Cut a Face with a Curve

Hello !

Unfortunatelly i never used BRepFeat_SplitShape, so i cannot say much about its usage.As far as i can tell GEOMAlgo_Splitter is a tottally different implementation for partinioning of "shapes".Even in complex shapes i have good results ! I think the usage of BRepAlgoAPI_Cut for partinioning is not the right method to use ! Anyway if you want you can build only the NTM... and the GEOMAlgo packages and give it a try !

Good luck !

PS:It would be good to post here if you will manage something out of it !

Correction: I meant NMT* and GEOMAlgo packages

In another branch I have told you that you need to make an extrusion of your edges to achieve a prism-like shape. If your edge is not a closed contour then it should be long enough in order its end points did not lie inside face. After making a prism, you will get a face or a shell. After that you should construct a solid with BRepPrimAPI_MakeHalfSpace. The reference point there defines the side of the shell that will be internal to the solid.
I agree with Fotis that it is not the best way for shape partitioning, I think in Salome the splitter was introduced to fill the lack of this functionality in OCC. But if you case is as simple as you tell, and you always need to get only the one part of the split shape, then cutting by prism is the right thing.
Good luck

Hello Thiago,
here is a sample code (extracted from something we do here, might not be exact but should be a really good starting point) to split a conical shell with a plane (could be a cylinder with another face build from your curve).
First I do a section between the two face, then a get a bunch of intersecting edge, and I do the link between those edges and faces for the Split operation. The faces that need to be split are split, and I get the result in the Shape() method, then I have to chose what to do with the new faces (in my case, I keep only faces on one side of the plane, but you might want to keep them on both sides...):

// do a section
BRepAlgoAPI_Section asect(aBody,aPlane, Standard_False);
asect.ComputePCurveOn1(Standard_True);
asect.Approximation(Standard_True);
asect.Build();

if (asect.IsDone()) {
// do a split
TopoDS_Shape R=asect.Shape();
QList edges;
BRepFeat_SplitShape asplit(aBody);
for (TopExp_Explorer Ex(R,TopAbs_EDGE); Ex.More(); Ex.Next()) {
TopoDS_Shape anEdge=Ex.Current();
TopoDS_Shape aFace;
if (asect.HasAncestorFaceOn1(anEdge,aFace)) {
TopoDS_Face F=TopoDS::Face(aFace);
TopoDS_Edge E=TopoDS::Edge(anEdge);
edges.append(E);
asplit.Add(E,F);
}
}
asplit.Build();

// no edge, shape stay the same, means no intersection between plane and body
if (edges.count()==0)
return;

// split is done
if (asplit.IsDone()) {

// go through all faces and keep those you want
// in my case, I keept all those that are one the positive side of my
// section plane
for (TopExp_Explorer iter(asplit.Shape(),TopAbs_FACE);iter.More();iter.Next()) {
if (iter.Current().ShapeType()==TopAbs_FACE) {

}
}
}
}

Good Luck,
francois.

hi do u know whether opencascade browser is supported by visual studio9.0.?

Dear Thiago,

I already told you in another discussion that SplitShape algorithm has some restrictions. It seems you did not understand that post. Therefore now I'll try to present a simple workaround of these restrictions in example draw script that plays with your data, and successfully splits face.brep.

restore face.brep f
axo;fit
vertex v1 50 -100 30
vertex v2 500 500 30
edge e v1 v2
# find extrema between f and e
distmini d f e
# make edge from projection vertices on the face boundaries
subshape d v 1
subshape d2 v 1
edge eproj d_1 d2_1
#split shape by projected edge
splitshape r f f eproj

And now no exception, the shape r contains two faces.

Thank you so much. I get it.

Hi, Sioutis. Thanks a lot. It works very well.
As you mentioned, prior to splitting, the projected edge should be obtained firstly. I use the BRepAlgoAPI_Section to get the projected edge. After lots of testing, there are some bugs with this API. That is, there are some unsuccessful instances. Therefore, I try to split a surface directly with a cylinder surface extracted from an edge, instead of getting the section edge(projected edge) firstly, by means of GEOMAlgo_Splitter. And it is working robustly. Here is the sample codes. Hopt it will be helpful.

Method #1: Create the projected firstly, then to split surface. (Sometimes it fails...)
-----------------------------------------------------------------------
TopoEdge aEdge = ...; // The original edge, which has not been projected yet.
gp_Vec vector = ...; // The axis vector of cylinder to be created;

// 1. Create the cylinder;
BRepSweep_Prism CylSurf (aEdge , vector, Standard_False);
TopoDS_Shape aCylinder = CylSurf.Shape();

// 2. Create the intersection curve;
BRepAlgoAPI_Section asect(aParentFace, aCylinder, FALSE);
asect.ComputePCurveOn1(TRUE);
asect.Approximation(TRUE);
asect.Build();
if(!asect.IsDone())
return;
TopoShape aInterCurve = asect.Shape();

// 3. Split surface with the intersection curve;
CArray edges;
edges.SetSize(0, -1);
BRepFeat_SplitShape asplit(aParentFace);
for (TopExp_Explorer Ex(aInterCurve,TopAbs_EDGE); Ex.More(); Ex.Next())
{
TopoDS_Shape anEdge=Ex.Current();
TopoDS_Shape aFace;
if (asect.HasAncestorFaceOn1(anEdge,aFace))
{
TopoDS_Face F=TopoDS::Face(aFace);
TopoDS_Edge E=TopoDS::Edge(anEdge);
edges.Add(E);
split.Add(E,F);
}
}
asplit.Build();

// 3.1 no edge: shape stay the same, means no intersection between plane and body;
if (edges.GetSize()==0)
return;

// 3.2 split is done;
if (asplit.IsDone())
{
...; // Do what you want...
}

-----------------------------------------------------------------------

Method #2: Split the surface by using cylinder directly (It work very robustly)
-----------------------------------------------------------------------
TopoEdge aEdge = ...; // The original edge, which has not been projected yet.
gp_Vec vector = ...; // The axis vector of cylinder to be created;

// 1. Create the cylinder;
BRepSweep_Prism CylSurf (aEdge , vector, Standard_False);
TopoDS_Shape aCylinder = CylSurf.Shape();

// 2. Split surface with created cylinder;
GEOMAlgo_Splitter PS;
PS.AddShape(aParentFace);
PS.AddTool(aCylinder);
PS.Perform();
TopTools_ListIteratorOfListOfShape iter(PS.Modified(aParentFace));
for (;iter.More();iter.Next())
{
TopoDS_Shape shape = iter.Value();
... // Do what you want...
}
-----------------------------------------------------------------------

By testing with lots of benchmark data, the Method #2 is much more robust than the Method #1.

By the way, does anyone know why ???