BRepAlgoAPI_Section and Shape-Shape Intersection Optimization

Hello, I was wondering if i could get some help/feedback/suggestions in a better understanding of the Mechanism by which BRepAlgoAPI_Section works, the philosophy, readings, who invented this technique?(Ivan Sutherland?, University of Utah?) is it documented on any siggraph papers? or any other proceedings.

the code runs fast, but i want to make it even faster, because I need to execute thousands of intersections and when summed up the accumulative time it takes will be in hours, would love to drop that total time to minutes.

so far i have been executing intersections between an imported IGS shell that may have more than one face, and another shell that may have more than one face (sphere).

I like Catia\'s vb CAA api, so if you have used the HybridShapeFactory, I built one for my self using Opencascade, Hence the HSF:: acronim.

here is a static function that returns the intersection:

TopoDS_Shape HSF::AddNewIntersectSrfW(TopoDS_Shape srf1,TopoDS_Shape srf2)
{
BRepAlgoAPI_Section asect(srf1,srf2,Standard_False);
asect.Approximation(Standard_True);
asect.Build();
TopoDS_Shape R = asect.Shape();
return R;
}

so as i said the code above runs fast, i just need to make it go faster.

So i looked into the way BRepAlgoAPI_Section works:

Stage 1(construction):
BRepAlgoAPI_Section subclasses BRepAlgoAPI_BooleanOperation
in the construction stage we feed the inputs to the base class BRepAlgoAPI_BooleanOperation(Sh1, Sh2, BOP_SECTION) letting the boolean operation know it is a BOP_SECTION.

If PerformNow is set to true it will build the curves at construction.

Stage2:
the base class BRepAlgoAPI_BooleanOperation calls preparefiller()
this creates a BOPTools_DSFiller if there is none
and it calls BOPTools_DSFiller::SetShapes(sh1,sh2)
this besides setting shape1 and shape 2 as references, checks their types and returns an error if the types are not compatible.

stage 3(build)

a call to BRepAlgoAPI_Section::Build()

after confirming there where no errors in the creation of the filler
a call to BOPTools_DSFiller::Perform passing the section attributes (computepcurveon1...aprox..)

in the perform function a BooleanOperations_ShapesDataStructure is created
then a reference is passed to BOPTools_InterferencePool.

then a BOPTools_PaveFiller object is created and the BOPTools_InterferencePool is passed with section attributes.

then the BOPTools_PaveFiller::perform is called

6 checks are performed:
vertex-vertex
vertex-edge
vertex-face
edge-edge
edge-face
face-face

and for each check, the respective geometry is created inside interference tables of the pavefiller.

back in the BRepAlgoAPI_Section::Build()

a BOP_Section object is created
BOP_SectionHistoryCollector object is created and passed the 2 shapes
then this is set as the history collector of the BOP_Section

a call to BOP_Section::dowithfiller passing a reference to the BOPTools_DSFiller made earlier

finally the shape can be extracted from the BOP_Section
by calling BOP_Section::Result()

**************************************************************************************

I have lots of questions about the whole process:

it seems the pseudo code is:

- check the type compatibility of the shapes to intersect.
- build a table of all the subshapes of shape 1 and shape 2
- check the shape ancestors and successors (dont know what this means)
- check the interference of the subshapes in shape1 with the subshapes in shape 2, based on ancestry.
(what is ment by interference? i noticed alot of mapping between a what and a with...)
- using this data gathered, calculate the apropiate intersection algorithm, vv,ve,vf,ee,ef,ff
- return results as a single shape.

so i thought if you already know before hand what shape 1 and shape 2 is you could jump to the corresponding ff or vv .. code

so i wrote this function, i had to make it work in the case shape1 has more than one face or shape2 has more than one face.

TopoDS_Shape HSF::AddNewIntersectFast(TopoDS_Shape srf1,TopoDS_Shape srf2)
{

TopoDS_Shape result;
BRepBuilderAPI_MakeWire thewire = BRepBuilderAPI_MakeWire();

TopExp_Explorer shapeX;
for (shapeX.Init(srf1,TopAbs_FACE); shapeX.More(); shapeX.Next())
{
TopoDS_Face aF1 = TopoDS::Face(shapeX.Current());

TopExp_Explorer shapeY;
for (shapeY.Init(srf2,TopAbs_FACE); shapeY.More(); shapeY.Next())
{
TopoDS_Face aF2 = TopoDS::Face(shapeY.Current());

double anApproxTol=1.e-7;

IntTools_FaceFace aFF;
aFF.SetParameters (true,
true,
true,
anApproxTol);

aFF.Perform(aF1, aF2);

if (aFF.IsDone()) {}
aFF.PrepareLines3D();
//
Standard_Integer aNbCurves, aNbPoints;
const IntTools_SequenceOfCurves& aCvs=aFF.Lines();
aNbCurves=aCvs.Length();

for (int i=1; i const IntTools_Curve& aIC=aCvs(i);
Handle(Geom_Curve) mycrv = aIC.Curve();
if (aIC.HasBounds())
{
double fp,lp;
gp_Pnt fpoint, lpoint;
aIC.Bounds(fp,lp,fpoint,lpoint);
TopoDS_Edge myedge = BRepBuilderAPI_MakeEdge(mycrv);
thewire.Add(myedge);
}
}
}
}
}

result = thewire.Shape();
return result ;
}

this code is running way slower than the BRepAlgoAPI_Section code, and it is not robust, if i intersect a sphere with a surface, in some cases it wont return a full loop distorted circle, it returns partial sections of the circle, the BRepAlgoAPI_Section code always returns the correct full circle.

Any Suggestions are welcome...

I uploaded a sample application that should run SFMQTGUI.EXE if you have occ installed, and if you dont have occ installed, call occinstall.bat to make the enviroment variables. the interface is Derived from the QTOCC by Peter Dolby.

https://rcpt.yousendit.com/1134128785/3b0a3255ab628ebfd3b2fa44b4525593?cid=tr-cv-web-stnd-dl-literec-null-c-19073&s=19073

the Hcount slider sets how many spheres in the horizontal row, and the vcount sets how many spheres in the vertical direction.

Thanks for Listening.

Best,

Alex

the code that generates this example :

void mus::update()
{

QTime time;
time.start();

INITPART

double hcount = get(hcount)
double vcount = get(vcount)

try
{

QoccInputOutput* io_man = new QoccInputOutput();
QString srfpath = QoccApplication::instance()->applicationDirPath() + QString(\"/MPP_SOU_DRV_ENV_SRF1.igs\");

Handle(TopTools_HSequenceOfShape) famshape1 = io_man->importIGES(srfpath);
TopoDS_Shape SRF = famshape1->Value(1);

vis(SRF)

double Xmin, Ymin, Zmin, Xmax, Ymax, Zmax;
Bnd_Box Bnd;
BRepBndLib::Add(SRF, Bnd);
Bnd.Get(Xmin, Ymin, Zmin, Xmax, Ymax, Zmax);

double width = abs(Xmax -Xmin);
double height = abs(Ymax -Ymin);

gp_Pnt centerp((Xmin+Xmax)/2,(Ymin+Ymax)/2,(Zmin+Zmax)/2);
gp_Vec up = gp::DZ();
gp_Pnt cog = HSF::GetCOG(SRF);

TopoDS_Shape rec = HSF::AddNewRectangle(cog,up,width,height);
TopoDS_Shape recfill = HSF::AddNewFillSurface(rec);
//vis (rec)
//vis(recfill)

TopoDS_Shape startcrv = HSF::AddNewIntersectSrfW(SRF,recfill);
vis(startcrv)

TopoDS_Shape startedge = HSF::getedgefromshape(startcrv);

double crvlen = HSF::GetLength(startedge);
crvlen = crvlen;
double minrad = floor(crvlen / hcount);
shapelist(spherelist1)

FOR(i,1,hcount)
try
{
gp_Pnt p1 = HSF::AddNewPointonCurve(startedge,(double) (i-1) / (hcount));
TopoDS_Shape sphere = HSF::AddNewSphereSurface(p1,minrad);
//vis(sphere);

TopoDS_Shape intsphere = HSF::AddNewIntersectSrfW(SRF,sphere);
//TopoDS_Shape intsphere = HSF::AddNewIntersectFast(SRF,sphere);

vis(intsphere);
addtolist(spherelist1,intsphere)
QString line;
QTextStream linestream(&line);
linestream ui::getInstance()->Statusbar(line);
}
catch(...)
{

}
END

shapelist(prevspheres)
prevspheres = spherelist1;
FOR(j,0,vcount)
shapelist(nextspheres)
FOR(i,0,prevspheres.size()-1)
try {
TopoDS_Shape sphere1 = prevspheres.at(i);
TopoDS_Shape sphere2 = prevspheres.at(i+1);

TopoDS_Shape p2 = HSF::AddNewIntersectionPointMulti(sphere1,sphere2);
vis(p2);
TopoDS_Shape point1 = HSF::GetLowestVertex(p2);
if (point1.IsNull()) continue;
gp_Pnt intp2 = BRep_Tool::Pnt(TopoDS::Vertex(point1));

TopoDS_Shape sphere = HSF::AddNewSphereSurface(intp2,minrad);
//vis(sphere);

TopoDS_Shape intsphere2 = HSF::AddNewIntersectSrfW(SRF,sphere);
//TopoDS_Shape intsphere2 = HSF::AddNewIntersectFast(SRF,sphere);
nextspheres vis(intsphere2);

QString line;
QTextStream linestream(&line);
linestream ui::getInstance()->Statusbar(line);

} catch (...) {}
END
prevspheres = nextspheres;
END

}

catch(...)
{

}

ENDPART

qGeomApp->myview->viewFront();
qGeomApp->myview->fitAll();

double milliseconds = time.elapsed();
double seconds = (milliseconds/1000);
double minutes = seconds/60;
double hours = minutes/60;

QString timetxt;
QTextStream linestream(&timetxt);
linestream

ui.time->setText(timetxt);
}