When hair emitter's "Curve Guide" Field Weight lies between 0 and 1, strands float toward origin. #71745

New Issue

tim · 2019-11-21T21:05:16+01:00

tim commented

2019-11-21 21:05:16 +01:00

Operating system: macOS Mojave 10.14.6
Bug occurs both in 2.8 and 2.81 beta

When a hair emitter's "Curve Guide" Field Weight lies between 0 and 1, strands float inexplicably toward origin. This renders the "Curve Guide" styling method incompatible with "Hair Dynamics."

The attached .blend was made with these steps:

enable "Hair" particle system on default cube.
add a bezier curve and enable "Curve Guide" Force Field physics.
stretch bezier curve into a wide arc emerging from the cube.
parent the curve to the cube.
translate the cube far from the origin.
In the Particle Settings' "Field Weights" tab, set Curve Guide to 0.5.
Hair particles float inexplicably toward origin.
(Optional)
set Curve Guide back to 1.0
in the curve's Physics tab, under Settings, set "Falloff Power" to nonzero. Encounter the same bug.

Thanks for your time!
curve-guided hair with field weights bug.blend

Operating system: macOS Mojave 10.14.6 Bug occurs both in 2.8 and 2.81 beta When a hair emitter's "Curve Guide" Field Weight lies between 0 and 1, strands float inexplicably toward origin. This renders the "Curve Guide" styling method incompatible with "Hair Dynamics." The attached .blend was made with these steps: 1. enable "Hair" particle system on default cube. 2. add a bezier curve and enable "Curve Guide" Force Field physics. 3. stretch bezier curve into a wide arc emerging from the cube. 4. parent the curve to the cube. 5. translate the cube far from the origin. 6. In the Particle Settings' "Field Weights" tab, set Curve Guide to 0.5. 7. **Hair particles float inexplicably toward origin.** (Optional) 8. set Curve Guide back to 1.0 9. in the curve's Physics tab, under Settings, set "Falloff Power" to nonzero. Encounter the same bug. Thanks for your time! [curve-guided hair with field weights bug.blend](https://archive.blender.org/developer/F8152552/curve-guided_hair_with_field_weights_bug.blend)

tim commented

2019-11-21 21:05:16 +01:00

Added subscriber: @teedotbee

tim changed title from ~~When hair emitter's "Curve Guide" Field Weights lie between 0 and 1, strands float toward origin.~~ to When hair emitter's "Curve Guide" Field Weight lies between 0 and 1, strands float toward origin.

2019-11-21 21:07:44 +01:00

tim commented

2019-11-22 00:31:23 +01:00

i'm pretty sure the issue lies in this function here...anyone care to confirm?
https://github.com/sobotka/blender/blob/master/source/blender/blenkernel/intern/effect.c#L915

EDIT: No, although most effectors' weights are factored in here, Curve Guides' aren't. They're done in the do_guides(...) function as per below

i'm pretty sure the issue lies in this function here...anyone care to confirm? https://github.com/sobotka/blender/blob/master/source/blender/blenkernel/intern/effect.c#L915 EDIT: No, although *most* effectors' weights are factored in here, Curve Guides' aren't. They're done in the `do_guides(...)` function as per below

tim commented

2019-11-22 21:50:36 +01:00

EDIT AGAIN: Patch request viewable here: https:*developer.blender.org/D6292

I'm back. Commenting out these two lines in particle.c's do_guides(...) function (L2107 and L2108) fixes it. https://github.com/sobotka/blender/blob/master/source/blender/blenkernel/intern/particle.c#L2107.

Also fixes it for particles, though may need normalization as valid range seems to lie between 0.001 and 0.005 (scales pleasingly for hair though, as the attached video shows)

EDIT: I'll be more precise--I didn't merely comment out the lines, I replaced

madd_v3_v3fl(veffect, guidedir, data->strength);```

with
```      madd_v3_v3fl(effect, vec_to_point, 1);

madd_v3_v3fl(veffect, guidedir, 1);```

Seems to have been allowing weight to impact global position, which explains the heading-to-origin. 

broken:
[BROKEN.mov](https://archive.blender.org/developer/F8156457/BROKEN.mov)
fixed:
[FIXED-.mov](https://archive.blender.org/developer/F8156461/FIXED-.mov)

Very happy as this means I can continue animating.

EDIT AGAIN: Patch request viewable here: [https:*developer.blender.org/D6292 ](https:*developer.blender.org/D6292) I'm back. Commenting out these two lines in `particle.c`'s `do_guides(...)` function (L2107 and L2108) fixes it. <https://github.com/sobotka/blender/blob/master/source/blender/blenkernel/intern/particle.c#L2107>. Also fixes it for particles, though may need normalization as valid range seems to lie between 0.001 and 0.005 (scales pleasingly for hair though, as the attached video shows) EDIT: I'll be more precise--I didn't merely comment out the lines, I replaced ``` madd_v3_v3fl(effect, vec_to_point, data->strength); ``` madd_v3_v3fl(veffect, guidedir, data->strength);``` ``` with ``` madd_v3_v3fl(effect, vec_to_point, 1); ``` madd_v3_v3fl(veffect, guidedir, 1);``` ``` Seems to have been allowing weight to impact global position, which explains the heading-to-origin. broken: [BROKEN.mov](https://archive.blender.org/developer/F8156457/BROKEN.mov) fixed: [FIXED-.mov](https://archive.blender.org/developer/F8156461/FIXED-.mov) Very happy as this means I can continue animating.

Philipp Oeser commented

2019-12-02 14:39:03 +01:00

Added subscribers: @JacquesLucke, @sebbas, @zazizizou, @lichtwerk

Philipp Oeser commented

2019-12-02 14:39:03 +01:00

Can only say that 2.79 behaved the same [that doesnt mean this is not looking a whole lot like a bug...]
I personally only ever used it with the Weights option [curve can then be tweaked to have less influence at the emitter], but...

Guess we have to wait for someone to understand what is supposed to happen here...

Can only say that 2.79 behaved the same [that doesnt mean this is not looking a whole lot like a bug...] I personally only ever used it with the `Weights` option [curve can then be tweaked to have less influence at the emitter], but... Guess we have to wait for someone to understand what is supposed to happen here...

Richard Antalik commented

2020-03-20 17:06:40 +01:00

Added subscriber: @iss

Richard Antalik commented

2020-03-20 17:06:40 +01:00

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

Richard Antalik commented

2020-03-20 17:06:40 +01:00

I will confirm this, since this definitely looks like a bug.

I don't think it makes sense to have this unconfirmed possibly indefinitely.

I will confirm this, since this definitely looks like a bug. I don't think it makes sense to have this unconfirmed possibly indefinitely.

Philipp Oeser removed the

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When hair emitter's "Curve Guide" Field Weight lies between 0 and 1, strands float toward origin. #71745