"Long Hair" children pulled to world origin when using Texture (2.93.4 regression) #91887

Open
opened 2021-10-01 19:55:02 +02:00 by sentharn · 13 comments

System Information
Operating system: Windows-10-10.0.19041-SP0 64 Bits
Graphics card: NVIDIA GeForce GTX 1080/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 471.11

Blender Version
Broken: version: 2.93.4, branch: master, commit date: 2021-08-31 09:23, hash: b7205031ce
Worked: 2.93.3

Short description of error
My particle hair system works as expected in 2.91+ up to 2.93.3:
hair_292.JPG

In 2.93.4, the children are pulled towards world (not object) origin:
hair_2934.JPG

This appears to only happen IF a texture is used to affect Density AND "Long Hair" is checked.

Exact steps for others to reproduce the error

  1. Move default cube away from origin
  2. Add particle system
  3. Enable interpolated children
  4. Check "Long Hair"
  5. Add a Texture (even a cloud texture works)
  6. Enable Density influence

bug3.JPG

Blend file with error:

Bug3.blend

**System Information** Operating system: Windows-10-10.0.19041-SP0 64 Bits Graphics card: NVIDIA GeForce GTX 1080/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 471.11 **Blender Version** Broken: version: 2.93.4, branch: master, commit date: 2021-08-31 09:23, hash: `b7205031ce` Worked: 2.93.3 **Short description of error** My particle hair system works as expected in 2.91+ up to 2.93.3: ![hair_292.JPG](https://archive.blender.org/developer/F10717928/hair_292.JPG) In 2.93.4, the children are pulled towards world (not object) origin: ![hair_2934.JPG](https://archive.blender.org/developer/F10717934/hair_2934.JPG) This appears to only happen IF a texture is used to affect Density AND "Long Hair" is checked. **Exact steps for others to reproduce the error** 1. Move default cube away from origin 2. Add particle system 3. Enable interpolated children 4. Check "Long Hair" 5. Add a Texture (even a cloud texture works) 6. Enable Density influence ![bug3.JPG](https://archive.blender.org/developer/F10718059/bug3.JPG) Blend file with error: [Bug3.blend](https://archive.blender.org/developer/F10718072/Bug3.blend)
Author

Added subscriber: @sentharn

Added subscriber: @sentharn

Added subscriber: @iss

Added subscriber: @iss

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

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

Can confirm bug in 7ec351c0d5 will try to bisect

Can confirm bug in 7ec351c0d531 will try to bisect

Added subscriber: @mont29

Added subscriber: @mont29

Caused by c2f74d6243

CC @mont29

Caused by c2f74d62435a25dc52d1a640db1c417ad7ec75d3 CC @mont29

@iss This cannot be 'caused' by c2f74d6243, but only revealed by this commit. Division by zero are never, ever acceptable.

Guess someone needs to spend some time in particle code to understand again what and why this is happening.

And also check if this is reproducible in current master as well.

@iss This cannot be 'caused' by c2f74d6243, but only revealed by this commit. Division by zero are never, ever acceptable. Guess someone needs to spend some time in particle code to understand again what and why this is happening. And also check if this is reproducible in current master as well.

In #91887#1230539, @mont29 wrote:
@iss This cannot be 'caused' by c2f74d6243, but only revealed by this commit. Division by zero are never, ever acceptable.

Sorry my bad, I automatically assumed commit broke something. Should have checked that this is actual cause.

And also check if this is reproducible in current master as well.

Yes it happens in master. I specify LTS only issues if they are critical and happen in LTS only.

> In #91887#1230539, @mont29 wrote: > @iss This cannot be 'caused' by c2f74d6243, but only revealed by this commit. Division by zero are never, ever acceptable. Sorry my bad, I automatically assumed commit broke something. Should have checked that this is actual cause. > And also check if this is reproducible in current master as well. Yes it happens in master. I specify LTS only issues if they are critical and happen in LTS only.
Author

The new (correct) behavior in project_v3_v3v3() is causing the issue.

psys_thread_create_path() in source/blender/blenkernel/intern/particle.c has a code path to create child paths. Around line 3017ish it calls project_v3_v3v3(dvec, off2- [x], (key- [x] + k)->vel);

For whatever reason, (key- [x] + k)->vel is a zero v3.

I don't know anything about the particle code (except how to misuse it) but I hope this helps track down the issue!

EDIT: 3017, not 3070

The new (correct) behavior in project_v3_v3v3() is causing the issue. psys_thread_create_path() in source/blender/blenkernel/intern/particle.c has a code path to create child paths. Around line 3017ish it calls **project_v3_v3v3(dvec, off2- [x], (key- [x] + k)->vel);** For whatever reason, **(key- [x] + k)->vel** is a zero v3. I don't know anything about the particle code (except how to misuse it) but I hope this helps track down the issue! EDIT: 3017, not 3070
Author

@iss the problem is still present in today's 3.1 Alpha build and 3.0 release. Should it be tagged as a 3.1 problem?

@iss the problem is still present in today's 3.1 Alpha build and 3.0 release. Should it be tagged as a 3.1 problem?

I think I have tagged this because it was regression in 2.93, but AFAIK these tags are supposed to signify when it should be fixed, so will remove the tag. Regression in LTS is clearly visible in broken/worked fields.

I think I have tagged this because it was regression in 2.93, but AFAIK these tags are supposed to signify when it should be fixed, so will remove the tag. Regression in LTS is clearly visible in broken/worked fields.

Added subscriber: @dfelinto

Added subscriber: @dfelinto

Thanks for the report. Marking this as known issue since it depends on the old particle settings which is end of life .

Thanks for the report. Marking this as known issue since it depends on the [old particle settings which is end of life ](https://developer.blender.org/tag/nodes_physics/).
Philipp Oeser removed the
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Reference: blender/blender#91887
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