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How to fing the normal to a curve ??

Omar Msaaf's picture

Hello, I am trying to calculate the normal and the tangent to a curve belonging to a surface in a given point. COuld some one point me out which package of classes to use ??? I tried using BRepLProp, and GeomProp, but it didn't work. Any help would be appreciated. Thanxs. Omar Msaaf

Thomas's picture

Hi Omar,

Have a look at the following classes: GeomLProp_CLProps and GeomLProp_SLProps.

For example: gp_Pnt p1(0,0,1);

gp_Pnt p2(1,2,2);

gp_Pnt p3(2,3,3);

gp_Pnt p4(4,3,4);

gp_Pnt p5(5,5,5);

TColgp_Array1OfPnt array1(1,5); // sizing array






Handle(Geom_BezierCurve) Curve1 = new Geom_BezierCurve(array1);

gp_Pnt P1 = Curve1->EndPoint();

GeomLProp_CLProps analyse1(Curve1,




gp_Dir T1;


It should be similar with a surface (GeomLProp_SLProps)

Regards, Thomas

Maxim ZVEREV's picture


The simpliest way is to use directly the methods of Geom_Curve and Geom_Surface:

for the Curves : see the method Geom_Curve::D1 (which returns vector of first derivative, i.e tangent).

for the Surfaces : see the method Geom_Surface::D1 (which returns two tangent vectors D1U and D1V). Normal is the vector product of two tangencies:

gp_Vec aNorm = aD1U^aD1V;

Best regards.

Michael Gandyra's picture

Normally you want to have the main normal. You are able to compute it with a given parameter u:

gp_Pnt aPnt;

gp_Vec d1u, d2u;

Handle_Geom_Curve aCurv=aCertainCurve;

Standard_Real u=aCertainValue;

// get 1st and 2nd derivative in u

aCurv->D2(u, aPnt, d1u, d2u);

Standard_Real nu_dot = d1u.Dot(d2u)/d1u.Magnitude();

gp_Vec t_vec = d1u.Divided(d1u.Magnitude());

// compute the main normal (not the bi normal)

gp_Vec mainn = d2u-(nu_dot*t_vec);

with regards,



University of Kaiserslautern

Research Group for Computer Application in Engineering Design


maneesh's picture

Can we compute the normal in case of line.
bcz for line the 2nd derivative becomes (0, 0, 0).

Pls. Help ??

Thanks in Advance.

Omar Msaaf's picture

NT = Nothing To Tell Omar Msaaf