Merge By Distance Node breaks Motion Blur in Cycles #96463

New Issue

Giff · 2022-03-14T20:16:08+01:00

Giff commented

2022-03-14 20:16:08 +01:00

System Information
Operating system: Win 10 Pro / Version: 10.0.19043 Build 19043
Graphics card: Nvidia 3080Ti / Driver Ver: 511.79

Blender Version
Broken: 3.1.0 Stable & 3.2 master_f6b73966101d

Geometry Nodes -> Merge By Distance Node breaks motion blur in Cycles. Works fine in EV.

Exact steps for others to reproduce the error

1: Take a mesh, split edges,
2. Set Position to give faces motion along their normal vector,
3. Extrude Mesh by faces,
4. Join Geometry with original faces,
5. Merge By Distance
6. Render in cycles with Motion Blur, Mesh Explodes. Now Remove Merge By Distance and render again.

MBTest.blend

**System Information** Operating system: Win 10 Pro / Version: 10.0.19043 Build 19043 Graphics card: Nvidia 3080Ti / Driver Ver: 511.79 **Blender Version** Broken: 3.1.0 Stable & 3.2 master_f6b73966101d Geometry Nodes -> Merge By Distance Node breaks motion blur in Cycles. Works fine in EV. **Exact steps for others to reproduce the error** 1: Take a mesh, split edges, 2. Set Position to give faces motion along their normal vector, 3. Extrude Mesh by faces, 4. Join Geometry with original faces, 5. Merge By Distance 6. Render in cycles with Motion Blur, Mesh Explodes. Now Remove Merge By Distance and render again. ![MotionBlurCyclesGeoNodes.png](https://archive.blender.org/developer/F12927715/MotionBlurCyclesGeoNodes.png) [MBTest.blend](https://archive.blender.org/developer/F12927717/MBTest.blend)

Giff commented

2022-03-14 20:16:08 +01:00

Added subscriber: @Giff-Boake

Richard Antalik commented

2022-03-14 23:39:43 +01:00

Added subscribers: @HooglyBoogly, @iss

Richard Antalik commented

2022-03-14 23:39:43 +01:00

Changed status from 'Needs Triage' to: 'Needs Developer To Reproduce'

Richard Antalik commented

2022-03-14 23:39:43 +01:00

Looking at spreadsheet editor, I see vertices jumping all over the place, which is why this does not work. This seems to be caused by merge by distance node, which shuffles vertex indices, so cycles thinks, that vertex number n jumped to far from where it was before.

I don't think you need merge by distance node for rendering, but still it would be nice if this could be prevented in first place. But currently it is working as intended and may not be considered as a bug.
@HooglyBoogly Should this be confirmed as known issue perhaps?

Looking at spreadsheet editor, I see vertices jumping all over the place, which is why this does not work. This seems to be caused by merge by distance node, which shuffles vertex indices, so cycles thinks, that vertex number n jumped to far from where it was before. I don't think you need merge by distance node for rendering, but still it would be nice if this could be prevented in first place. But currently it is working as intended and may not be considered as a bug. @HooglyBoogly Should this be confirmed as known issue perhaps?

Giff commented

2022-03-15 00:55:00 +01:00

Merge by distance is not needed above but you would definitely need it when using Sub Surface which I think would be a pretty common use case.

Jacques Lucke commented

2022-03-22 10:43:45 +01:00

Added subscriber: @JacquesLucke

Jacques Lucke commented

2022-03-22 10:43:45 +01:00

The Merge by Distance node can not output a consistent topology when the input mesh changes, as would be necessary for motion blur. There could potentially be workarounds using an id on vertices, but that doesn't really work yet, I think. Generally it would be better to do the topology changing operation before the animation here.

Jacob Merrill commented

2022-03-25 05:08:56 +01:00

Added subscriber: @JacobMerrill-1

Jacob Merrill commented

2022-03-25 05:08:56 +01:00

Being able to write your own velocity vector could work here too*

maybe we can expose a 'last frame' data node ?

(to compare a objects pos/rot from last frame to generate your own velocity even if faces merged etc)

Being able to write your own velocity vector could work here too* maybe we can expose a 'last frame' data node ? (to compare a objects pos/rot from last frame to generate your own velocity even if faces merged etc)

Behe eddine commented

2022-12-19 13:21:57 +01:00

Added subscriber: @BEHE

Philipp Oeser added the

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Merge By Distance Node breaks Motion Blur in Cycles #96463