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Light Path Expressions

Light Path Expressions (LPEs) describe the propagation of light through a scene starting from a source of light, bouncing around between the objects of the scene and ultimately ending up at the eye. The paths that light takes through a scene are called light transport paths.

For example, LPEs may be used to extract specific light contributions from Iray into separate render elements for use in compositing and post-processing.

In essence, LPEs are regular expressions. The alphabet of LPEs consists of descriptions of interactions between light, the scene and objects within.

The separate Render Elements functionality allows less flexibility to create such layers of information but may often give the required result. Some types of information, useful for post-processing, are only available using Render Elements, for example, Material ID. LPEs will be rendered out at the same time as any Render Elements defined.

If Irradiance mode is selected then this limits the information available to create certain LPEs — although not enforced, it is advised to use Beauty mode for reliable results with LPEs.

✱  Note: LPEs are only available in production rendering (Production Rendering Mode).

Using Light Path Expressions

  1. Open Render Setup > Render Elements
  2. Toggle on/off the checkbox for Enable LPEs.
  3. At the bottom of the Light Path Expressions – Production rollout select New.
  4. In the New LPE dialog, choose between an Alpha or RGB Buffer, enter a Name and your light path expression (LPE) and select OK. The LPE will appear in the table above. Choose Cancel to abort entry.
    5. ✱  Note:
    The LPE name is a label for your own convenience, they can be named anything (e.g. "Diffuse Layer", "My LPE" etc). This label will be appended to the filename when you save out a render. For example, if you are rendering your scene out to My_Render.png and add an LPE called "My_LPE" you will also get My_Render - My_LPE.png.

To edit or rename an LPE that you have already added to the render elements, follow these steps:
  1. Select in the table the LPE you wish to edit
  2. Select Edit and make any changes to the name or LPE
  3. Click OK to accept the changes or Cancel to abort.

To delete an LPE that you have already added to the render elements, follow these steps:

  1. Select in the table the LPE you wish to delete
  2. Click the Delete button.

LPE Examples


LPE Description Notes
L.{2,}E Indirect Lighting Displays all light that has bounced more than once
E.*Le Environment Lighting Displays all light contributed by the environment
E.*La Area Lighting Displays all light contributed by Area Lights
E<RS.>.*L Specular Reflection Displays all specular reflection that hits the camera

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