Full Version: Make face with more than 4 edges

It simple when I want to make face on 4 or three edges. How to do it when you have more that 4 edges

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If edges are in the same plan and closed : Planar
else
Just Join some edges for have "4 curves" then use Network! ;)

Better if you post your 3dm file!

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Hi bartosh well a NURBS surface is made up of a "row and column" grid of control points so every surface inherently has 4 edges (some of which may be collapsed down to a point). But in NURBS modeling there is the additional concept of and "underlying surface" and "trim edges", which are boundary curves that live on a surface and can cut holes in it. The combination of an underlying surface along with trim curves makes what is called a "face". So you produce something with more than 4 edges by some process that makes a trimmed face. That can be a command like a boolean or also some commands build surfaces with trim curves from the start like Construct > Planar.

There's some more information on how NURBS modeling works with "underlying surfaces" in this FAQ entry here:
http://moi3d.com/wiki/FAQ#Q:_Why_does_show_points_work_for_some_objects_but_not_others.3F

- Michael

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Hi bartosh, here's one example. Here are 7 coplanar curves:



You can take these curves and construct one surface from them using Construct > Planar, which gives this result:



That's a trimmed surface - if you select it and turn on surface control points you'll see the control points for the "underlying surface" are just 4 corner points making a simple plane:



There is also the concept of an "untrimmed surface" - that's a surface that doesn't have any additional trim curves on it other than the underlying surface's own boundaries. Often times the process of construction will involve building some untrimmed surfaces to start with like with sweep, loft, extrude, or whatever, and then doing boolean operations on them which then introduces new trim curves. This concept of building an initial simplified extended piece of "stock material" which then gets cut by some boolean operations is pretty central to NURBS modeling. So booleans are usually the main way to do things in NURBS modeling, which is a very different strategy from sub-d modeling where you've learned to avoid booleans entirely. There are some tips here for people coming from a poly modeling background which may be useful: http://moi3d.com/forum/index.php?webtag=MOI&msg=4865.2 . One of the key things is that you don't start by drawing in all the 3D edge curves that make up your model, instead you should mostly be working with 2D curves and generating solids from some 2D curves by extrusions for example, and then use other 2D curves or other solids to cut away portions. So some of the final edges in the 3D model are not ones that you draw directly, they're trim curves that are the result of intersection and cutting operations.

The NURBS strategy of "build a sheet of stock material and then cut it with booleans" tends to work well with real world manufacturing processes and so it's a very good strategy to use for mechanical man made type parts.

This concept of having an "underlying surface" and then trim edges that live on that surface is why booleans work much much better in NURBS modeling than they do in polygon modeling. In NURBS modeling when 2 solids are booleaned together, it only involves the creation of new trim curves, the "underlying surfaces" of each surface remain the same, they aren't fragmented into tons more little tiny pieces like will happen when doing poly modeling booleans.

Also you can recover the full "underlying surface" that is part of a trimmed face by selecting all the edges of a single trimmed surface that is not joined to anything else and using Delete.

- Michael

Image Attachments:
bartosh_trimmed_face1.jpg  bartosh_trimmed_face2.jpg  bartosh_trimmed_face3.jpg 

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Thanks Michael. On flat face I`ve done it as you descibed. That question should be like - how create more than 4 edges face basing on curved edges around it.

I`ve made my shield:


But I did some faces as many 4 edges networks. It is time consuming. I would like to lear how do it faster.


Please look on that face. It has been created with using boolean substract:



I had problem with making this solid, So I`ve checked edges and I know that face is bad (this is example only). So I delete it to make that face from scratch to fit boundary edges. I try to rebuild that face manually (only one face). I`ve done it as many 4 edges networks. I`m looking for faster metod for making faces like this. It should be flat face on curvy edges.

Image Attachments:
Przechwytywanie01.JPG  Przechwytywanie02.JPG  Przechwytywanie03.JPG  tarcza.JPG 

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Hi bartosh, for the face in your example there I'd rebuild it by first selecting this edge shown in red here:



Then run the Extrude command and snap the point to extrude to onto this endpoint here:



That will make a surface covering an area like this:



In order to incorporate the notches you'd then use the Edit > Trim command to trim the extruded surface by these edges here:



Now you'll have a trimmed surface that matches the hole and you can use Edit > Join to join it in and make the object a solid again.

There is some more information about techniques like this in the object repair tutorial here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=446.17

But often times problems in a particular face can be in the trimming boundaries and the underlying surface is actually ok. So instead of deleting that face like you show above it would be good to first extract that face and untrim it because that will give you the full surface and you won't have to remake that part. Then you can retrim it. That process of extract, untrim, and retrim is covered in that object repair tutorial.

At any rate, anytime you have a boundary with more than 4 edges, the technique for filling it is going to be to initially build an extended 4 sided surface and then use trims or booleans to trim away areas of it.

Hope that helps - if you have another example that you have a question about please also post the 3DM file so I can use your file to help illustrate what to do.

- Michael

Image Attachments:
rebuild_face1.jpg  rebuild_face2.jpg  rebuild_face3.jpg  rebuild_face4.jpg 

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Thanks, Michael.

I`ve already done it (by network with using 4 edges patches) but your last method is a lot faster :)

So process of rebuilding flat faces based on boundary edges is:

3 or 4 edges - select edges and click network button
more than 4 edges (flat) - select edges, click planar button, fire script: "ShrinkTrimmedSrf" (why there is no button for that?)
more than 4 edges (curved edges - flat face) - select longest curved edge and extrude it, trim rest and cut out

Thank you very much.

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Hi bartosh,

re:
> 3 or 4 edges - select edges and click network button
> more than 4 edges (flat) - select edges, click planar button, fire script: "ShrinkTrimmedSrf" (why there is no button for that?)
> more than 4 edges (curved edges - flat face) - select longest curved edge and extrude it, trim rest and cut out

It's usually best to focus more on the shape of the surface than the number of edges. If it's all on a single plane it's better to use Construct > Planar on that for all numbers of edges, including for 1,2,3, or 4 edges. That makes a simple plane for the underlying surface which can then get special case handling for intersection calculations.

Network is more of a last resort method, if nothing like Planar, Extrude, Revolve are a match. If you use Network on flat curves it will make a surface there but it won't be a simple 4 corner point plane and so it won't be able to get special case intersections with it.

re: ShrinkTrimmedSrf, there isn't any button set up for that because it's not generally very useful, if you have a flat area just use Construct > Planar and don't use ShrinkTrimmedSrf afterwards unless you have some special situation like you want to modify the surface control points. It's normal for an underlying surface to extend out a ways, it's not something you would normally need to try and avoid.

- Michael

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Hello,

This model can be constructed very quickly using Extrude and Boolean tools. Unless I'm missing something there should be no need for Network commands. Can you share the model or dimensions?

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