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Essentially, the syntax follows the direction in which a ray travels in the scene and capture captures information along the ray path.

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For example, if you need to capture GI bounced off of one object onto another, light will travel from the light, onto the object that casts GI, then onto the object that receives GI and end at the camera. But in LPEs, just like in ray tracing, the expression will be in reverse, starting from the camera, asking for what object receives GI, then what object casts the GI and end at the light source. 

LPE Symbols

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The LPE alphabet consists of the symbols representing a ray event/type and additional regular expression symbols (“.”, “+”, “*”, “<”, “>”, “[“, “]”, “|”, “?”, “^” and additional label strings).

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The Misc and masking presets are useful in common-use - cases.

Any preset can be used as a starting point for building other expressions and like any LPE, they can be combined with boolean operations.

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