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Light Path Expressions or LPEs are a very powerful and advanced tool for extracting specific lighting events from the scene to a separate channel. This allows for a very fine control of the image in compositing.

For example, LPEs allow breaking the GI down to different types of light sources like lights, the environment and self-illuminating objects, or breaking down the GI to its separate bounces, or capturing only self-reflections, or the SSS that's only seen behind glass and similar for compositing control of only this aspect of the image.

The right-click menu offers a list of all symbols in LPEs, as well as presets to get you started.

There are preset equivalents to already available render elements (like Beauty, Lighting, GI, Light Select types etc). There are also Misc and masking presets for the GI coming from the environment or for the GI coming from a specific object.

Also available is a list of boolean operations that allow more complex expressions to be rendered in a single channel. For example, in this way some compositing operations like union or subtraction can be done in the renderer, instead of in the compositing application.