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Cycles 3D Curves missing some intersections using Metal #123634

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opened 2024-06-23 12:46:54 +02:00 by Weizhen Huang · 4 comments
Weizhen Huang commented 2024-06-23 12:46:54 +02:00
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System Information
Operating system: macOS-14.5-arm64-arm-64bit 64 Bits
Graphics card: Metal API Apple M2 Max 1.2

Blender Version
Broken: version: 4.2.0 Beta, branch: blender-v4.2-release, commit date: 2024-06-23 04:43, hash: 485a4dbf0fe2
Worked: Not sure, even 3.3 is broken

Short description of error
Some hair segments are missing when rendering 3D curves with Metal. The behavior depends on the viewing angle.

CPU GPU
hair_cpu.png hair_gpu.png

Exact steps for others to reproduce the error
Add Empty Hair with a length of more than 0.4 or 0.5m, change Curve Shape to 3D Curves, render with GPU, observe some hair segments are missing.

**System Information** Operating system: macOS-14.5-arm64-arm-64bit 64 Bits Graphics card: Metal API Apple M2 Max 1.2 **Blender Version** Broken: version: 4.2.0 Beta, branch: blender-v4.2-release, commit date: 2024-06-23 04:43, hash: `485a4dbf0fe2` Worked: Not sure, even 3.3 is broken **Short description of error** Some hair segments are missing when rendering 3D curves with Metal. The behavior depends on the viewing angle. | CPU | GPU | | -------- | -------- | |![hair_cpu.png](/attachments/3887fea6-60fe-4c4f-87eb-10a9b5778f8b)|![hair_gpu.png](/attachments/47b3d514-fb04-4f0c-a02f-78242864f5bd)| **Exact steps for others to reproduce the error** Add Empty Hair with a length of more than 0.4 or 0.5m, change Curve Shape to 3D Curves, render with GPU, observe some hair segments are missing.
hair_missing_intersection.blend
930 KiB
hair_cpu.png
134 KiB
hair_gpu.png
189 KiB
Alaska commented 2024-06-23 14:00:04 +02:00
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I can confirm this issue. Just adding that this issue doesn't occur when using MetalRT, which points towards it being a BVH2 issue. But I could not reproduce the issue with other backends that use BVH2 (CUDA with BVH2, and CPU with BVH2). So it seems it's a Metal + BVH2 issue.

Since the issue is view dependant, it points to a problem in the curve intersection functions.

Apple M1 Pro

I can confirm this issue. Just adding that this issue doesn't occur when using MetalRT, which points towards it being a BVH2 issue. But I could not reproduce the issue with other backends that use BVH2 (CUDA with BVH2, and CPU with BVH2). So it seems it's a Metal + BVH2 issue. Since the issue is view dependant, it points to a problem in the curve intersection functions. Apple M1 Pro
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 labels 2024-06-23 14:00:14 +02:00
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Alaska commented 2024-06-23 15:06:34 +02:00
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I did some quick investigating. The issue reported here only seems to occur when the hair strands are straight. As they start to curve, the gaps disappear.

But what's interesting is that as you continue adjusting the hair shape, sometimes gaps will appear again, and I assume these two issues are related to each other.

However in the case of curved hair, the issue can be reproduced on multiple backends that use BVH2 (Tested BVH2 on Metal and CPU). So hopefully this helps with debugging the issue.

CPU Embree CPU BVH2 Metal BVH2
CPU Embree.jpg BVH2.jpg BVH2.jpg

missing_hair_intersection_curved.blend
Note: I can only reproduce the issue in this file in the viewport. Although the appreance of the original file in the report adjusts based on final vs viewport rendering, so I assume this is the same thing.

I did some quick investigating. The issue reported here only seems to occur when the hair strands are straight. As they start to curve, the gaps disappear. But what's interesting is that as you continue adjusting the hair shape, sometimes gaps will appear again, and I assume these two issues are related to each other. However in the case of curved hair, the issue can be reproduced on multiple backends that use BVH2 (Tested BVH2 on Metal and CPU). So hopefully this helps with debugging the issue. |CPU Embree|CPU BVH2|Metal BVH2| |-|-|-| |![CPU Embree.jpg](/attachments/8e700f6d-c2aa-4b68-bd8d-81708ec12f9e)|![BVH2.jpg](/attachments/90d9ea74-882a-46ff-9f84-43f68d1f7876)|![BVH2.jpg](/attachments/90d9ea74-882a-46ff-9f84-43f68d1f7876)| [missing_hair_intersection_curved.blend](/attachments/63403df7-f41e-4aef-a7ed-ab00487fe7f2) Note: I can only reproduce the issue in this file in the viewport. Although the appreance of the original file in the report adjusts based on final vs viewport rendering, so I assume this is the same thing.
BVH2.jpg
39 KiB
CPU Embree.jpg
50 KiB
missing_hair_intersection_curved.blend
962 KiB
Weizhen Huang added
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 labels 2024-06-24 14:33:42 +02:00
david494 commented 2024-08-09 22:32:02 +02:00
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I am pretty sure that the curved variant is a different issue, and I have a proposed fix for it in #126157. I cannot reproduce the original problem, maybe something changed, fixing it by accident?

I am pretty sure that the curved variant is a different issue, and I have a proposed fix for it in #126157. I cannot reproduce the original problem, maybe something changed, fixing it by accident?
Alaska commented 2024-08-11 05:29:51 +02:00
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Thanks for opening the pull request David.

I can confirm the original issue Weizhen reported and the one I reported are different, sorry for grouping them together.

On that note, I can still reproduce the issue reported by Weizhen here on my M1 Pro. I tried investigating that issue, but I can't figure out the cause yet. From my debugging it seems that the GPU ray correctly hits the curve, and schedules a INTEGRATOR_SHADE_SURFACE kernel. But a INTEGRATOR_SHADE_BACKGROUND kernel gets processed instead.

Edit: It seems that when I try and debug the issue, the intersect occurs, but if I don't try to debug it, the intersection doesn't. This lead to some confusion and the incorrect (now crossed out) information above

Thanks for opening the pull request David. I can confirm the original issue Weizhen reported and the one I reported are different, sorry for grouping them together. On that note, I can still reproduce the issue reported by Weizhen here on my M1 Pro. I tried investigating that issue, but I can't figure out the cause yet. ~~From my debugging it seems that the GPU ray correctly hits the curve, and schedules a `INTEGRATOR_SHADE_SURFACE` kernel. But a `INTEGRATOR_SHADE_BACKGROUND` kernel gets processed instead.~~ Edit: It seems that when I try and debug the issue, the intersect occurs, but if I don't try to debug it, the intersection doesn't. This lead to some confusion and the incorrect (now crossed out) information above
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Reference: blender/blender#123634
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