# surface modeling and solid modeling

• posted

hello,

how do solid modeler and surface modeler algorithmically differ?? i.e what i perceived so far after reading about them is

1)the surface modeler can draw surfaces so that you can tell whether a point is on it or not and 2)you can draw solids so that you can tell whether a point is inside,outside or on the solid or not??

now algorithmically the solid modeler just adds the point normal information and compares it with the surface normals..so does it mean that the developer of surface modeler are lazy to put just a point normal information and make their modeler as solid modeler???

regds, yogesh

• posted

The only real difference between surface and solid modeling is that solid modeling functions maintain a volume definition throughout the process whereas in a surface modeler discrete surfaces are created to define a volume (if a solid is the desired end, it's often not) and then joined at the end of the process. Until paired edges are joined to the extent necessary to create a closed quilt there is no way for the software to differentiate between "inside" and "outside".

More or less, anyway.... There's more to it; void lumps, etc. 8~)

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• posted

Surfaces have surface normals .... do they point out or in? They can easily be reversed .... but do the BREP solids modelers even use them?

If someone well versed in kernels wanted to post an essay on them it would probably help on the subject.

AFAIK Most solids modelers have kernels but surface-only modelers do not need them.

• posted

"Cliff Huprich" wrote

Uh, huh. If I understand what you are getting at.... For a shape like a sphere (single closed surface) I think the "solid" distinction would amount to some database entity types and functions. The coincident edges [would have to be / might have to be (I'm just guessing here)] flagged as being "joined" (a database entity) and functions added to do such things as determine if a point in space falls within the volume defined by the surface (as Yogesh mentioned) or not. For something more complicated; a quilt, polysurface, set of surfaces that enclose a volume the joined edges (co-edges in some kernels' jargon) are necessary to make the volume definition "official" before the software can determine if a point is inside or outside. Another function of the solid modeler is integrity checking; e.g. the edges and corner vertices must conform to a set of rules, most notably coincidence tolerances, in order to be joined.

Yes, but to what extent ....? Void lumps probably rely on normals for their definition. I guess they could be used to determine how boolean operations are calc'd (add, subtract, union, etc.)

(I have, on a few occasions, imported objects into adesk inventor that displayed many of the characteristics of a solid but (1) I was looking through the near side faces at the inside of the far side faces (solid backfaces aren't rendered) and (2) it couldn't calculate the infinite volume outside of the part. Deleting and replacing a face caused a recalc of the surface normals and all was well.)

Yes. Sadly most of what's floating around is "trade rag" garbage and uneducated speculation (like this post ).

They all have geometry engines (I guess that's a good name) of some sort to generate surfaces, calculate intersections, etc. and I guess you could say that the geometry engine is the foundation of a kernel.

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• posted

Hi,

Generally speaking, I think solid model is better than surface model. For example, solid has weight, you can know a part's weight before any prototype or product be made. You can do FEA strentgh analysis and know if it is strong enough for solid. These can not be achieved with surface model. However, in the modeling process, surface sometimes is necessary.

Regards,

Raymond

• posted

We used to do this all the time with closed surface models.

That was often done as well. Solids is fairly new, using kernels. FEM and mass properties are not. All available back in the days of CADDS-III (and probably other systems as well), more than two decades ago.

• posted

I know closed surface model can be regarded as solid in a meaning, and SolidWorks just do this when it open a closed surface model created by Rhino etc.. My opinion is just appealing industrial designer to give us, the structure designer and mold designer, solid model, or closed surface model with high modelling quality. I hate to open a surface model with bad modelling quality, it is sometimes too difficult to convert it into solid and do further design to it.

If you want to modify the model, you will find that solid is much better than surface. For example, if you want to make a sinked holes somewhere, it is much quicker using solid than surface.

For molds designing, solid is much better than surface. You can use boolean fuction with solid model to form cavities.

Raymond

• posted

This was all done long before NURBS solids modelers. My point was that for the things mentioned to be done solids are not a requirement.

While it may make your life simple it's always a computational resource hog it seems . Probably you also need to know less.

That's a problem with bad "designers", not the systems they used, usually.

Does that not depend?

Suppose you have an airfoil or something with 10,000 control points that define it?

All solids modelers actually do, at the core of it, is to manipulate certain surfaces .

(I'm still waiting for jb to wonder what's *inside* a solid model .)

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