Unknown black lines on model in Cycles render. The fix for this suggests it's a bug or an unforeseen problem. #79679

New Issue

Blake Fishman · 2020-08-10T04:13:07+02:00

Blake Fishman commented

2020-08-10 04:13:07 +02:00

System Information
Operating system: Windows-10-10.0.18362-SP0 64 Bits
Graphics card: GeForce GTX 970/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 446.14

Blender Version
Broken: version: 2.82 (sub 7), branch: master, commit date: 2020-03-12 05:06, hash: 375c7dc4ca
Worked: (optional)

Short description of error
The model renders with black lines that correspond with edges on the model. No texture on the model. There's nothing wrong with render settings, normals, etc. The object has custom normals but they're fine. The issue persists even in brand-new projects. The solution is to cull backfaces:

Backfaces should not be creating these shadows anyways, and the fact that this is the fix suggests that Blender implemented some weird change that possibly created this problem. I'm not sure but it warrants a bug report for further investigation.

Exact steps for others to reproduce the error
A .blend file with the issue is attached. The error is already present in the file and all you have to do to see it is to render it. The model renders like so until backfaces are culled:

File black lines problem sword example.blend

Model by itself sword.obj

**System Information** Operating system: Windows-10-10.0.18362-SP0 64 Bits Graphics card: GeForce GTX 970/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 446.14 **Blender Version** Broken: version: 2.82 (sub 7), branch: master, commit date: 2020-03-12 05:06, hash: `375c7dc4ca` Worked: (optional) **Short description of error** The model renders with black lines that correspond with edges on the model. No texture on the model. There's nothing wrong with render settings, normals, etc. The object has custom normals but they're fine. The issue persists even in brand-new projects. The solution is to cull backfaces: ![problem FIXn.png](https://archive.blender.org/developer/F8764604/problem_FIXn.png) Backfaces should not be creating these shadows anyways, and the fact that this is the fix suggests that Blender implemented some weird change that possibly created this problem. I'm not sure but it warrants a bug report for further investigation. **Exact steps for others to reproduce the error** A .blend file with the issue is attached. The error is already present in the file and all you have to do to see it is to render it. The model renders like so until backfaces are culled: ![black lines problem sword example . blend RENDER.png](https://archive.blender.org/developer/F8764600/black_lines_problem_sword_example_._blend_RENDER.png) File [black lines problem sword example.blend](https://archive.blender.org/developer/F8764567/black_lines_problem_sword_example.blend) Model by itself [sword.obj](https://archive.blender.org/developer/F8764630/sword.obj)

Blake Fishman commented

2020-08-10 04:13:07 +02:00

Added subscriber: @BlakeF24

anu manu commented

2020-08-10 08:36:09 +02:00

Added subscriber: @zaster

anu manu commented

2020-08-10 08:36:09 +02:00

There is no bug at all, that is just lazy modeling. Once you apply the scale and get the object to a reasonable size there are no issues.

Evan Wilson commented

2020-08-10 15:26:11 +02:00

Added subscriber: @EAW

Evan Wilson commented

2020-08-10 15:26:11 +02:00

Changed status from 'Needs Triage' to: 'Archived'

Evan Wilson closed this issue

2020-08-10 15:26:11 +02:00

Evan Wilson self-assigned this 2020-08-10 15:26:11 +02:00

Evan Wilson commented

2020-08-10 15:26:11 +02:00

Thank you for the report. The sword is has a scale under 1% and is far from the origin. This causes glitches due to floating point arithmetic.

I have applied the scale (Ctrl-A) and moved all of the objects to the origin (Shift-S then 7) in your file.

black lines problem sword example_fixed.blend

This fixes the issue.

Thank you for the report. The sword is has a scale under 1% and is far from the origin. This [causes glitches](https:*www.youtube.com/watch?v=wGhBjMcY2YQ) due to [floating point arithmetic](https:*en.wikipedia.org/wiki/Floating-point_arithmetic#Accuracy_problems). ![problem_FIXn_cropped.png](https://archive.blender.org/developer/F8766057/problem_FIXn_cropped.png) I have applied the scale (`Ctrl-A`) and moved all of the objects to the origin (`Shift-S` then `7`) in your file. [black lines problem sword example_fixed.blend](https://archive.blender.org/developer/F8766051/black_lines_problem_sword_example_fixed.blend) This fixes the issue. ![#79679.jpg](https://archive.blender.org/developer/F8766052/T79679.jpg)

Nathan Vasil commented

2020-08-10 17:46:30 +02:00

Added subscriber: @vasiln

Nathan Vasil commented

2020-08-10 17:46:30 +02:00

It's shocking to me that a few hundred units (meters) is leading to precision issues. (Applying scale doesn't fix the problem, just moving closer to world origin.) It would be easy to imagine animators that wanted to cover that distance in a single take. It's also surprising to me that Cycles is calculating in world space rather than camera space. (Maybe to maintain a static BVH or something? But then, there ought to be some options for which space it operates in.) Moving geometry is not always an option, because of things like physics (inertia is tied uncontrollably to the world.)

Evan Wilson commented

2020-08-11 01:46:07 +02:00

I took a second look at this file. I was only partially correct about this being a floating point precision issue.
The mesh has issues, including 1170 non-manifold edges and 886 intersecting faces.

I took a second look at this file. I was only partially correct about this being a floating point precision issue. The mesh has issues, including 1170 non-manifold edges and 886 intersecting faces. ![meshanalysis.png](https://archive.blender.org/developer/F8767581/meshanalysis.png) ![Inside_sword_2.png](https://archive.blender.org/developer/F8767583/Inside_sword_2.png)

Evan Wilson commented

2020-08-11 02:40:59 +02:00

Closed as duplicate of #43835

Evan Wilson closed this issue

2020-08-11 02:40:59 +02:00

Nathan Vasil commented

2020-08-11 20:39:23 +02:00

While it does, that's not the cause of the problem. While investigating this for OP, one of the things I did to troubleshoot it was to create brand new geometry at the same positions, with the same angles-- just two quad faces, sharing an edge, triangulated (but that doesn't matter), with vertices snapped to the same positions as the original geometry. This new geometry showed the same problems, with the same fixes, even when giving it a solidify modifier to make it fully manifold.

It seems to me that there are two fixes for this issue: move it closer to the origin or cull backfaces. Other stuff isn't relevant to it. Making the geometry manifold and non-intersecting , and/or applying scale, doesn't solve the problem. (And there are certainly times to use intersecting and non-manifold geometry, particularly with a low poly model like the one linked. It's not like that's a hard no-no or anything, just something where you have to aware of the issues it can potentially cause.)

While it does, that's not the cause of the problem. While investigating this for OP, one of the things I did to troubleshoot it was to create brand new geometry at the same positions, with the same angles-- just two quad faces, sharing an edge, triangulated (but that doesn't matter), with vertices snapped to the same positions as the original geometry. This new geometry showed the same problems, with the same fixes, even when giving it a solidify modifier to make it fully manifold. It seems to me that there are two fixes for this issue: move it closer to the origin or cull backfaces. Other stuff isn't relevant to it. Making the geometry manifold and non-intersecting , and/or applying scale, doesn't solve the problem. (And there are certainly times to use intersecting and non-manifold geometry, particularly with a low poly model like the one linked. It's not like that's a hard no-no or anything, just something where you have to aware of the issues it can potentially cause.)

Evan Wilson commented

2020-08-12 03:54:50 +02:00

@vasiln Thank you for the additional investigation and resulting information. Further tracker searching lead me to discover that there is a task #78007 and a patch D6250 to fix this issue that I had forgotten to mention.

@vasiln Thank you for the additional investigation and resulting information. Further tracker searching lead me to discover that there is a task #78007 and a patch [D6250](https://archive.blender.org/developer/D6250) to fix this issue that I had forgotten to mention.

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Unknown black lines on model in Cycles render. The fix for this suggests it's a bug or an unforeseen problem. #79679