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• Variable - Specify the name and type of the AOV, e.g. varying normal Nn.
• Source - Set the source string for the LPE.  If using a built-in LPE (see below), you can leave this field empty; otherwise, you must provide the appropriate LPE expression, e.g. color lpe:C<RD>[<L.>O].
• Display - Set this to framebuffer or a specify a filename.
• Display Device - Specify the type of output. If you want to see your AOVs interactively, you would set it to houdini (for Render View) or framebuffer (for 'it').  
• Quantize - Set the quantitze for your output file.  For openexr, keep the default 0 0 0 0.
• Dither - This is unnecessary for floating point outputs such as an EXR.
• Filter - For color AOVs, the recommended filter is Gaussian. Data AOVs should be unfiltered.
• Filter Width - As above, when using color AOVs, a filter width of 2 2 using Gaussian matches the default render settings.
• Statistics - This field is only used if you are specifying AOVs manually for Denoising.

Built-in

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AOV

Instead of specifying a variable, you can select one of the following

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PxrSurface's AOV

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Custom AOV

Image AddedSome Bxdfs such as PxrSurface require user LPE setup.