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Overview


Now that we've looked at how LPEs work and know the basics, let's look at some advanced concepts and more ways to use LPEs.

 

 

Operations


Light path expressions may also include the modifiers placed after event symbols:

? for 0 or one repetitions (or an optional event)

+ for 1 or more repetitions (or indirect). See the example below.

* for 0 or more repetitions (any)

 

These can be used in combination with the . symbol. It can signify an arbitrary ray spawn event or an arbitrary scattering type, simply put it stands for any symbol.

 

Look at the table to the right to see how using these modifiers change the expressions.

 

Symbols in an expression can be grouped.

[] groups event, scatterings and labels

<> groups event, scatterings and labels

[^] inverts events, scatterings and labels

'' labels for masking objects or materials are placed inside single quotations.

| OR (pipe symbol)

() groups symbols.

 

 

ExpressionEquivalent (Description)Captured ray paths
C.*LFull light selectcamera (C) → any event (.) that repeats zero or more times (*) → light (L)

CRL

  • simplified for C<R.>L
Direct light select

camera → diffuse reflection → light
camera → glossy reflection → light
camera → singular reflection → light

CR.+LIndirect light selectcamera → any event (.) that repeats one or more times (+) → light

CR?L

 ?

camera → diffuse reflection → light
camera → glossy reflection → light
camera → singular reflection → light
camera → light

C<RD>L

  • <> groups events R and D

Lighting (Direct diffuse reflections)

camera → diffuse reflection → light

C<R[GS]>L

  • [ ] groups scatters G and S
Specular (Direct glossy and singular reflections)camera → glossy reflection → light
camera → singular reflection → light

C<RD'1'>L

  • object label 1 is placed in single quotations
Lighting for object label 1camera → diffuse reflection for object with label 1 → light

C<RD[^'1']>L

  • ^ inverts label '1.'Тhis part is placed inside [ ]
Lighting for all objects other than object label 1camera → diffuse reflection for objects that don't have label 1 → light

C<RD>(.+L|.*[OB])

  • .+L means any event, that repeats more than once, until a light hit (L)
  • .*[OB] means any event, zero or more times, until hitting emissive objects (O) or the background (B)
  • | asks for .+L OR .*[OB]
  • [ ] groups O and B
  • ending part is in brackets ( )
GI (Indirect diffuse reflections)camera diffuse reflection any event (.) that repeats one or more times (+) light
camera diffuse reflection any event (.) that repeats zero or more times (*) emission
camera diffuse reflection any event (.) that repeats zero or more times (*) background

 

Example: The + modifier

Previously, we looked at C<T[GS]><RD>L to capture diffuse reflection behind refraction. This will work behind a single refractive pane, as we only ask for a single refractive hit with <T[GS]> 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 C<T[GS]><T[GS]><RD>L. If there's no risk of capturing other random refraction hits, we can use <T[GS]>+ to ask for 1 or more repetitions of <T[GS]>


C<T[GS]><T[GS]><RD>L captures the diffuse surface behind exactly 2 panes of refractive surfaces.

C<T[GS]>+<RD>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.

 

LPE labels for objects


If we take C.* as an example, which gives us the Beauty, C.'1'.* will filter the Beauty only for objects with LPE label 1.

C.'cube'.* will be the Beauty only for objects with LPE label cube.

LPE labels to objects are added with V-Ray User Attributes, added from the extra V-Ray attributes menu. The user attribute name needs to be lpe_label and its value can be text or numbers: for instance

lpe_label=1 or lpe_label=cube.

 

LPE labels can be anything. You can label an object or material 1, or 2, or myObject, or myMaterial.

When used in expressions, labels need to be placed inside single quotes: C.'cube'.*

Note that Maya attributes cannot be used as LPE labels.

 

Adding a Light Path Expression label to geometry

LPE labels for materials


Similarly, C.'m1'.* can be used to separate the Beauty only for materials with material label 1.

C.'mmyMaterial'.* will be the Beauty only for materials with LPE label myMaterial.

LPE labels are added to materials with the Light Path Expression Label attribute from the extra V-Ray attributes menu. The label can take numeric or text values and when used in an expression it needs to be preceded by m and placed inside in single quotes.

 

LPE labels can be anything. You can label an object or material 1, or 2, or myObject, or myMaterial.

When used in expressions, material labels require prefixing the label with m and placing the label inside single quotes:  C.'m1' or C.'mmyMaterial'.

 

Adding a Light Path Expression label to materials


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<RD>.+L, C<RD>.*O and C<RD>.*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<RD>.+L. Then, we subtract C<RD>.+L from the GI in compositing, grade C<RD>.+L and add it back to the composite.

Alternatively we can render GI and an LPE for GI minus C<RD>.+L. In this way, we'll only need to add the graded C<RD>.+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<RD>.+L)|(C<RD>.*O)|(C<RD>.*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 C<RD>L. If we additionally want to capture direct lighting falling onto object(s) labeled cube, we need to render an LPE for C<RD'cube'>L. We can then subtract C<RD'cube'>L from the Lighting channel in compositing, grade it and add it back in.

Alternatively, we can render the C<RD'cube'>L expression and a second one that does the subtraction right in the renderer: (C<RD>L)-(C<RD'cube'>L). 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<TD>+L
  • SSS directly visibly OR occluded (behind reflections and refractions) C<[RT][SG]>*<TD>.+L
  • SSS only from occluded objects C<[RT][SG]>+<TD>.+L

Self-reflections by material label 1

  • C<R[GS]'m1'>.'m1'+L

Emissive

  • Emissive (self-illuminating) objects only - direct camera rays CO
  • The GI coming from emissive objects C<RD>.*O
  • The union of GI, coming from emissive object and the directly visible to camera emissive objects (C<RD>.*O)|(CO)

GI

  • Full GI C<RD>(.+L|.*[OB])
  • GI from the environment only C<RD>.*B
  • GI minus GI from environment C<RD>(.+L|.*[OB])-(C<RD>.*B)

VRayMtl Sheen and Coat

  • Coat Specular C<R[GS]'mcoat'>L

  • Coat Reflection C<R[GS]'mcoat'>.+L

  • Sheen Specular C<R[GS]'msheen'>L

  • Sheen Reflection C<R[GS]'msheen'>.+L