Can Tessellation typed Representation hold items from another group?

Developers
1

Hello,
I need Your help.

During development, my program should understand IFC files exported by another CAD softwares.

Now I have found the following situation (copied only the relevant part).

#13597= IFCEXTRUDEDAREASOLID(#13593,#13596,#20,1.54);
(…)
#14443= IFCPOLYGONALFACESET(#14417,.T.,(#14393,#14395,#14397,#14399,#14409,#14411),$);
(…)
#14595= IFCSHAPEREPRESENTATION(#111,‘Body’,‘Tessellation’,(#13597,#14443));

As you can see, there is a Body Representation for an element, which has Tessellation type.
It has 2 items, one is a Polygonal Faceset, it is OK since it is in the “IfcTessellatedItem” group.
But the other one is a AreaExtrusion, which is not a Tessellation-like entity, but it belongs to the IfcSolidModel group.

The documentation for IfcShapeRepresentation has a Formal Proposition:

CorrectItemsForType: Checks the proper use of Items according to the RepresentationType .

It is worth noting that this whole representation is a Mapped thing:
#14598= IFCREPRESENTATIONMAP(#14597,#14595);
#14605= IFCMAPPEDITEM(#14598,#13514);
#14607= IFCSHAPEREPRESENTATION(#111,‘Body’,‘MappedRepresentation’,(#14605));
#14609= IFCPRODUCTDEFINITIONSHAPE($,$,(#14607));

In summary, the question again:
Should a Representation hold only matching items?

Thanks in advance

1 Like
2

CorrectItemsForType refers to FUNCTION IfcShapeRepresentationTypes, which basically contains a mapping from the RepresentationType strings to a set of allowed geometrical entity types. The number of items is compared to the number of items that match the type criteria and these should be equal (RETURN (Count = SIZEOF(Items));). So the answer is yes.

I think the way to go is to add an additional entry to the IfcProductDefinitionShape. I assume the tesselation and the body describe the same geometry (?) so importing applications can then chose the description they support best.

3

Thank you Thomas for the clarification.

Yeah, it would be great, more (alternative) BODY definitions…
But in my case it is an IfcWindow, where the frame is defined by Tessellation-type bodies, while the glass is a simple Sweptsolid :frowning:

Anyway, I will send a note to the creators of the original software about this problem.

4

Maybe this needs consideration. If you check the documentation of https://standards.buildingsmart.org/IFC/DEV/IFC4_2/FINAL/HTML/link/ifccsgsolid.htm you can see it mentions you can nest tessellated shapes into that solid definition, but looking at the select, it doesn’t reflect this.

5

I would agree with the consensus above. I also, on import, wouldn’t trust or care what that value said, since there is nothing that prevents invalid use of that label. A shared open source toolkit might validate that, but currently it is just on trust. That said, I’m going to investigate the original issue.

1 Like
6

The docs make sense to me. The description is referring to the operands for boolean operations. Therefore I don’t think any change needs to be made here.