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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. |
Boolean
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Operations
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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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