Surface Deform modifier - distortion on bind (depending on scale) #73348

New Issue

Jose Conseco · 2020-01-23T21:47:43+01:00

Jose Conseco commented

2020-01-23 21:47:43 +01:00

System Information
Operating system: Windows-10-10.0.18362 64 Bits
Graphics card: GeForce GTX 660/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 436.15

Blender Version
Broken: version: 2.81 (sub 16), branch: master, commit date: 2019-11-20 14:27, hash: 26bd5ebd42
Same on blender 2.82 daily build
Worked: (optional)

Short description of error
When pressing 'Bind' in surface deform modifier, some small distortion is applied to mesh, even though is should not change at all.

Exact steps for others to reproduce the error

Press 'Bind' on surface deform modifier. Mesh gets distorted slightly on fingers. This is problematic.
Blend file:
bind_problem.blend
Gif showing problem:

Simplified test case:
#73348.blend

**System Information** Operating system: Windows-10-10.0.18362 64 Bits Graphics card: GeForce GTX 660/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 436.15 **Blender Version** Broken: version: 2.81 (sub 16), branch: master, commit date: 2019-11-20 14:27, hash: `26bd5ebd42` Same on blender 2.82 daily build Worked: (optional) **Short description of error** When pressing 'Bind' in surface deform modifier, some small distortion is applied to mesh, even though is should not change at all. **Exact steps for others to reproduce the error** 1. Press 'Bind' on surface deform modifier. Mesh gets distorted slightly on fingers. This is problematic. Blend file: [bind_problem.blend](https://archive.blender.org/developer/F8297785/bind_problem.blend) Gif showing problem: ![bind_distortion.gif](https://archive.blender.org/developer/F8297789/bind_distortion.gif) Simplified test case: [#73348.blend](https://archive.blender.org/developer/F8299067/T73348.blend)

Jose Conseco commented

2020-01-23 21:47:43 +01:00

Added subscriber: @JoseConseco

Philipp Oeser commented

2020-01-24 10:39:47 +01:00

Added subscriber: @lichtwerk

Philipp Oeser commented

2020-01-24 10:39:47 +01:00

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

Philipp Oeser commented

2020-01-24 10:39:47 +01:00

Can confirm, checking...

Philipp Oeser commented

2020-01-24 10:56:48 +01:00

Seems to be a precision problem somewhere, if I scale the whole thing by 10, issues seem to vanish...

Philipp Oeser changed title from ~~Surface Deform modifier - distortion on bind~~ to Surface Deform modifier - distortion on bind (depending on scale)

2020-01-24 11:13:24 +01:00

Philipp Oeser commented

2020-01-24 13:10:15 +01:00

Added subscriber: @LucaRood-3

Philipp Oeser commented

2020-01-24 13:10:15 +01:00

Well, even if this can be worked around by scaling, will still keep open, since there seems to be some mismatch going on [which really shouldnt be the case]

@LucaRood-3 : Known Isssue?

Well, even if this can be worked around by scaling, will still keep open, since there seems to be some mismatch going on [which really shouldnt be the case] @LucaRood-3 : Known Isssue?

Demeter Dzadik commented

2020-02-06 11:04:17 +01:00

Added subscriber: @Mets

Cody Winchester commented

2020-02-07 03:09:40 +01:00

Added subscriber: @CodyWinchester

Cody Winchester commented

2020-02-07 03:09:40 +01:00

Looking into this and I found the problem lies in the Ngon method bindings. It is the only method that returns errors on the small scale that go away once scaled up.

In MOD_surfacedeform.c the line:

        interp_weights_poly_v2( 
             sdbind->vert_weights, bpoly->coords_v2, bpoly->numverts, bpoly->point_v2);

is where things go wrong. The weight gets calculated incorrectly due to the small size of the geometry, which causes the calculations to be off.

Inside of interp_weights_poly_v2 is the line:

       ht = mean_value_half_tan_v2(&d_curr, &d_next);

in this function if the area it calculates is too small it returns 0.0f which causes the weights to be wrong. Since the geometry is so small some polys return an area of 0.0f when scaled up they are correct.

If I scale the data put into interp_weights_poly_v2 and interp_weights_tri_v3 and then unscale it afterwards the bug goes away as the weight calculation is using much larger values and doesn't fail due to float precision. Depending on the factor I scale the data is how much smaller the geometry can be.

If I remove the Ngon method as an option entirely it works with no errors at the current scale, but if scaled down again by .1 it still fails.

Looking into this and I found the problem lies in the Ngon method bindings. It is the only method that returns errors on the small scale that go away once scaled up. In MOD_surfacedeform.c the line: ``` interp_weights_poly_v2( sdbind->vert_weights, bpoly->coords_v2, bpoly->numverts, bpoly->point_v2); ``` is where things go wrong. The weight gets calculated incorrectly due to the small size of the geometry, which causes the calculations to be off. Inside of interp_weights_poly_v2 is the line: ``` ht = mean_value_half_tan_v2(&d_curr, &d_next); ``` in this function if the area it calculates is too small it returns 0.0f which causes the weights to be wrong. Since the geometry is so small some polys return an area of 0.0f when scaled up they are correct. If I scale the data put into interp_weights_poly_v2 and interp_weights_tri_v3 and then unscale it afterwards the bug goes away as the weight calculation is using much larger values and doesn't fail due to float precision. Depending on the factor I scale the data is how much smaller the geometry can be. If I remove the Ngon method as an option entirely it works with no errors at the current scale, but if scaled down again by .1 it still fails.

Cody Winchester self-assigned this 2020-02-07 22:51:12 +01:00

Luca Rood commented

2020-02-08 11:58:56 +01:00

@lichtwerk, kinda known issue. The whole SDef code is kinda unstable, and in ideal circumstances I'd like to completely rewrite it. I've had some thoughts on better algorithms, though no time for that at the moment.
But @CodyWinchester seems to have tracked down where this issue lies, and it seems like it could be a simple fix. I expressed my thoughts at D6782.

@lichtwerk, kinda known issue. The whole SDef code is kinda unstable, and in ideal circumstances I'd like to completely rewrite it. I've had some thoughts on better algorithms, though no time for that at the moment. But @CodyWinchester seems to have tracked down where this issue lies, and it seems like it could be a simple fix. I expressed my thoughts at [D6782](https://archive.blender.org/developer/D6782).

blender-admin commented

2020-02-11 08:19:34 +01:00

This issue was referenced by c5d0a23204

This issue was referenced by c5d0a2320498d1feb7cc2c68fc6504bb187d5a8d

Campbell Barton commented

2020-02-11 08:21:42 +01:00

Changed status from 'Confirmed' to: 'Resolved'

Campbell Barton closed this issue

2020-02-11 08:21:42 +01:00

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Surface Deform modifier - distortion on bind (depending on scale) #73348