GetPlane

from Rhino import *
from Rhino.DocObjects import *
from Rhino.Commands import *
from Rhino.Input import *
from Rhino.Geometry import *
from scriptcontext import doc

def RunCommand():
  #First, collect all the breps to split
  rc, obj_refs = RhinoGet.GetMultipleObjects("Select breps to split", False, ObjectType.Brep)
  if rc <> Result.Success or obj_refs == None:
    return rc

  # Get the final plane
  rc, plane = RhinoGet.GetPlane()
  if rc <> Result.Success:
    return rc

  #Iterate over all object references
  for obj_ref in obj_refs:
    brep = obj_ref.Brep()
    bbox = brep.GetBoundingBox(False)

    #Grow the boundingbox in all directions
    #If the boundingbox is flat (zero volume or even zero area) 
    #then the CreateThroughBox method will fail.
    min_point = bbox.Min
    min_point.X -= 1.0
    min_point.Y -= 1.0
    min_point.Z -= 1.0
    bbox.Min = min_point
    max_point = bbox.Max
    max_point.X += 1.0
    max_point.Y += 1.0
    max_point.Z += 1.0
    bbox.Max = max_point

    plane_surface = PlaneSurface.CreateThroughBox(plane, bbox)
    if plane_surface == None:
      #This is rare, it will most likely not happen unless either the plane or the boundingbox are invalid
      RhinoApp.WriteLine("Cutting plane could not be constructed.")
    else:
      breps = brep.Split(plane_surface.ToBrep(), doc.ModelAbsoluteTolerance)
      if breps == None or breps.Length == 0:
        RhinoApp.Write("Plane does not intersect brep (id:{0})", obj_ref.ObjectId)
        continue
      for brep_piece in breps:
        doc.Objects.AddBrep(brep_piece)
      doc.Objects.AddSurface(plane_surface)
      doc.Objects.Delete(obj_ref, False)

  doc.Views.Redraw()
  return Result.Success

if __name__ == "__main__":
  RunCommand()

Imports Rhino
Imports Rhino.DocObjects
Imports Rhino.Commands
Imports Rhino.Input
Imports Rhino.Geometry

Namespace examples_vb
  Public Class SplitBrepsWithPlaneCommand
    Inherits Rhino.Commands.Command
    Public Overrides ReadOnly Property EnglishName() As String
      Get
        Return "vbSplitBrepsWithPlane"
      End Get
    End Property

    Protected Overrides Function RunCommand(doc As RhinoDoc, mode As RunMode) As Result
      'First, collect all the breps to split
      Dim obj_refs As ObjRef() = Nothing
      Dim rc = RhinoGet.GetMultipleObjects("Select breps to split", False, ObjectType.Brep, obj_refs)
      If rc <> Result.Success OrElse obj_refs Is Nothing Then
        Return rc
      End If

      ' Get the final plane
      Dim plane As Plane
      rc = RhinoGet.GetPlane(plane)
      If rc <> Result.Success Then
        Return rc
      End If

      'Iterate over all object references
      For Each obj_ref As ObjRef In obj_refs
        Dim brep = obj_ref.Brep()
        Dim bbox = brep.GetBoundingBox(False)

        'Grow the boundingbox in all directions
        'If the boundingbox is flat (zero volume or even zero area) 
        'then the CreateThroughBox method will fail.
        Dim min_point = bbox.Min
        min_point.X -= 1.0
        min_point.Y -= 1.0
        min_point.Z -= 1.0
        bbox.Min = min_point
        Dim max_point = bbox.Max
        max_point.X += 1.0
        max_point.Y += 1.0
        max_point.Z += 1.0
        bbox.Max = max_point

        Dim plane_surface = PlaneSurface.CreateThroughBox(plane, bbox)
        If plane_surface Is Nothing Then
          'This is rare, it will most likely not happen unless either the plane or the boundingbox are invalid
          RhinoApp.WriteLine("Cutting plane could not be constructed.")
        Else
          Dim breps = brep.Split(plane_surface.ToBrep(), doc.ModelAbsoluteTolerance)
          If breps Is Nothing OrElse breps.Length = 0 Then
            RhinoApp.Write("Plane does not intersect brep (id:{0})", obj_ref.ObjectId)
            Continue For
          End If
          For Each brep_piece As Brep In breps
            doc.Objects.AddBrep(brep_piece)
          Next
          doc.Objects.AddSurface(plane_surface)
          doc.Objects.Delete(obj_ref, False)
        End If
      Next

      doc.Views.Redraw()
      Return Result.Success
    End Function
  End Class
End Namespace