# NURBS surfaces should be called NURBS membranes

(David Rutten) #25

There’s two common ways to approach this:

1. A 2 dimensional object embedded in a 2+ dimensional space.
2. An N-1 dimensional object embedded in an N dimensional space.

Let’s keep it simple for the time being and limit ourselves to 2-d surfaces and 3-d embedding spaces, as that is what almost all CAD programs deal with. In such a setting a surface can be defined as the set of loci along with adjacency relations (in topology I think these are called ‘neighbourhoods’ or ‘localities’, not 100% sure those are exactly the same thing.) When you have adjacencies, you can ‘walk’ along a surface by stepping from one locus onto any nearby locus, and as such travel along a path from one point to another. Not every mathematical surface is continuous, it can be made up of disjoint regions that share no neighbourhoods between them.

Add a distance metric to this entity and you graduate from topological surfaces to geometric surfaces.

Yes it’s an example. As are Nurbs surfaces and meshes. They are all types of surface, but the core notion of a surface is far broader than you could achieve with either nurbs or t-splines.

See me response to point #2.

Implicit surface equations and iso-surfaces are also types of surfaces that you often cannot represent with a single nurbs surface.

I was specifically talking about mapping of open sets to other open sets within a mathematical space. Trims are a B-Rep implementation details well beyond the generality of what surfaces are, and the fact that there’s a command called `_Patch` is entirely incidental and unrelated.

(Tom) #26

Translating surface into german “Oberfläche”, you basically can describe everything as a surface which describes a border to something touchable. Its basically a boundary to a physical object.

That actually means you can name everything expressing this, like it. However my guess is, that the reason why predominantly NURBS-surfaces are called “surfaces” is because you explicitly model a surface directly, and not by creating it out of regular forms (solids) or by modifying “boxes” - traditional mesh-box-modeling.
Furthermore a shape described in NURBS-patches(in automotive this is equivalent to a single untrimmed/unfaced and trimmed/faced NURBS-surface, so David proposal is quite right I guess), can exactly represent any point on that shape, whereas a mesh can only do this exactly at the vertex location… (unless the mesh has a link to a formula, which then would be a true surface anyway).
I see meshes as way to visualise a surface, whereas a NURBS-surface is a surface, owing a rendermesh). Its like comparing apples and peaches.

(David Rutten) #27

Would Oberfläche work for a soap-film stretched within a frame?

(Tom) #28

yes , I think so

(Tom) #29

but who knows, maybe in some decades we can model a rhinoceros in soap-film-surfaces only

(David Rutten) #30

Not trying to argue with you here, German isn’t a native language of mine, but in what sense is it still ‘ober’ if it’s just hanging in space? What exactly is it that’s underneath?

(Tom) #31

even a soap film has a thickness, but yes, sometimes words are not excat… “ober”/over is more like the topmost particles I guess…

#32

Well if you think that’s superficial, then underneath may be subficial?

(David Rutten) #33

(David Cockey) #34

Computation Geometry for Design and Manufacture by Faux and Pratt is/was a standard reference which was originally published in 1979. It uses “curve” and “surface” as the general terms. It also uses “patch” a topologically rectangular two parameter surface, which may be part of a larger surface.

(Wim Dekeyser) #35

I didn’t think I’d get into this thread… oh well.
In 't nederlands is 't toch ook ‘oppervlakte’ - en een zeepbel heeft ook een oppervlakte, met oppervlaktespanning…

(David Rutten) #36

Een zeepbel wel, die heeft een binnen en buitenkant, maar ik zou “oppervlak” niet gebruiken voor een niet-gesloten, eindig vlak. Wel met betrekking tot vierkante meters, maar niet met betrekking tot de vorm. Maar ja, een beter woord heb ik ook niet. “Vlak” is wiskundig gezien correct, maar klinkt alsof het niet te veel gebogen mag zijn.

Vlies? Ook niet echt ideaal.

(Wim Dekeyser) #37

Interessant. Dus als je een zeepbel in 2 zou kunnen delen, zou die opeens geen oppervlakte hebben?

Maar je hebt in elk geval gelijk - zover ik kan zien, is er geen één woord dat helemaal dekkend is voor het begrip. Als je het mij vraagt, is het daarom dat men bij conventie het eens wordt (zonder daarom het aan iedereen te vragen) en gewoon een bepaald woord gebruikt.
heb nog een prettige avond!

(Nathan 'jesterKing' Letwory) #38

Ha, so funny reading discussion about people trying to find words for abstracts concepts to express actual things. That is bound to get the brain in a knot.

Een zeepbel gedeeld in twee is gelijk aan twee zeepbellen. Dat betekent dat je dan twee binnenkanten en twee buitenkanten hebt.

#39

We really need a Rhino Membrane and Base 12 edition.

(Nathan 'jesterKing' Letwory) #40

I’d be much better to have a variable radix, it’d be too easy otherwise.

#41

base 12 Rhino already exists.
(as in— make a line 1 inch long… array it end to end 12 times… measure the resulting length… it will be 1)

only a couple of characters need the tweak

(David Rutten) #42

Hij zou wel een oppervlakte hebben, alleen geen oppervlak meer zijn.

#44

i don’t know… both skin and membrane, by definition, have a thickness… which to me, makes both of those words no good for 3D modeling surfaces… (unless you offset the surfaces into a solid in which case sure, skin/membrane could work)

#45

Spatial structures made from tensioned surfaces are called membranes. Technically a NURBS surface also is a tensile structure formed by the “pulling forces” of its control points. (Although the weights may also be negative… but that’s a different story.) Following that logic a NURBS surface also is kind of a membrane.

But since the control points can be placed anywhere the resulting surface can have any shape. In most cases NURBS surfaces do not look like natural membranes in between its boundaries (also known as minimal surfaces). Therefore I think calling NURBS membranes would simply be confusing. I mean there are still designers around who believe that one needs to know mathematical equations to model with NURBS…