# Can Rhino model any object which polygonal packages like Maya are able to create?

#161

Not everyone needs production, someone can just use a printer.

#162

Well, if all you want is some model for 3D printing, where nothing much matters apart from fitting into the build chamber, you should have mentioned that at the outset.

(Gustavo Fontana) #163

I just wanted to say that i was up until 3am last night because I was trying to add a simple mechanical features with good continuity to a part that my client had decided to take the SubD conversion and take it straight to tooling.

So if you are going to keep working downstream detailing remember this: keep going forward with just a SubD conversion it will bite you in the ass eventually. This is why we usually rebuild when we are done having fun.

I need a nap now.

G

(Edited all the autocorrect nonsense. This phone has a mind of its own!)

#164

If you look at Wikipedia, you can see the following. NURBS is a mathematical form, and SubD (mesh) is only a collection of polygons. These are completely different systems!

#165

Err, sort of. Depends on the flavor.

#166

Did you skin that cat from the other thread and tasted it? BBQ or stew?

#167

#168

Iād suggest you google āJoe Stam Limit Surfaceā for some interesting researchā¦
_
c.

(David Cockey) #169

Subdivision and NURBS surfaces are related in several ways.

NURBS is based on an algebraic (formulas) description of a surface. The control points of a NURBS surface form a mesh. This mesh is very similar to the initial polygons of a subdivision surface. One advantage of NURBS surfaces is that it is easy and efficient to evaluate the NURBS formulas to obtain the xyz coordinates of a point anywhere on the surface.

When a NURBS surface is to be used for output, whether graphics on a screen or a physical part, a mesh of some sort based on the NURBS formulas is almost always created as in intermediate step. The ease and efficiency of evaluating NURBS simplifies the ability of a mesh creation algorithm to directly control the local mesh density by placing vertices as desired.

Subdivision surfaces are based on repeated division of a set of polygons. Subdivision meshes have associated limit surfaces which would be the results of repeating the division process until the limit surface is defined with the desired accuracy, and the limit surfaces are usually equivalent to generalized NURBS surfaces. However there is not as simple and efficient a process for determining the xyz coordinates of a specific point on the limit surface from the initial polygons as there is for NURBS surfaces.

Intermediate meshes between the subdivision initial polygon mesh and the subdivision limit surface, including for use in output processes, can be created by repeating the division process until the mesh is sufficiently fine. A disadvantage of a subdivision mesh is the mesh may have excess vertices in areas where they are not needed. It should be feasible to create a fine mesh and then decimate the mesh until the local mesh density is as desired.

Back to the claim that subdivision and NURBS are fundamentally different. In some respects they are, but in many respects they are similar. Both have initial points which form meshes. Both have corresponding surfaces. And meshes for use in output processes can be created from both.

#170

Here I do not understand something ā¦ Can I be more precise?

Later I will get acquainted.

(Gustavo Fontana) #171

hereās a basic explanation thatās very useful:

(Tom) #172

As said, I question āfastā. To give you generic answer to a generic question: I donāt know if you already work in a job where CAD is applied. But we should always be aware that we spend a lot of time of our lifes on the computer. When they introduced CAD, they said development gets simplified with computer drawings. No matter if you are an architect, designer, engineer. I know hardly anybody who believes the computer did speed up development. Most CAD veterans will tell you the opposite. Since CAD modelling became a full time job, development became much more complicated. Nowadays they ideally want you to be able to code, know all regulations and laws and being a businessman and marketing expert. Getting Business calls even on Christmas. People around you having burnouts, life crises and health issues. If everything is "simple, why paying you like being expert, when Autodesk and Co, telling people its so easy, even 10 years old can do it.

I truly believe we should slow down our lifes. Iām not a lazy person, I just like to protect me from this fatal development of this global insanity.

I was probably saying this three times in this thread, not to mention other thread-repliers. we shouldnāt cycle on this one again.
However I wouldnāt fully agree on the statement that they have more in common as differences. From a mathematical standpoint they may have some similarities. However:

Sub-D is based on approximating a shape by polygons, whereas NURBS explicitly determines the mathematical description. So if you subdivide a mesh you always change the curvature per iteration, whereas if you tessellate a Nurbs-Patch you keep the curvature in exact the same position, but you improve the discretisation for rendering or production (based on the curvature). This is a fundamental difference.
Furthermore, ābox modelling (sub-d)ā limits you in curvature control, because it determines how parts are connected in between and where (in a relative sense). It is very hard to control curvature, if you have to. āWavy Highlightsā are much more likely and hard to smoothen out, if you are bound by technical or design critical demands.

That is the reason why even developer themselves tell you its a technique for āconceptingā, because you cannot model accurate. However there are lot of industries, where accuracy isnāt so important. In the end it is you who decides which path to follow. You can even become a master in both worlds. And if you decide to become a speed modeller, just do it. You can be a person who does quantity or a person who does quality. Both worlds have legitimation and can led to success.

#173

This notorious topology change and shrinkage per iteration does not occur with the Limit Surface based SubD approach (as already used inside several CAD systems and planned by McNeel). Here one always edits either the control cage or the smooth Limit Surface.

Looking at these smooth limit surfaces in isolation they should be precise enough, even for you :o).

But several roadblocks for precision work do remain in place. One can not trim SubD-models without spoiling their topology. One for the same reason may not combine several SubD objects (as Booleans would also involve trimming). If one wants clean looking shape intersections SubD Modelers often completely redo the control cage in order to conform the new shape ā a very time-consuming manual process.

In order to deal with this huge limitation one has come up with various strategies:
Inside mesh modelling applications thereās products like Mesh Fusion in Modo (thereās similar tools for Blender, Maya etc.). This finally gives access to Trimming / Booleans on sets of SubD input meshes, without time-consuming workarounds. Output however is a quad dominant mesh similar to a Nurbs render mesh. Other strategies are to use SubD for laying out base geometry and to switch over to very high resolution meshes in later stages, which again no longer obey the Catmull Clark scheme. That happens in Zbrush for instance.

Inside very expensive, parametric feature base CAD-systems one runs two separate geometry Layers, the SubD Cage plus a Live-converted Nurbs version where one may trim, apply fillets and the like. This approach is kind of cool, but one can say that it is out of reach for McNeel for foreseeable future. Also patch layout and overall surface quality of the auto converted Nurbs certainly do never match serious production needs,
as pointed out by you. One uses this in all ways crazy expensive dual workflow in order to yield conceptual geometry.

The other large limitation I see is transition control. With meshes thatās simple ā one adds control loops (more faces) and slides them around until things look good. But as also pointed out by you ā thatās display only and only looks good in conjunction with openGL smoothing tricks.

With Limit Surface based approaches one rather would not want to add needless complexity to faces. The only thing one has at disposal here is point weighting (creasing) but this alone would be way too limited for serious work. New Tools like Xirus propose alternative interfaces to control surface flow with their Tangency Handles but this all still looks extremely rough to me.

Even if one had all of this solved and had a precise limit surface based SubD model: One still had to convert to Nurbs ā and thatās a great problem as well. Thus far I have not seen a tool which outputs clean, human editable patch layouts: Output is overly complex, hard to edit. While compatible for milling now its no good input either ā in the sense that every other crappy Nurbs model may be a pain (think creating helper geometry, curve and surface offsets etc ā this all sucks with weird surface layouts).

Likely it will be the Tool maker who redoes at least parts of the model to make things work.

Almost T-Joint/Star-Joint how would you model this?
#174

THIS

#175

The crux of the matter! (and the opportunity)

Right. Typically, your conversion is frozen. Need to edit; back to your cage, then convert again.

#176

Not called the ārat raceā for nothing, brother. In existence since the beginning of time, and only the characters and constructs change. Dictatorship, communism, socialism, liberal democracy, full bore Ayn Ran: it matters not in terms of pecking order. Thereās always a pyramid, and weāre just wild animals with mouse pads and iPhones. Make your own reality.

When you find utopia, do ring me, por favorā¦

Now, back to deciding whether to nurbs or subD that next part, how much it is going to cost, why it beats the competition, and how to convince the dude(s) who signs off to nod his block head up and downā¦and get paid.

#177

And

Or will I have enough video from @gustojunk

#178

If you want, you can, but is it efficient to model this?

There is no mathematical continuity between the polygons.

That is, the Limit Surface system is useless (almost)?

I decided to try the conversion to Rhino:

(David Cockey) #179

Did you use MeshToNURB to convert the subdivision model to Rhino? If so thatās not what is being discussed as converting the a Subdivision surface to NURBS. MeshToNURB is just a simple conversion of a mesh to degree 1 NURBS surface which will be faceted. A Subdivision surface is generally equivalent to degree 3 NURBS surfaces.

(Tom) #180

I believe you misinterpret my intention. Iām well aware of this pyramid, and Iām definitely one of those rats. But actually its nothing to be sad of. Iām still having a good life, but thatās a result of my own mind-set, not based on my bank account. I believe I could perform better, but Iām not ready to pay the full price for it . I hope that most guys doing CAD, are not in the worst situation.

However Iām rather talking about the āspeedā or āartificial hecticā of our society we live in, which is mentality. We donāt develop anything faster, nor does technology improves faster, nor we get better living standard, just because we work faster and faster. We are now at a point where you can buy a new car or phone every year. I mean everyone has to decide if he/she needs that annually. To me its just a waste of money and time, which then lacks at the other end. So people rather buy less quality but more often.
In my experience, best projects had enough kappa and money. If the person you are working for doesnāt want to spend money, he simply gets less quality and a high risk of failure. Now why is that:
Modelling is just a tiny step. What you do most is thinking about solving a problem, not dumb drawing.
Whatās the point if you can model faster, but in the end you need more time in correcting mistakes. Mistakes done because you havenāt even thinking much about the actual problem you are trying to solve. Quality comes from reflecting and iterating over and over again.

Unfortunate people nowadays want raw consumption. Its some modern form of greed.
There is so much fake innovation to keep this wheel spinning. This has nothing to do with our political views, its not left, right, gay or whatever: again it is a matter of mentality and a fast paced lifestyle of the majority of people (who are influenced by commercial warfare as well -> so at some point its political, yes).But you can change that for yourself. You donāt have to be that guy competing by saying Iām the new Lucky Luke of CAD. Be the guy doing the best and feed the people on top of that pyramid.