From LuxRender Wiki
Passes (also called Arbitrary Output Variables or AOVs) allow to obtain various extra information about the rendered image, like the distance a surface is away from the camera.
LuxRender supports passes through its new LuxCore interface. More in-depth information (e.g. for exporter developers) is available here.
BIASPATH (OpenCL and CPU) and PATH (OpenCL and CPU) are the only engines currently supporting passes.
BIASPATH is the only engine to support all passes.
The OpenCL engines only support one BY_MATERIAL_ID and MATERIAL_ID_MASK pass.
BIDIR (CPU) supports only the ALPHA and RGB(A)* passes.
The RGB_TONEMAPPED pass and its RGBA version are the only passes where the LuxCore imagepipeline is applied.
They are the equivalent of the "Combined" or "Beauty" pass.
RGB and RGBA (HDR)
These passes contain the raw color information of the film (without imagepipeline operations like tonemapping) in HDR format.
We're using a different scene for this example:
Normals without interpolation (smoothing).
Interpolated normals (with smoothing).
One color per material.
One color per object, similar to MATERIAL_ID
A black/white mask of one material.
The result of the RGB_TONEMAPPED pass, but masked out to only contain parts where the selected material is visible.
The radiance group pass is the equivalent of light groups, please refer to the examples at the LuxRender Light groups page.
Shows how much computation time (fired rays) is spent on each pixel. Since the images on this page were rendered with the tile-based Biased Path engine in adaptive mode, some tiles clearly show as significantly brighter.