modify primitive normals?

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Hi!

I have a bunch of non-connected primitives. Some have their normals in the wrong direction and I'm trying to change that.

I tried tranfering the prim normals from another mesh to this one with a wrangle (xyzdist), but the normals just dont change. I also tried setting the point normals and transfer these point normals to my primitives (attribute transfer sop) but it still doesn't change.

For fun, I used a primitive wrangle and did:

@N = set(0,1,1);

…but it still doesn't modify the prim normals. Why is that?

-Olivier
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Hi, Olivier,

I just tried the wrangle you quoted and it works for me (looking at the geo spreadsheet). However, there seems to be a bug in the normal display in 3d-view, those don't get updated. They don't get updated when you use a normal-node and invert primitive normals. So you may have to use the spreadsheet for the moment until the bug is fixed (I filed it).


Marc
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Out of here. Being called a dick after having supported Houdini users for years is over my paygrade.
I will work for money, but NOT for "you have to provide people with free products" Indie-artists.
Good bye.
https://www.marc-albrecht.de [www.marc-albrecht.de]
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Ok, so I was looking in the viewport to see if my prim normals changed and didn't notice any change. But if I look in the geometry spreadsheet, it does change!

It's all good!

-Olivier
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you can't change geometric primitive normals, they are defined by winding of the geometry (making primitive v@N won't affect them, but it can be used for shading)
so if you want to flip or reverse normals of a certain polygon group, use Reverse SOP (also possible with Primitive SOP if you set Poly/Hull/Vertex to Reverse)

lso Poly Doctor or Facet SOP have options to fix winding of the olygons (orientation) in case you have connected polygons with mixed winding
Edited by tamte - March 22, 2020 11:05:28
Tomas Slancik
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> making primitive v@N won't affect them, but it can be used for shading

… well, according to the geo spreadsheet, you can change them.

Marc
---
Out of here. Being called a dick after having supported Houdini users for years is over my paygrade.
I will work for money, but NOT for "you have to provide people with free products" Indie-artists.
Good bye.
https://www.marc-albrecht.de [www.marc-albrecht.de]
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malbrecht
> making primitive v@N won't affect them, but it can be used for shading

… well, according to the geo spreadsheet, you can change them.

Marc

yes, you can and as mentioned that attribute and values will be used in shading as shading normals similar to vertex and point v@N
but having primitive v@N attribute will not change implicit geometric primitive normal that is defined by topology, in any software for that matter, Houdini uses left handed winding rule
Edited by tamte - March 22, 2020 11:12:46
Tomas Slancik
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The word “implicit” is the most important in your statement:
The 3d viewport “display primitive normals” does not mention “implicit” (nor does it say “defined by winding order of points in the primitive”), so one must assume that the same “attribute quality” as with point normals or vertex normals is meant, not an implicit (as in “NOT attribute”) quality.

It may not be a “bug”, but it clearly looks like an irritating if not misleading description in the UI.

Marc
---
Out of here. Being called a dick after having supported Houdini users for years is over my paygrade.
I will work for money, but NOT for "you have to provide people with free products" Indie-artists.
Good bye.
https://www.marc-albrecht.de [www.marc-albrecht.de]
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