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[Jan Stránský <honzik.stransky@xxxxxxxxx>]

** Changed in: yade
       Status: New => Fix Released

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https://bugs.launchpad.net/bugs/1427380

Title:
  Inertia calculation is not correct for box bodies!

Status in Yet Another Dynamic Engine:
  Fix Released

Bug description:
  Hi,

  I'm using Yade 2015-02-20.git-0e42755 version.

  
  The inertia calculation for box bodies in utils.py has a problem. 

  We need to have the following equation as we have in Shop_01.cpp:

  body->state->inertia=Vector3r(mass*(4*extents[1]*extents[1]+4*extents[2]*extents[2])/12.,mass*(4*extents[0]*extents[0]+4*extents[2]*extents[2])/12.,mass*(4*extents[0]*extents[0]+4*extents[1]*extents[1])/12.);

  
  However, in py/utils.py the equation is using density instead of mass. This, obviously, give an wrong value of inertia for the boxes. 

  
  There're two issues:
  1- It doesn't include the 1/12 coefficient:


  def box(center,extents,orientation=Quaternion(1,0,0,0),dynamic=None,fixed=False,wire=False,color=None,highlight=False,material=-1,mask=1):
  	"""Create box (cuboid) with given parameters.

          :param Vector3 extents: half-sizes along x,y,z axes

  	See :yref:`yade.utils.sphere`'s documentation for meaning of other parameters."""
  	b=Body()
  	b.shape=Box(extents=extents,color=color if color else randomColor(),wire=wire,highlight=highlight)
  	V=8*extents[0]*extents[1]*extents[2]
  	geomInert=Vector3(4*(extents[1]**2+extents[2]**2),4*(extents[0]**2+extents[2]**2),4*(extents[0]**2+extents[1]**2))
  	_commonBodySetup(b,V,geomInert,material,pos=center,dynamic=dynamic,fixed=fixed)
  	b.state.ori=orientation
  	b.mask=mask
  	b.aspherical=True
  	return b


  2- It is using density instead of mass.


  def _commonBodySetup(b,volume,geomInertia,material,pos,noBound=False,resetState=True,dynamic=None,fixed=False):
  	"""Assign common body parameters."""
  	if isinstance(material,int):
  		if material<0 and len(O.materials)==0: O.materials.append(defaultMaterial());
  		b.mat=O.materials[material]
  	elif isinstance(material,str): b.mat=O.materials[material]
  	elif isinstance(material,Material): b.mat=material
  	elif callable(material): b.mat=material()
  	else: raise TypeError("The 'material' argument must be None (for defaultMaterial), string (for shared material label), int (for shared material id) or Material instance.");
  	## resets state (!!)
  	if resetState: b.state=b.mat.newAssocState()
  	mass=volume*b.mat.density
  	b.state.mass,b.state.inertia=mass,geomInertia*b.mat.density
  	b.state.pos=b.state.refPos=pos
  	b.bounded=(not noBound)
  	if dynamic!=None:
  		import warnings
  		warnings.warn('dynamic=%s is deprecated, use fixed=%s instead'%(str(dynamic),str(not dynamic)),category=DeprecationWarning,stacklevel=2)
  		fixed=not dynamic
  	b.state.blockedDOFs=('xyzXYZ' if fixed else '')

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