use IfcOpenShell to draw ifc in OpenGL

[Ttofuuu]

Currently I've builded IfcOpenShell ver 0.7.0, and tried ifcConvert.exe to trans an ifc file into obj file. The conversion works great but I have trouble in the next step: my goal is to read an ifc file, and draw it through OpenGL. So my problem comes up: how can I get the geometry data from ifc, and trans them to OpenGL vertex buffer object directly, instead of transing them into obj then read obj into vertex buffer object? I wonder if there is a obvious data stream we can cite to boost my target.
I'll appreciate it if somebody know that or show me codes about this. Too fresh to be a coder that I nearly have no ability to handle this🤣🤣

[aothms]

IfcConvert is just one possible use of the "geometry iteration" mechanism we have in IfcOpenShell. Using IfcGeom::Iterator which you can iterate over to give you a bunch of IfcGeom::TriangulationElement. The data model is something you can almost directly feed to OpenGL.

If you want I think it'd make a nice tutorial series for academy.ifcopenshell.org/ so I can help code review and provide feedback if we get a tutorial out of it.

[aothms]

which is a cuboid, described by IfcExtrudedAreaSolid and has no property of length/width/height

Yes this is always the choice you have to make. Slabs are generally extrusions, so you might assume it's an IfcExtrudedAreaSolid, but it doesn't have to be. So reverse engineering those parameters from triangulation (or BRepElement using OpenCASCADE would be preferable but another API to learn) is a general solution that always works, but can be a bit painful to implement.

[Ttofuuu]

or BRepElement using OpenCASCADE would be preferable but another API to learn

Do you mean this sir?

[aothms]

Yes, and then by calling as_compound() you get a single OpenCASCADE element for a product, that you can use for all possible analysis https://ifcopenshell.github.io/docs/rst_files/class_ifc_geom_1_1_representation_1_1_b_rep.html#_CPPv4NK7IfcGeom14Representation4BRep11as_compoundEb

[Ttofuuu]

that you can use for all possible analysis

I'll read about TopoDS_Compound and find what it is soon. But before that, let me show you some paragraph in an ifc file:

#14961= IFCCOLUMN('a1a813ce-1a41-46c0-b816-ca39dcb8c376',#5,'\X2\77E95F6267F1\X0\',$,$,#14960,#14957,$); 

#14963= IFCPROPERTYSINGLEVALUE('\X2\5BBD5EA60042\X0\',$,IFCLABEL('\X2\003800300030\X0\'),$);   
#14964= IFCPROPERTYSINGLEVALUE('\X2\9AD85EA60048\X0\',$,IFCLABEL('\X2\003400300030\X0\'),$);   
#14965=IFCPROPERTYSET('4fc8e74c-2a29-4192-8f48-8657861db31b',#5,'\X2\53C26570\X0\',$,(#14963,#14964));
#14966= IFCRELDEFINESBYPROPERTIES('ec09a468-3ba2-4495-8fca-da16a807ae1e',#5,$,$,(#14961),#14965);  

I'm using code like this:

auto rels = this_ifc_column_element->product()->get_inverse("IsDefinedBy");
for (auto& rel : *rels) {
  IfcUtil::IfcBaseClass* inst = rel->get("RelatingPropertyDefinition");
}

Then what should I do next to get the \X2\5BBD5EA60042\X0\'s data in #14963 line?
Learnt a lot from your replies already, really thanks.

[aothms]

You still need to do HasProperties (returns aggregate_of_instance::ptr) loop over these to get NominalValue (returns IfcUtil::IfcBaseClass*) and then just call attr = nv.data.getArgument(0). Check attr->type() == Argument_STRING. `std::string attr_value = *attr;