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Liu Ailin's picture

When we build a face with BRepOffsetAPI_MakeOffsetShape,
The offset shape is constructed:

- outside S, if Offset is positive,
- inside S, if Offset is negative;
Now, my problem is how to know that a side of face is the outside or the inside before making offset.
We can get the normal direction aNormal of aFace by below method.

Standard_Real aUMin, aUMax, aVMin, aVMax;
BRepTools::UVBounds (aFace, aUMin, aUMax, aVMin, aVMax);
gp_Pnt2d aUV((0.5 * (aUMin + aUMax)),(0.5 * (aVMin + aVMax)));

Handle(Geom_Surface) aSurface = BRep_Tool::Surface(aFace);
gp_Vec aD1U, aD1V;
gp_Pnt aStartPnt;
aSurface->D1(aUV.X(), aUV.Y(), aStartPnt, aD1U, aD1V);
gp_Vec aNormal = aD1U^aD1V;

What relation is there between this normal direction and inside or outside of a face?

Thanks in advance

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