Compare if two Geom_Curves are identical

Hello Paul,

I think I have found a solution. It is computationally quite expensive I guess, but might work for most of the cases.

The idea is the following:
Get the two Geom_Curves (Curve A and Curve B), and make a quick check if the two endpoints of the curves are the same. If this is not the case, they are not equal for sure. If this condition is satisfied, then loop through all the points of Curve B, and make a projection to Curve A from these points. If the distance of the projected point on Curve A to the point on Curve B is higher then a given tolerance, then the curves are not equal. If for all the points the distance of projection stays under the limit, they are geometrically equal.

Yeah, basically we do not loop through all the points of curve B, we only use a specific resolution.

Here is the code:

bool checkForGeometricEquality(Handle(Geom_Curve) curveA, Standard_Real startParamA,Standard_Real endParamA, Handle(Geom_Curve) curveB,Standard_Real startParamB, Standard_Real endParamB)
//The function takes curveA and its parameter range, and curveB with its parameter //range
{
//How many projections do we want
int resolution = 10;
Standard_Real paramIncrement = ((startParamB-endParamB)/double(resolution));

GeomAPI_ProjectPointOnCurve projector;

//Get the start end endpoints of the curves
gp_Pnt startPointA = curveA->Value(startParamA);
gp_Pnt startPointB = curveB->Value(startParamB);
gp_Pnt endPointA = curveA->Value(endParamA);
gp_Pnt endPointB = curveB->Value(endParamB);

// Check if there are matching pairs
if(startPointA.IsEqual(startPointB,Precision::Confusion()) ||
startPointA.IsEqual(endPointB,Precision::Confusion()) ||
endPointA.IsEqual(startPointB,Precision::Confusion()) ||
endPointA.IsEqual(endPointB,Precision::Confusion()))
{
//Loop through points of curveB and project them on curveA
for(int i=0;iValue(startParamB+i*paramIncrement);
projector.Init(checkPnt,curveA);
//If the distance exceeds the limit, return false
if(projector.LowerDistance()>Precision::Confusion())
{
return false;
}
}
//If the for cycle finished, the curves are equal
return true;
}

return false;
}

So if you have two TopoDS_Edges (topoEdgeA and TopoEdgeB), this is how the function is called:

Standard_Real startParamA,endParamA;
Standard_Real startParamB,endParamB;

Handle(Geom_Curve) curveA = BRep_Tool::Curve(topoEdgeA,startParamA,endParamA);
Handle(Geom_Curve) curveB = BRep_Tool::Curve(topoEdgeB,startParamB,endParamB);

if(checkForGeometricEquality(curveA,startParamA,endParamA,curveB,startParamB,endParamB)
{
//They are equal
}
else
{
// They are not equal
}

I haven't done extensive testing, but it worked for matching Geom_Lines and Geom_BezierCurves. And it also worked for my original problem (compare two TopoDS_Edges geometrically.)

Hope this helped! :)

Best regards,
László

Dear Forum supervisor,

thanks for the note. The research group here uses version 6.3, so I guess I won't be using this IsEqual() function of the Geom_BSplineCurve for a while.

However, if it is there, and working, then a good way for comparing curves might be to convert the curves to be compared to b-spline curves, and check their equality using the above mentioned function.

I will check out the link also.

Best regards,
László

Also, you might check the bounding boxes of the two objects as a pre-filter for this function. It looks like this:

bool checkBndBoxes(TopoDS_Edge edgeA, TopoDS_Edge edgeB)
{
//The function compares if the bounding boxes of edgeA and edgeB are touching or intersecting
//each other
Bnd_Box boxA;
Bnd_Box boxB;

BRepBndLib::Add(edgeA,boxA);
BRepBndLib::Add(edgeB,boxB);

if(boxA.Distance(boxB) <= Precision::Confusion())
{
return true;
}
return false;
}