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Bug in Extrema_ExtPC

Steph's picture

Hi, I'm encountering some computing problems with the class Extrema_ExtPC.

I use it to compute the projection of a point over a TopoDS_Wire.
I need the point but also the parameter of the projected point on the wire.

The way i use it is the next :


TopoDS_Wire originalWire --> "CURV2_P_Wire.brep"

BRepAdaptor_CompCurve wireAdaptor(originalWire);

Extrema_ExtPC extremaPoint(point,wireAdaptor,1E-1);
if ( extremaPoint.IsDone() && extremaPoint.NbExt() > 0 )
const Standard_Integer iNbExtremaPoints = extremaPoint.NbExt();

for (Standard_Integer i=1;i {
bool bIsMin = extremaPoint.IsMin(i);
const Extrema_POnCurv& currentPOnCurv = extremaPoint.Point(i);
const gp_Pnt& currentPoint = currentPOnCurv.Value();
const Standard_Real& currentParameter = currentPOnCurv.Parameter();
const Standard_Real& currentDistance = currentPoint.Distance(point);

In this case the algorithm find 3 solutions :

Point 1 : IsMin=true; point=(1000.0,1000.0,0.0); param=0.0; distance=501.99
Point 2 : IsMin=false; point=(1986.98,1170.38,0.0); param=1.79; distance=1041.0
Point 3 : IsMin=false; point=(1000.0,1500.0,0.0); param=4.0; distance=1.99

The point 3 is the correct point and i expect this point to answer true to the IsMin method.
But it is not the case.
Moreover thanks to the distance it is easy to view that there is a problem.

Thanks to say if you can reproduce this case. Moreover if someone can explain me what represent the tolerance parameter. With the default constructor value 1E-10 in this case found only one solution (point 2 !!)

Thanks you.

Steph's picture

Sorry i forgot to put the point : const gp_Pnt point(999.99,1501.99,0.0);

So i expect to get as nearest point the (1000.0,1500.0,0) point.

Hennig's picture

hey steph,

i cant tell you something about the parameter, but this would interest me too... somebody an idear?

what i can tell you is that the function IsMin do not answer true, when your distance is the minimum... it answers True when your Extremafunction is a Minimum and not a Maximum. And of course there can be only one absolute minimum but more than one local minimum between your point and the curve. Thats why i check all distances to get the absolute minimum.

greetings paul

Steph's picture

Thanks Paul for the answer. I understand better.
However if i have found a better way to compute the projection. In my case i'm not interested in getting all the extrema but only the projection to the nearest point over a wire.
So i use ShapeAnalysis_Curve::Project.

I've encountered some strange case of bad computing but increasing brutally the precision value (last parameter of the project method) it works in all the cases. So it work but i don't understand the precision parameter.