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Flipping Normals

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gallerykim
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While progressing with the data received in Unreal Engine, I encounter situations where certain normal directions are flipped, as shown in the image. If I want to procedurally flip only the inverted faces, what should I do?

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스크린샷 2023-07-04 203801.png (422.7 KB)

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ajz3d
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PolyDoctor's "Correct Winding of Polygons to Majority in their Manifold Patch" is one way to do it.
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gallerykim
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Polydoctor seems to operate only on the flipped faces within connected polygons. It doesn't seem to affect polygons that are detached or separate.
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gallerykim
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To elaborate further, after going through several processes, when I merge convert lines and perform a polygon fill, occasionally I encounter flipped normals. What should I do to prevent flipped normals from occurring?
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ajz3d
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Do prims that you're generating always lie on a flat plane, like on the screenshot?
Edited by ajz3d - 2023年7月5日 08:56:20
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gallerykim
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ajz3d
Do prims that you're generating always lie on a flat plane, like on the screenshot?
Due to obtaining surface information from the low hills of the landscape, slopes can occur in some cases.
Edited by gallerykim - 2023年7月5日 21:28:21
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ajz3d
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If this is the case, then perhaps it will be enough to reverse all primitives which have normals within a specific spread angle from the (0, -1, 0) vector? It should work for this kind of data.
Edited by ajz3d - 2023年7月6日 06:30:17

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normal_spread_angle.hipnc (166.4 KB)

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gallerykim
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ajz3d
If this is the case, then perhaps it will be enough to reverse all primitives which have normals within a specific spread angle from the (0, -1, 0) vector? It should work for this kind of data.

Do I need to modify @N to change the normal vector?
When instructed to multiply by -1 when @N.y < 0, in the spread sheet, the N value became positive, but the actual normal did not flip.
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ajz3d
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gallerykim
Do I need to modify @N to change the normal vector?
Depends.
  • If you have no existing vertex normals and it is only primitive winding that is wrong, then obviously no.
  • If you have existing vertex normals, then regardless of primitive winding you will also need to reverse vertex normals.

When instructed to multiply by -1 when @N.y < 0, in the spread sheet, the N value became positive, but the actual normal did not flip.
It would be best if you could post a sample scene.
Maybe it's a viewport bug? Or perhaps you're multiplying only one vector component instead of negating the full vector?

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fix_reversed_vx_N.hipnc.zip (31.3 KB)

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gallerykim
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Thank you for providing the sample. I've tried incorporating a portion of my scene into the sample file, but I'm still encountering issues with the normals that need to be resolved before performing the final extrude operation.

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fix_reversed_vx_N_stash.zip (144.9 KB)

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ajz3d
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I added two Reverse SOPs (one per method) to the network that are reversing the candidate group, and now PolyExtrude returns correct result.

There were ill-formed primitives in the input geometry, so I cleaned them up by recreating primitives from their edges. This might not be a perfect solution, but it works on geometry data you provided.

Attachments:
fix_reversed_vx_N_stash.hipnc.zip (46.2 KB)

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gallerykim
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The issue has been resolved cleanly.
Thank you very much!!
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ajz3d
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You're welcome.
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