Mesh to Part/fr

Converting Part objects to Meshes/fr

La conversion des objets de haut niveau tels que les objets (formes) en objets simples comme les mailles (Mesh) est une opération facile, où, toutes les faces d'un Objet Part deviennent une composition de triangles (exemple sur le site de coin3d un des moteurs de FreeCAD).
Le résultat de cette triangulation (tessellation) est ensuite utilisé pour construire un maillage (Mesh):

 #let's assume our document contains one part object
 import Mesh
 faces = []
 shape = FreeCAD.ActiveDocument.ActiveObject.Shape
 triangles = shape.tessellate(1) # the number represents the precision of the tessellation)
 for tri in triangles[1]:
     face = []
     for i in range(3):
         vindex = tri[i]
         face.append(triangles[0][vindex])
     faces.append(face)
 m = Mesh.Mesh(faces)
 Mesh.show(m)

Parfois, la triangulation de certaines faces offertes par OpenCascade sont assez laid. Si une face a un forme rectangulaire et ne contient pas de trous ou n'est pas limité par des courbes, vous pouvez également créer un maillage sur cette forme:

 import Mesh
 def makeMeshFromFace(u,v,face):
 	(a,b,c,d)=face.ParameterRange
 	pts=[]
 	for j in range(v):
 		for i in range(u):
 			s=1.0/(u-1)*(i*b+(u-1-i)*a)
 			t=1.0/(v-1)*(j*d+(v-1-j)*c)
 			pts.append(face.valueAt(s,t))
 
 	mesh=Mesh.Mesh()
 	for j in range(v-1):
 		for i in range(u-1):
 			mesh.addFacet(pts[u*j+i],pts[u*j+i+1],pts[u*(j+1)+i])
 			mesh.addFacet(pts[u*(j+1)+i],pts[u*j+i+1],pts[u*(j+1)+i+1])
 
 	return mesh

Conversion de Mailles en Part objects

Converting Meshes to Part objects is an extremely important operation in CAD work, because very often you receive 3D data in mesh format from other people or outputted from other applications. Meshes are very practical to represent free-form geometry and big visual scenes, as it is very lightweight, but for CAD we generally prefer higher-level objects that carry much more information, such as the idea of solid, or faces made of curves instead of triangles.

Converting meshes to those higher-level objects (handled by the Part Module in FreeCAD) is not an easy operation. Meshes can be made of thousands of triangles (for example when generated by a 3D scanner), and having solids made of the same number of faces would be extremely heavy to manipulate. So you generally want to optimize the object when converting.

FreeCAD currently offers two methods to convert Meshes to Part objects. The first method is a simple, direct conversion, without any optimization:

 import Mesh,Part
 mesh = Mesh.createTorus()
 shape = Part.Shape()
 shape.makeShapeFromMesh(mesh.Topology,0.05) # the second arg is the tolerance for sewing
 solid = Part.makeSolid(shape)
 Part.show(solid)

The second method offers the possibility to consider mesh facets coplanar when the angle between them is under a certain value. This allows to build much simpler shapes: (let's assume our document contains one Mesh object)

 # let's assume our document contains one Mesh object
 import Mesh,Part,MeshPart
 faces = []
 mesh = App.ActiveDocument.ActiveObject.Mesh
 segments = mesh.getPlanes(0.00001) # use rather strict tolerance here
 
 for i in segments:
   if len(i) > 0:
      # a segment can have inner holes
      wires = MeshPart.wireFromSegment(mesh, i)
      # we assume that the exterior boundary is that one with the biggest bounding box
      if len(wires) > 0:
         ext=None
         max_length=0
         for i in wires:
            if i.BoundBox.DiagonalLength > max_length:
               max_length = i.BoundBox.DiagonalLength
               ext = i
 
         wires.remove(ext)
         # all interior wires mark a hole and must reverse their orientation, otherwise Part.Face fails
         for i in wires:
            i.reverse()
 
         # make sure that the exterior wires comes as first in the lsit
         wires.insert(0, ext)
         faces.append(Part.Face(wires))
 
 shell=Part.Compound(faces)
 Part.show(shell)
 #solid = Part.Solid(Part.Shell(faces))
 #Part.show(solid)

Topological data scripting

Scenegraph

Index