Reverse face U-direction maintaining the normal direction

Rhino Developer
1

Hi there,
I have a closed Brep, and I would like to reverse some of the faces in U-direction but keeping the normals outwards, this is important for the CAM software which I used for CNC milling. I tried to reverse the underlay surface and then create a new BrepFace like this :

var s = f.Reverse(0, true);
 Brep bf = Brep.CreateTrimmedSurface(f, s, 0.01);

however, the Brep is always turns out “Inavlid” , is there another work around?

2

Hi @torabiarchitect,

Again, without a sample file and sample code we can run here, we can only speculate as to what is going on.

But perhaps you should be using BrepFace.DuplicateFace?

– Dale

3

here is the full code in C# script in grasshopper ,
reverseface direction test.gh (11.3 KB) surface direction test.3dm (126.6 KB)

// list of points to anchor the vectors in the middle of the face
List<Point3d> a = new List<Point3d>();
// list of the vectors in U-direction of the face
List<Vector3d> b = new List<Vector3d>();
// surface derivetives
Vector3d[] d;
// point at the mid of the domain
Point3d p;
List<Brep> breps = new List<Brep>();
foreach (BrepFace f in x.Faces)
{
  // extract the boundary
  var c = f.Loops[0].To3dCurve();
  // find the center of the boundary
  var amp = AreaMassProperties.Compute(c, 0.01);
  // evaluatet the face in the middle
  double u = f.Domain(0).Mid;
  double v = f.Domain(1).Mid;
  f.Evaluate(u, v, 1, out p, out d);
  // normal of the face (face is assumed to be planar)
  Vector3d norm = f.NormalAt(u, v);
  if (norm.IsParallelTo(top, Rhino.RhinoDoc.ActiveDoc.ModelAngleToleranceRadians) != 0)
  {
    //do not change the faces that are perpendicular to the 'top' vector
    breps.Add(f.DuplicateFace(false));
    continue;
  }
  // we need the U direction of the face to be consisten with the input vecotr 'top'
  // top vector is Z axis of the CNC XY plane
  Vector3d currectU = Vector3d.CrossProduct(top, norm);
  // if the face U-vector is not in the same direction of the milling direction then
  // reverse the face U direction
  if (currectU * d[0] < 0)
  {
    // surface with reverse U
    var s = f.Reverse(0, true);

    // create a new trimmed surface with the boundary of the f and surface s
    Brep bf = Brep.CreateTrimmedSurface(f, s, 0.01);
    // add bf to the list
    breps.Add(bf);
  }else{
    // U-direction of the face was OK! so we add the face to the list
    breps.Add(f.DuplicateFace(false));
  }
  // save center point to list a
  a.Add(amp.Centroid);
  // save the original u-vector in list b
  b.Add(d[0]);
}
// pass the data to output
A = a;
B = b;
C = breps;
4

Since you are already using Reverse with inPlace set to true, you don’t need to further modify the faces.