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Arbitrary Output Variables (AOVs) are the secondary images produced by the renderer. There can be any number of them generated simultaneously and each one may go to a different file, a different display driver, or use different pixel filter settings. In some cases, special pixel filter modes may be used to avoid mixing values from different samples in a non-sensical way; typically these select a single sample to be representative of the whole pixel.
The built-in AOVs display mostly geometric information pertaining to points on the visible surfaces seen by the camera. These values are automatically generated by the renderer and are available as AOVs regardless of the active integrator. Here is the current list of available builtin-ins:
FIX NEEDED: These didn't have images so they weren't included: float VLen, float incidentRaySpread, float time, vector dPdtime and float outsideIOR