Scaled cylinder yields invalid shape

[aothms]

Hi all,

Please find attached a trivial example program that yields an invalid shape by using TopoDS_Shape::Move() on a shape with a Geom_CylindricalSurface and a gp_Trsf with scale factor.

#include <gp_Trsf.hxx>
#include <Geom_Circle.hxx>

#include <BRepBuilderAPI_MakeWire.hxx>
#include <BRepBuilderAPI_MakeEdge.hxx>
#include <BRepBuilderAPI_MakeFace.hxx>
#include <BRepBuilderAPI_Transform.hxx>

#include <BRepPrimAPI_MakePrism.hxx>

#include <TopoDS_Iterator.hxx>

#include <BRepCheck.hxx>
#include <BRepCheck_Result.hxx>
#include <BRepCheck_Analyzer.hxx>
#include <BRepCheck_ListOfStatus.hxx>
#include <BRepCheck_ListIteratorOfListOfStatus.hxx>

#include <BRepTools.hxx>

int main(int argc, char** argv) {
    Handle(Geom_Circle) circle = new Geom_Circle(gp_Ax2(), 400.);
    BRepBuilderAPI_MakeWire wire;
    wire.Add(BRepBuilderAPI_MakeEdge(circle));
    TopoDS_Shape cylinder = BRepPrimAPI_MakePrism(BRepBuilderAPI_MakeFace(wire, false), gp_Vec(0, 0, 4000)).Shape();

    {
        BRepCheck_Analyzer valid(cylinder);
        bool validity = valid.IsValid();
        std::cout << validity << std::endl;
    }

    gp_Trsf trsf;
    trsf.SetScaleFactor(0.1);
    TopoDS_Shape scaled_cylinder = cylinder.Moved(trsf);

    {
        BRepCheck_Analyzer valid(scaled_cylinder);
        bool validity = valid.IsValid(); 
        std::cout << validity << std::endl;
    }

    scaled_cylinder = BRepBuilderAPI_Transform(cylinder, trsf);

    {
        BRepCheck_Analyzer valid(scaled_cylinder);
        bool validity = valid.IsValid();
        std::cout << validity << std::endl;
    }
}

Results in:

1
0
1

(in other words, the unscaled cylinder is valid, the scaled cylinder with TopoDS_Shape::Move() is not, BRepBuilderAPI_Transform does yield a valid shape on the same operands)

I did some debugging and it boils down to

1. a BRepCheck_UnorientableShape being issued for the 'cylindrical' TopoDS_Face
2. This is due to a BRepCheck_InvalidCurveOnSurface being issued for one of its edges
   

   oce/src/BRepCheck/BRepCheck_Analyzer.cxx

   Line 257 in c0cdb53

   ste == BRepCheck_InvalidCurveOnSurface ||

3. This is due to the distance between points on the pcurve and 3d curve being a factor 10 away from each other in the Z-axis. Which seems to indicate the somehow the transformation does get applied to only one of them.
   

   oce/src/BRepCheck/BRepCheck_Edge.cxx

   Line 718 in c0cdb53

   if (pref.SquareDistance(pother) > Tol2) {

I'm in way over my head here. Appreciate some help. Thanks all.

[a-betenev]

Hello Thomas,

Unfortunately, transformations with non-unit scale factor are not
supported in shape locations.
The reason is that elementary surfaces and curves (such as line or
cylinder) in OCCT are parametrized 'by length'.
When you apply scale factor, parametrization changes, and all the data
that refer to it need to be recomputed (such as p-curves on surfaces).
Though this potentially could be accounted for, it is not done in
practice (would be overly complex).
Thus, just do not use non-unit scales in this way (use Transform to get
new shape instead).

Andrey

On 01.05.2016 14:16, Thomas Krijnen wrote:

Hi all,

Please find attached a trivial example program that yields an invalid
shape by using |TopoDS_Shape::Move()| on a shape with a
|Geom_CylindricalSurface| and a |gp_Trsf| with scale factor.

|#include <gp_Trsf.hxx> #include <Geom_Circle.hxx> #include
<BRepBuilderAPI_MakeWire.hxx> #include <BRepBuilderAPI_MakeEdge.hxx>
#include <BRepBuilderAPI_MakeFace.hxx> #include
<BRepBuilderAPI_Transform.hxx> #include <BRepPrimAPI_MakePrism.hxx>
#include <TopoDS_Iterator.hxx> #include <BRepCheck.hxx> #include
<BRepCheck_Result.hxx> #include <BRepCheck_Analyzer.hxx> #include
<BRepCheck_ListOfStatus.hxx> #include
<BRepCheck_ListIteratorOfListOfStatus.hxx> #include <BRepTools.hxx>
int main(int argc, char** argv) { Handle(Geom_Circle) circle = new
Geom_Circle(gp_Ax2(), 400.); BRepBuilderAPI_MakeWire wire;
wire.Add(BRepBuilderAPI_MakeEdge(circle)); TopoDS_Shape cylinder =
BRepPrimAPI_MakePrism(BRepBuilderAPI_MakeFace(wire, false), gp_Vec(0,
0, 4000)).Shape(); { BRepCheck_Analyzer valid(cylinder); bool validity
= valid.IsValid(); std::cout << validity << std::endl; } gp_Trsf trsf;
trsf.SetScaleFactor(0.1); TopoDS_Shape scaled_cylinder =
cylinder.Moved(trsf); { BRepCheck_Analyzer valid(scaled_cylinder);
bool validity = valid.IsValid(); std::cout << validity << std::endl; }
scaled_cylinder = BRepBuilderAPI_Transform(cylinder, trsf); {
BRepCheck_Analyzer valid(scaled_cylinder); bool validity =
valid.IsValid(); std::cout << validity << std::endl; } } |

Results in:

|1 0 1 |

(in other words, the unscaled cylinder is valid, the scaled cylinder
with |TopoDS_Shape::Move()| is not, |BRepBuilderAPI_Transform| does
yield a valid shape on the same operands)

I did some debugging and it boils down to

1. a |BRepCheck_UnorientableShape| being issued for the 'cylindrical'
   |TopoDS_Face|
2. This is due to a |BRepCheck_InvalidCurveOnSurface| being issued
   for one of its edges
   

   oce/src/BRepCheck/BRepCheck_Analyzer.cxx

   Line 257 in c0cdb53

   ste == BRepCheck_InvalidCurveOnSurface ||

3. This is due to the distance between points on the pcurve and 3d
   curve being a factor 10 away from each other in the Z-axis. Which
   seems to indicate the somehow the transformation does get applied
   to only one of them.
   

   oce/src/BRepCheck/BRepCheck_Edge.cxx

   Line 718 in c0cdb53

   if (pref.SquareDistance(pother) > Tol2) {

I'm in way over my head here. Appreciate some help. Thanks all.

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#615

[aothms]

Dear Andrey,

Thanks for clarifying this. I wasn't aware of this, although admittedly the fact that the method is called Move() should have made me think longer. Maybe for future ignorant users like me it might be good to throw an exception when a location with gp_Trsf::ScaleFactor() != 1. is encountered, but I'm sure you have your reasons. Thanks again!

Kind regards,
Thomas

[a-betenev]

Dear Thomas,

I agree, now recorded as issue in OCCT Mantis:
http://tracker.dev.opencascade.org/view.php?id=27457

Best Regards,
Andrey

On 01.05.2016 15:00, Thomas Krijnen wrote:

Dear Andrey,

Thanks for clarifying this. I wasn't aware of this, although
admittedly the fact that the method is called |Move()| should have
made me think longer. Maybe for future ignorant users like me it might
be good to throw an exception when a location with
|gp_Trsf::ScaleFactor() != 1.| is encountered, but I'm sure you have
your reasons. Thanks again!

Kind regards,
Thomas

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#615 (comment)

[aothms]

Dear Andrey,

Great! Thanks.

Kind regards,
Thomas