Da Mesh a Parte e viceversa
Convertire oggetti Parte in Mesh
La conversione di oggetti di alto livello come le forme di Parte in oggetti semplici come gli oggetti Mesh è una operazione piuttosto semplice, nella quale tutte le facce di un oggetto Parte vengono triangolate (suddivise in maglie di una rete). Il risultato di tale triangolazione (tassellatura) viene poi utilizzato per costruire un oggetto mesh: (supponiamo che il nostro documento contenga un oggetto Parte)
#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)
A volte la triangolazione di alcune facce offerta da OpenCascade è abbastanza brutta. Se la faccia ha una forma rettangolare e non contiene buchi o altre curve di taglio è possibile creare una tassellatura da soli:
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
Convertire oggetti Mesh in Parte
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)