Levels of Details (LOD's)

A part may have multiple polygonal representations. In this case a part is considered having levels of details (LOD's), i.e. granularity or accuracy of approximation of a part.

By convention, LODs must be sorted from the most accurate (fine) to the least accurate (coarse) representations.

The following example demonstrates a part with one BRep (as a sheet body) and three polygonal representations:

BRep

Fine LOD

Medium LOD

Coarse LOD

Polygonal representations can be either explicitly created bottom-up or generated from B-Rep with the help of meshers (see Mesh Generation).

The following code snippet demonstrates creation of a part with BRep and two polygonal representations:

ModelData_Solid aSolid = ...; //exact solid shape
ModelData_BRepRepresentation aBRep (aSolid);
//automatically generates polygonal representations from BRep
ModelData_PolyRepresentation aFinePoly   (aBRep, ModelAlgo_BRepMesherParameters::Fine);
ModelData_PolyRepresentation aMediumPoly (aBRep, ModelAlgo_BRepMesherParameters::Medium);
ModelData_Part aPart (aBRep, aFinePoly, aMediumPoly, "my_part");

Semantics of LOD is application-specific. The data model only provides a mechanism to define multiple polygonal representations.

B-Rep representation contains ModelData_BodyList, which is a list of bodies (ModelData_Body), which define exact part geometry. Each ModelData_Body can be solid, sheet, wireframe or acorn (see ModelData_BodyType enumeration).

Each polygonal representation contains ModelData_PolyShapeList, which is a list of vertex sets. Each vertex set (subclass of ModelData_PolyVertexSet) defines either a set of triangles (ModelData_IndexedTriangleSet), polylines (ModelData_PolyLineSet) or just a point cloud ( ModelData_PolyPointSet).

Traversal over the representations

The part representations can be traversed as follows:

with the help of direct search;
with the help of iterator;
with the help of visitors.

The following examples demonstrate using each approach:

ModelData_Part aPart = ...;
 
//find B-Rep representation
ModelData_BRepRepresentation aBRep = aPart.BRepRepresentation();
if (aBRep) {
    const ModelData_BodyList& aList = aBRep.Get(); //retrieve root bodies
    for (ModelData_BodyList::SizeType i = 0; i < aList.Size(); ++i) {
        const ModelData_Body& aBody = aList[i];
        //...
    }
}
 
//find first polygonal representation
ModelData_PolyRepresentation aPoly = aPart.PolyRepresentation (ModelData_RM_FineLOD);
if (aPoly) {
    const ModelData_PolyShapeList& aList = aPoly.Get(); //retrive root vertex sets
    for (ModelData_PolyShapeList::SizeType i = 0; i < aList.Size(); ++i) {
        const ModelData_PolyVertexSet& aPVSet = aList[i];
        //...
    }
}

class MyRepresentationVisitor : public ModelData_Part::RepresentationVisitor
{
public:
    virtual void operator() (const ModelData_BRepRepresentation& theRep) override { /*...*/ }
    virtual void operator() (const ModelData_PolyRepresentation& theRep) override { /*...*/ }
};
 
MyRepresentationVisitor aVisitor;
aPart.Accept (aVisitor);

In the above example a respective operator() of MyRepresentationVisitor will be invoked for each representation encountered in the part.

ModelData_Part::RepresentationIterator anIt (aPart);
while (anIt.HasNext()) {
    const ModelData_Representation& aRep = anIt.Next();
    if (aRep.TypeId() == ModelData_PolyRepresentation::GetTypeId()) {
        const ModelData_PolyRepresentation& aPoly = static_cast<const ModelData_PolyRepresentation&> (aRep);
    } else if (aRep.TypeId() == ModelData_BRepRepresentation::GetTypeId()) {
        const ModelData_BRepRepresentation& aBRep = static_cast<const ModelData_BRepRepresentation&> (aRep);
    }
}

Table of Contents

Levels of Details (LOD's)

Traversal over the representations