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Maniphest T92832

No Backfacing data on Displacement
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Description

System Information
Operating system: Windows-10-10.0.19043-SP0 64 Bits
Graphics card: NVIDIA GeForce GTX 960M/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 465.89

Blender Version
Broken: version: 3.1.0 Alpha, branch: master, commit date: 2021-11-01 19:48, hash: rB75f5edcaf338
Worked: -

Short description of error
In the Shader Nodes, Geometry Node's Backfacing output is always zero when used in a Displacement Node.

Exact steps for others to reproduce the error
Open the attached blend file that has a plane and a demo material
Switch to render view (displacement takes effect)
Compare the backfacing value on emission and displacement

no_backfacing_on_displacement.blend797 KBDownload

Event Timeline

Berk Canlı (bekorn) created this task.Nov 4 2021, 5:54 PM
Philipp Oeser (lichtwerk) changed the task status from Needs Triage to Needs Information from Developers.Nov 5 2021, 11:35 AM
Philipp Oeser (lichtwerk) added a project: Render & Cycles.
Philipp Oeser (lichtwerk) added a subscriber: Philipp Oeser (lichtwerk).

I can confirm the behavior, but I think this might actually be expected behavior (havent checked code though).
(just thinking of this sort of "dependency cycle" where result of displacement could actually modify/change if stuff is backfacing or not)

Will let wiser people have a word here though.

Brecht Van Lommel (brecht) closed this task as Archived.Nov 5 2021, 6:31 PM
Brecht Van Lommel (brecht) added a subscriber: Brecht Van Lommel (brecht).

This is by design, the mesh surface can't be displaced in a different direction depending on which side you view it from. For bump mapping that's possible, but for real displacement that can't work.