They are different in target market, not math. All CAD systems are based on the same concepts.
It doesnāt work with revolve because what you are describing is a degenerate surface. And the developers decided to not allow that.
You can make degenerate single surfaces that are disjoint or hollow in Rhino:
degenerate_objects.3dm (1.3 MB)
i believe we are talking past each other here. and how equal the concept of Rhino and Solidworks are is not up to me to decide. there is an official definition what a solid modeller is. Rhino is not. but lets leave it there before it run even more loops.
what is a degenerate surface? when a seam rotates around its own axis for instance why would it be degenerate when it simply folds the surface round it? it renders well like that, it creates that one solid one would want without any downsides as far as i can tell, unless you want to keep modeling further with it you might have to understand where the seam is if that causes issues at all i mean.. other than that i dont see a reason to disallow the creation of such surfaces. by the way your example is not very neat, it creates a hole which one would want to avoid, how did you create this?
Degenerate for this case means collapsing a surface or part of a surface into a line.
In general the problem with degenerate geometry is that the math tends to fall apart. Things like computing intersections or surface normals become undefined or produce incorrect results
Apparently the reason to disallow it is that McNeel prefers to do that rather than the alternative which is to warn the user about the problem.
This is already a solid, but Rhinoās definition of a closed solid does differ from some solid CAD/CAM application. Here is a relevant techinical article with detailed explanations - hope it proves helpful.
The point I was trying to make earlier is that this now a more common thing/request, and I usually favor practicality over theoretical rigor, so I would support this type of construction being facilitated. Maybe a new object type needs to be createdā¦?
Anyway, hollow objects with no openings to the outside are fairly common, whether they be produced by welding/fusing multiple parts together to enclose a space - think tennis/pingpong balls - or by a production process that allows this in one piece - like rotomolding or now 3D printing. These objects exist, therefore they should be āmodel-ableā in CAD.
As demonstrated in this thread, they can.
I guess we need 3dm samples of situations where downstream commands fail on such geometry to see if those situations can be fixed. This would be a command-by-command process, with possible case-by-case models as well.
-wim
Hi Wim,
I know they can, but the procedure with NonManifoldMerge and CreateRegions is a bit arcane - hence the number of posts here asking for this. Just wondering if it couldnāt be simplified/made more intuitive.
Hi Mitch -
I suppose thatās as simple as creating a macro. Do you have a name suggestion for a possible new command?
-wim
well testing on the example i posted further above, i found no real issue working with the geometry further, i split it, trimmed it, made boolean operations and deleted the resulting face then used cap to close that, intersected the object with a clipping plane and had always good geometry, so there is no potential downside i can see so far. maybe there are some issues but when creating a wall like that there are no actual inherent issues like when creating singularities on a surface for instance because at this stage with wall thickness one would not change crucial parts anymore but anyway
thinking about it.. maybe that is equivalent to having knots or mulitknots on a curve or singularities on a surface. they are generally not good for everything but they can exist. depreciating them entirely is not necessary because they can still fulfill essential parts in modelling.
CreateHollowSolid ? Hmm, just thingking out loud here, could this be integrated into the existing CreateSolid command?
i dont think that for modelling purposes it is a big advantage though it might make it a bit simpler in some instances, it is also not an issue to print right out of the box with two nested solids, i think any slicer recognises this as is.
what it might boil down to the most is the visually corresponding attributes. like clipping planes etc that dont know what do do with these hollow objects. but also maybe even more importantly rendering these objects is always a nuisance.
images like this below always need workarounds or in this case i used sweep as posted further above which creates this kind of object without nonmanifold edges or any other hoop jumping processes. why not allow the same for revolve and railrevolve? for OffsetSrf the issue is a different i guess because there one would have to find the internal seam manually unless there is a way to script or programm a short path for these.
yes, i am actually just experimenting on a process for square objects either, but i dont have much hope. for now only round objects work.
Hi Mitch -
ā RH-89969 CreateHollowSolid
I tried that before asking for a name for a new command. I created a box, exploded that, and scaled pairs of opposing surfaces so that they all intersect. Then copied all of those and scaled them down to fit inside the first batch, but still intersecting in places. Running CreateSolid on that doesnāt create two closed polysurfaces, and I guess that command would need to get a lot smarter before a non-manifold process could be added to it. It sounds prone to errorā¦
-wim
@user2849 one should maybe ask what you are trying to do actually. if you dont need the actual closed capsule and will cut that off anyway why not trim the part of first then use cap and then use shell?
I fail to see what the OPās problem is. Hollow single part objects are either blow moulded or rotationally moulded. The wall-thickness forms as part of the process. If one knew what kind of product the OP wants to manufacture and what manufacturing process shall be usedā¦ the whole thing rather sounds like a typical XY-problem.
If only rendering is concerned, just model the tangible exterior surfaces and offset to suit.
