute a . (period) for the events and types.

...

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

Camera to <[reflection OR transmission] OF [diffuse OR specular]> ALL light bounces TO [lights OR mesh/glow objects]

You can now understand each of the parts above and how specifying the right tokens (or even simple swapping them out) can give you the results you need for compositing a frame after making adjustments.

The above LPE can be used to output not only the beauty, but can be used to output something like the beauty of a particular light group, this is a very common "ask" by artists. Below we'll dig deeper into more advanced workflows.

Per Light LPE

For artists rendering complex stills, it might be important to separate lights and light groups to change contribution later instead of re-rendering or creating many wedge tests. For animation, per-light AOVs are much less common as complex VFX composites already have many layers on top of AOVs for those layers. Splitting this further into selected light passes can further multiply the number of passes a compositor will handle to adjust the beauty. This is possible and should your pipeline handle this extra set of data, LPE can provide the control you need.

In RenderMan for Maya we handle this using the lights drop down menu in the AOVs tab where you define your LPE Display Channels. But other software may require you manually insert the light groups.

First, you must specify which lights belong to which groups. This is done at the light shader level. You notice a string (text) field in the light parameters for an LPE Group. This is where you define your groups in plain text. For example, in the classic three-point lighting setup you have three lights or groups: key, fill, and rim lighting. You can specify this in the light shaders and then call these groups using the LPE syntax of a name in single quotes, like 'key' or 'fill'.

lpe:C<[RT][DS]>*[<L.'key'>O]


Shorthand
lpe:C<..>*[<L.'key'>O]

The above LPE will render a beauty of the scene lit by the lights tagged as "key". Replacing 'key' with another group would render the other group as specified like 'rim' or maybe you need the HDRI as 'environment' separated into an AOV.