# Carve Sop Question

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Is it possible to control the start point when carving closed curves?

I tried hard but nothing seems to work, the "0" point is freezed and even "sort" SOP doesn't change it.

I tried bith nurbs and polys.

There are closed sections of some solid and I'd like them to grow from random points on perimeter.

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Use two Carve SOPs. The first one determines the start point. The second gives you the cuts offset from the first positoin.

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Try using a reverse sop set to shift mode and shift the U value

The carve sop will start at the first vertex not the first point

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Yahoo!

This was too easy, I had to be blind:)

I really love houdini for all these combinations.

Thank you guys! They both work great and in different ways.

Sometimes I get stuck on very specific usage of nodes and don't suspect them of being more than that.

Carve sop IS one of my favourites!

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One very important thing to know about sops is that some are point centric and some are vertex centric. As a rule I tend to try and keep points and vertices in line. So if I do a shift with the reverse sop I follow it up with a sort by vertex order. It's not strictly necessary but feels neater...

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One very important thing to know about sops is that some are point centric and some are vertex centric. As a rule I tend to try and keep points and vertices in line. So if I do a shift with the reverse sop I follow it up with a sort by vertex order. It's not strictly necessary but feels neater...

27974[/snapback]

Thanks for a note, it spawned another question:

To reverse normals I can apply a point sop with "-" added or apply a reverse sop and facet with "post comp. normals" checked.

After applying a reverse, all polys become reversed (black) but normals stay untouched (because reverse works on vrtices yes?) but then after applying facet, normals flip but polys probably also since they are no longer black.

I assume after reverse I get reversed vertices (vertex normals) and after a facet, point normals and vertex normals are flipped, so what I get after a reverse/facet operation Is only point normals flipped. Does that make any sense?

I undersatnd it's the best to keep both types of normals consistent and pointing in the same side.

Formerly I had some troubles with rendering glass and fiddling with these options and that was where I became to sense a mysterious invisible force behind point normals .

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I assume after reverse I get reversed vertices (vertex normals) and after a facet, point normals and vertex normals are flipped, so what I get after a reverse/facet operation Is only point normals flipped. Does that make any sense?

I undersatnd it's the best to keep both types of normals consistent and pointing in the same side.

Formerly I had some troubles with rendering glass and fiddling with these options and that was where I became to sense a mysterious invisible force behind point normals .

27977[/snapback]

Ok, so the reverse sop reverses the prim normal, you can check this by turning on primitive normals in the viewport. If you don't facet anything or add point normals then at render time point normals will be calculated using the primitive normal. It is very rare that I actually add point normals, I tend to only do this for modelling. If you need to facet objects to get crisp edges you can use the cusp option of the facet sop without actually adding point normals.

If however point normals are applied they will be used for the shading even though they may not match the primitive (vertex) normals.

Again for neatness I try to make sure primitive (vertex) normals are always pointing outwards and in general remove all point normals.

Having primitive normals pointing in the correct direction will save you all sorts of headaches when shading.

For a glass with thickness the outside primitive normals should face away form the glass and the inside face should point into the middle of the glass.

I often even quickly convert NURBs surfaces to polys just to check the direction of the primitives, which are often hard to tell with a Nurbs primitive. Try looking at the prim normals for a Nurbs tube and a reversed Nurbs tube, then convert it to polys and do the same thing.

Of course there are always ways in the shader to flip normals the right way round but again I just think its good practise to make them right in the first place.

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Thanks for explanation Simon, till this moment I was concerned mostly about point normals, and used their reference for shading.

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One very important thing to know about sops is that some are point centric and some are vertex centric. As a rule I tend to try and keep points and vertices in line. So if I do a shift with the reverse sop I follow it up with a sort by vertex order. It's not strictly necessary but feels neater...

27974[/snapback]

I think of SOPs as one of the following three:

- point centric (the foundation for all geometry)

- vertex - primitive centric (one needs the other)

- parametric centric (NURBs, Bezier, faces, works in s and t space)

Some examples:

- Carve SOP is parametric. Works in uv domain space 0-1 of the face(s) or surface(s).

- Primitive SOP is one of those vertex-primitive SOPs. You have access to vertex local variables along with prim local variables. See the help card > local variables tab.

- Attribute SOP is whatever type of attribute you are creating. If you are working with points then you can't access the prim-vertex default local attributes. If you are creating a prim or vertex attribute, you do.

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