--- title: "Advanced Light Path Expressions" canonical: "https://documentation.chaos.com/space/VRAYNUKE/110626238/Advanced%20Light%20Path%20Expressions" format: markdown --- ## **Overview** --- Now that we've looked at [how LPEs work](https://docs-chaos.atlassian.net/wiki/spaces/VRAYNUKE/pages/110626236) and know the basics, let's look at some advanced concepts and more ways to use LPEs.     ## **Operations** --- > Macro (anchor)   > Macro (rw-ui-expands-macro) > > > Macro (rw-expand) > > Previously, we looked at **CL** to capture diffuse reflection behind refraction. This will work behind a single refractive pane, as we only ask for a single refractive hit with **** that is either Glossy or Singular. > > If the refractive object is (for example) a cube, then the ray needs to enter and exit the cube, hitting the refractive surface twice before reaching the diffuse surface behind. > > In this case, we need to ask for two refraction hits **CL**. If there's no risk of capturing other random refraction hits, we can use **+** to ask for **1 or more repetitions of ** > > > ![image](media://35d25746-a3be-497a-b0a0-24198287d291) > > CL captures the diffuse surface **behind exactly 2** panes of refractive surfaces. > > C+L captures the diffuse surface **behind 1 or more** refractive hits. ## **Capture objects or materials** --- LPEs can be modified to record information for only specific objects or materials by adding a label.     **Inverting LPE labels** --- Labels can be easily inverted in expressions by adding** ^** in front of them and then placing the inversion symbol and the label inside square brackets **[ ]**. For example, **C.[^'m1'].*** will give us the Beauty for all materials that do not have material label **1**. We can invert multiple labels at the same time, for example **CR[^'1''2'].+L **filters the indirect illumination for all object that do not have the labels **1** and **2**.   ### **Reserved labels: coat and sheen** --- The material labels coat and sheen are reserved for the additional VRayMtl Coat and Sheen layers. You cannot add a custom Light Path Expression Label with values **coat** or **sheen**. You can use **mcoat** and **msheen** directly in expressions to capture the reserved labels. Boolean operations between expressions can be used along with the reserved coat or sheen labels to extract those layers exclusively for specified materials or objects. Example: **(C.*)-(C.'mcoat'.*)** captures the Beauty without the contribution of the VRayMtl Coat layer. *You can read more about boolean operations in the next section.*   ## **Boolean operations** --- Boolean operations allow us to combine expressions. We can complement (**^**), intersect (**&**) or unite (**|**) expressions, or we can subtract (**-**) one expression from another. Let's look at some examples. GI can be broken down to different types of light sources: lights, self-illuminating objects and the environment. If we need to control each of these separately in compositing, we can use the built-in presets, respectively: **C.+L**, **C.*O** and **C.*B**. The combination of these in compositing (with a plus) replaces the GI channel in a back-to-beauty composite. However, if we only need to grade the GI from Lights, we can render a GI render element and **C.+L**. Then, we subtract **C.+L** from the GI in compositing, grade **C.+L** and add it back to the composite. Alternatively we can render GI and an LPE for GI minus **C.+L**. In this way, we'll only need to add the graded **C.+L** channel to the GI in compositing, saving us some compositing complexity. For the purpose of this example, we can do a union of the GI coming from all three types of light sources to get the GI using the pipe symbol for union: **(C.+L)|(C.*O)|(C.*B)**   Boolean operations can also be very useful when using LPEs with object or material labels. Let's look at a simple example with direct lighting. V-Ray already has a render element for this (**Lighting**) and the equivalent expression is **CL**. If we additionally want to capture direct lighting falling onto object(s) labeled **cube**, we need to render an LPE for **CL**. We can then subtract **CL** from the Lighting channel in compositing, grade it and add it back in. Alternatively, we can render the **CL** expression and a second one that does the subtraction right in the renderer: **(CL)-(CL)**. We then grade each channel as needed and add them with a plus in the composite.     ## **Additional examples** --- SSS: - SSS from objects directly visible to the camera **C+L** - SSS directly visibly OR occluded (behind reflections and refractions) **C<[RT][SG]>*.+L** - **SSS only from occluded objects  C<[RT][SG]>+.+L ** Self-reflections by material label 1 - **C.'m1'+L** Emissive - Emissive (self-illuminating) objects only - direct camera rays **CO** - The GI coming from emissive objects **C.*O** - The union of GI, coming from emissive object and the directly visible to camera emissive objects **(C.*O)|(CO)** GI - Full GI **C(.+L|.*[OB])** - GI from the environment only **C.*B** - GI minus GI from environment **C(.+L|.*[OB])-(C.*B)**