Cycles: Displacement splits flat-shaded faces #114028

New Issue

Sun Kim · 2023-10-22T08:23:45+02:00

Sun Kim commented

2023-10-22 08:23:45 +02:00

System Information
Operating system: Windows-10-10.0.19045-SP0 64 Bits
Graphics card: NVIDIA GeForce RTX 2060 SUPER/PCIe/SSE2 NVIDIA Corporation 4.6.0 NVIDIA 536.40

Blender Version
Broken: version: 4.1.0 Alpha, branch: PR111187, commit date: 2023-10-20 17:39, hash: 05ff4ce98bce

Caused by something between 4.1.0 Alpha 2023-10-19 22:55 a53dd6e93cfe and 4.1.0 Alpha 2023-10-20 22:28 0bde01eab51c.

Short description of error

Cycles displacement split flat-shaded faces if the mesh has a mix of smooth- and flat-shaded faces.

If all faces are flat-shaded they don't split.

Test file: displacement_split_cycles.blend

**System Information** Operating system: Windows-10-10.0.19045-SP0 64 Bits Graphics card: NVIDIA GeForce RTX 2060 SUPER/PCIe/SSE2 NVIDIA Corporation 4.6.0 NVIDIA 536.40 **Blender Version** Broken: version: 4.1.0 Alpha, branch: PR111187, commit date: 2023-10-20 17:39, hash: `05ff4ce98bce` Caused by something between 4.1.0 Alpha 2023-10-19 22:55 `a53dd6e93cfe` and 4.1.0 Alpha 2023-10-20 22:28 `0bde01eab51c`. **Short description of error** Cycles displacement split flat-shaded faces if the mesh has a mix of smooth- and flat-shaded faces. If all faces are flat-shaded they don't split. <video src="/attachments/8d55299e-a1f2-4023-8d2d-df29df2bf2f6" title="displacement_split_cycles.mp4" controls></video> Test file: [displacement_split_cycles.blend](/attachments/0fbc0309-a194-4932-b942-cf43a59e44f5)

displacement_split_cycles.blend

1.0 MiB

displacement_split_cycles.mp4

6.3 MiB

Sun Kim added the

labels 2023-10-22 08:23:45 +02:00

Sun Kim changed title from ~~Cycles: displacement split flat-shaded faces~~ to Cycles: Displacement splits flat-shaded faces

2023-10-22 08:25:33 +02:00

Sun Kim referenced this issue

2023-10-22 08:29:58 +02:00

EEVEE Next: Displacement splits flat-shaded faces #114029

Jesse Yurkovich commented

2023-10-23 03:32:50 +02:00

Most likely caused by 89e3ba4e25.

CC @HooglyBoogly

Most likely caused by 89e3ba4e25c. CC @HooglyBoogly

Jesse Yurkovich referenced this issue

2023-10-23 03:32:52 +02:00

EEVEE Next: Displacement splits flat-shaded faces #114029

Jesse Yurkovich added

and removed

labels 2023-10-23 03:33:32 +02:00

Brecht Van Lommel added this to the 4.1 milestone 2023-10-23 12:19:27 +02:00

Hans Goudey commented

2023-11-01 11:19:00 +01:00

I don't have a great idea of how to fix this honestly. Maybe displacement could be tweaked to keep these vertices "merged".

The other option is that we're forced to add a "No Split Normals" flag to meshes, but I'd really like to avoid that since it would make edge sharpness disfunctional, meaning we have some of the complexity of auto smooth again.

I don't have a great idea of how to fix this honestly. Maybe displacement could be tweaked to keep these vertices "merged". The other option is that we're forced to add a "No Split Normals" flag to meshes, but I'd really like to avoid that since it would make edge sharpness disfunctional, meaning we have some of the complexity of auto smooth again.

Hans Goudey closed this issue

2023-11-01 11:19:03 +01:00

Blender Bot added

and removed

labels 2023-11-01 11:19:05 +01:00

Hans Goudey reopened this issue

2023-11-01 11:19:07 +01:00

Blender Bot added

and removed

labels 2023-11-01 11:19:12 +01:00

Hans Goudey added

and removed

labels 2023-11-01 11:19:19 +01:00

Hans Goudey commented

2024-01-08 21:56:58 +01:00

To make it clear, the new state we have now that we didn't have before is that we can have "split" normals (which in Cycles causes splitting of mesh topology at the seams) without explicitly turning on "auto smooth". Options are:

Not a bug. This is just the existing limitation that used to be hidden behind the auto smooth flag.
Somehow use normals averaged per vertex for displacement. Not sure how feasible this is.
Somehow add a post-displacement step for merging vertices-- would require keeping track of vertex indices before the split
Stop splitting edges for split normals and upload per-corner normals to Cycles. Would that require extra branching in shaders?
Always upload per-corner normals to Cycles. Lots of memory usage, yikes!

I don't really view going back to the previous state of "turn on some option if you want this behavior" just hide the problem a proper solution. It would be helpful to have some input from the Cycles team here. I'd also note there might be performance benefits to not altering mesh topology for split shading requirements.

To make it clear, the new state we have now that we didn't have before is that we can have "split" normals (which in Cycles causes splitting of mesh topology at the seams) without explicitly turning on "auto smooth". Options are: 1. Not a bug. This is just the existing limitation that used to be hidden behind the auto smooth flag. 2. Somehow use normals averaged per vertex for displacement. Not sure how feasible this is. 3. Somehow add a post-displacement step for merging vertices-- would require keeping track of vertex indices before the split 4. Stop splitting edges for split normals and upload per-corner normals to Cycles. Would that require extra branching in shaders? 5. Always upload per-corner normals to Cycles. Lots of memory usage, yikes! I don't really view going back to the previous state of "turn on some option if you want this behavior" just hide the problem a proper solution. It would be helpful to have some input from the Cycles team here. I'd also note there might be performance benefits to not altering mesh topology for split shading requirements.

Brecht Van Lommel commented

2024-01-09 11:49:43 +01:00

@HooglyBoogly I'll look into adding native support for per-corner normals in Cycles, to avoid splitting the mesh.

👍 1

Brecht Van Lommel referenced this issue from a commit

2024-01-24 19:56:16 +01:00

Fix #114028: Cycles displace splits faces with smooth and flat faces

Brecht Van Lommel closed this issue

2024-01-24 19:56:17 +01:00

Blender Bot added

and removed

labels 2024-01-24 19:56:20 +01:00

Hans Goudey added

and removed

labels 2024-01-24 20:01:33 +01:00

Brecht Van Lommel commented

2024-01-24 20:19:33 +01:00

I implemented corner normals in Cycles, but there is a significant increase in memory usage in benchmarks that I'm unhappy about. There are many existing models that have autosmooth enabled for no good reason, and a handful of edges get marked as sharp which affects memory usage for the entire mesh.

I ended up implementing a simpler fix for this specific regression, since I don't want to spend much more time on this now.

There is still a pre-existing issue where sharp edges can also cause artifacts like this bug report. However that is not a regression, so not high priority.

In general, displacement is not really meant to be used with low poly models where you would use these kinds of sharp edges and faces.

I implemented corner normals in Cycles, but there is a significant increase in memory usage in benchmarks that I'm unhappy about. There are many existing models that have autosmooth enabled for no good reason, and a handful of edges get marked as sharp which affects memory usage for the entire mesh. I ended up implementing a simpler fix for this specific regression, since I don't want to spend much more time on this now. There is still a pre-existing issue where sharp edges can also cause artifacts like this bug report. However that is not a regression, so not high priority. In general, displacement is not really meant to be used with low poly models where you would use these kinds of sharp edges and faces.

Sign in to join this conversation.

No Label

Download

What's New

Blender Studio

Manual

Developers Blog

Documentation

Benchmark

Blender Conference

Development Fund

One-time Donations

Cycles: Displacement splits flat-shaded faces #114028