How to compute surface normal in OpenCASCADE

OCCUtils provides a convenient utility to compute the Normal of a surface (represented by a GeomAdaptor_Surface) in OpenCASCADE:

#include <occutils/Surface.hxx>

using namespace OCCUtils;

GeomAdaptor_Surface surf = /* ... */;

gp_Ax1 surfaceNormal = Surface::Normal(surf);

// ...or just get the direction
gp_Dir surfaceDirection = Surface::NormalDirection(surf);

This function computes the normal vector at specific U/V coordinates which default to (0,0). You can also give custom U/V coordinates:

gp_Ax1 normal = Surface::Normal(surf, 1.0 /* u */, -4.5 /* v */);

In case you can’t use OCCUtils and you need to do it manually, here’s how you can do it:

#include <GeomLProp_SLProps.hxx>

GeomLProp_SLProps props(surf.Surface(), u, v, 1 /* max 1 derivation */, precision);
gp_Ax1 axis(props.Value(), props.Normal());

props.Value() returns the point on the surface at the given U/V coordinates.