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[Jan Stránský <question688822@xxxxxxxxxxxxxxxxxxxxx>]

Question #688822 on Yade changed:
https://answers.launchpad.net/yade/+question/688822

    Status: Open => Answered

Jan Stránský proposed the following answer:
well, it is not tough (MWE below, probably could be simplified by a meshing/triangulation library)
I just found it unnecessary without enough information
cheers
Jan

###
import stl # pip install stl

class Triangle:
   def __init__(self,p1,p2,p3):
      self.points = [Vector3(p) for p in (p1,p2,p3)]
   def scale(self,factor):
      self.points = [p*factor for p in self.points]
      return self
   def shift(self,v):
      self.points = [p+v for p in self.points]
      return self
   def refine(self): # split 1 triangle into 4
      p1,p2,p3 = self.points
      p4 = (.5*(p1+p2)).normalized()
      p5 = (.5*(p2+p3)).normalized()
      p6 = (.5*(p3+p1)).normalized()
      return [
         Triangle(p1,p4,p6),
         Triangle(p2,p5,p4),
         Triangle(p3,p6,p5),
         Triangle(p4,p5,p6),
      ]
   def toStlFacet(self):
      minieigen2stl = lambda v: stl.Vector3d(*tuple(v))
      p1,p2,p3 = self.points
      normal = (p2-p1).cross(p3-p1).normalized()
      normal = minieigen2stl(normal)
      vertices = [minieigen2stl(p) for p in self.points]
      return stl.Facet(normal,vertices)
   
def triangulateUnitSphere(): # octahedron, you can use whatever instead, e.g. icosahedron
   xm,xp = [(v,0,0) for v in (-1,+1)]
   ym,yp = [(0,v,0) for v in (-1,+1)]
   zm,zp = [(0,0,v) for v in (-1,+1)]
   return [
      Triangle(xp,yp,zp),
      Triangle(xm,yp,zp),
      Triangle(xp,ym,zp),
      Triangle(xm,ym,zp),
      Triangle(xp,yp,zm),
      Triangle(xm,yp,zm),
      Triangle(xp,ym,zm),
      Triangle(xm,ym,zm),
   ]

def triangulateOneSphere(sph,smooth=1):
   ret = triangulateUnitSphere()
   #
   for ismooth in range(smooth):
      ret2 = []
      for t in ret:
         ret2.extend(t.refine())
      ret = ret2
   #
   c = sph.state.pos
   r = sph.shape.radius
   for t in ret:
      t.scale(r).shift(c)
   #
   return ret

def triangulateSpheres(smooth=1):
   ret = []
   for b in O.bodies:
      if not isinstance(b.shape,Sphere): continue
      ret.extend(triangulateOneSphere(b,smooth))
   return ret

def spheres2stl(fName,smooth=1):
   triangles = triangulateSpheres(smooth)
   triangles = [t.toStlFacet() for t in triangles]
   solid = stl.Solid()
   solid.facets = triangles
   with open(fName,"w") as f:
      solid.write_ascii(f)

O.bodies.append((
   sphere((0,0,0),1),
   sphere((5,4,3),2),
   sphere((0,1,4),3),
))

spheres2stl("sphs.stl")
spheres2stl("sphs-smooth.stl",smooth=3)
###

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