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Overview
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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
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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.
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LPE labels for objects
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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.
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Adding a Light Path Expression label to geometry |
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LPE labels for materials
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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.
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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.
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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
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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 into 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 Combining 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 minus C<RD>.+L. In this This way, we 'll only need to add the graded 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 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
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