This example shows how to retrieve and set transformations on object nodes.

1. Open $HFS/houdini/help/files/hom_cookbook/obj_xform.hip.

   The file contains a series of boxes chained together in a transformation hierarchy. You can translate and rotate a box object in the middle of the chain to see that the outputs of that box inherit the transformations.

2. Choose Window ▸ Python Shell and observe the effects on box_object4 in the viewer and the network editor as you type the following statements:

   # Store the hou.ObjNode for /obj/box_object4 into a Python variable.
   >>> box_object4 = hou.node("/obj/box_object4")
   >>> box_object4
   <hou.ObjNode of type geo at /obj/box_object4>
   
   # Ask for box_object4's world transformation.  This transformation matrix
   # includes the parm transformation (determined from the translate, rotate,
   # scale, etc. parameters on the object), pretransformations, parent
   # transformations, transformations on the containing object, etc.
   >>> xform = box_object4.worldTransform()
   >>> xform
   <hou.Matrix4 [[0.942106, -0.147233, -0.301261, 0],
   [0.134676, 0.988938, -0.062158, 0],
   [0.3070 8, 0.017987, 0.951514, 0],
   [-5.04816, -0.769449, -9.42593, 1]]>
   
   # Disconnect box_object4's input.  box_object4 will move, since it no longer
   # inherits its input's transformations.  If we print out its new transformation
   # matrix, we see that it's different.
   >>> box_object4.inputs()
   (<hou.ObjNode of type geo at /obj/box_object3>,)
   >>> box_object4.setFirstInput(None)
   >>> box_object4.inputs()
   ()
   >>> box_object4.worldTranform()
   <hou.Matrix4 [[0.966956, 0, 0.254942, 0],
   [0, 1, 0, 0],
   [-0.254942, 0, 0.966956, 0],
   [0.885497 , 0, -3.01977, 1]]>
   
   # Restore box_object4's transformation to its previous value.  We'll leave
   # its input disconnected, so we'll have unparented it but kept its position.
   # setWorldTransform will adjust the objects parameters to achieve the
   # desired end result, accounting for inputs, pretransforms, etc.
   >>> box_object4.setWorldTransform(xform)
   

Next, we’ll put the logic to unparent a node and keep its position into a function in a shelf tool.

1. Reload $HFS/houdini/help/hom/cookbook/obj_xform.hip to reset the scene.

2. Right-click an empty area of the shelf, choose New tool, and name the tool "Unparent". Paste the following into the Script tab:

   def unparentAndKeepPos(obj_node):
       """
           Unparent an object node, but keep it in the same position as it was
       when it was parented.
       """
   
       xform = obj_node.worldTransform()
       obj_node.setFirstInput(None)
       obj_node.setWorldTransform(xform)
   
   unparentAndKeepPos(hou.node("/obj/box_object4"))
   

3. Click the tool in the shelf to unparent box_object4 without changing its position.

Next, we’ll generalize the shelf tool to work on the selected node, instead of being hard-coded to unparent box_object4.

1. Right click the Unparent shelf tool and choose Edit tool.

2. Paste the following into the Script tab:

   # The toolutils module ships with Houdini and is used to perform common
   # shelf tool operations.  It can be found in $HFS/houdini/pythonX.Xlibs.
   import toolutils
   
   def unparentAndKeepPos(obj_node):
       """
           Unparent an object node, but keep it in the same position as it was
       when it was parented.
       """
   
       xform = obj_node.worldTransform()
       obj_node.setFirstInput(None)
       obj_node.setWorldTransform(xform)
   
   # If an object is already selected, we'll use it.  If nothing is selected,
   # we'll prompt the user to select something.  If multiple objects are selected,
   # we'll use the last one, since that should correspond to the parameter pane
   # contents.
   viewer = toolutils.sceneViewer()
   obj_node = viewer.selectObjects(quick_select=True)[-1]
   unparentAndKeepPos(obj_node)
   

Finally, we’ll create another shelf tool that will reparent the selected node, prompting the user for the new parent node.

1. Right-click an empty area of the shelf, choose New tool, and name the tool "Reparent". Paste the following into the Script tab:

   # The toolutils module ships with Houdini and is used to perform common
   # shelf tool operations.  It can be found in $HFS/houdini/python2.7libs.
   import toolutils
   
   def parentAndKeepPos(obj_node, new_parent):
       """
           Parent an object node, but keep it in the same position as it was
       when it was unparented.
       """
   
       xform = obj_node.worldTransform()
       obj_node.setFirstInput(new_parent)
       obj_node.setWorldTransform(xform)
   
   # We'll select the object to reparent in the same way the unparent tool works.
   # We'll prompt the user to select the new parent in the viewer.
   viewer = toolutils.sceneViewer()
   node_to_reparent = viewer.selectObjects(quick_select=True)[-1]
   
   new_parent = viewer.selectObjects(use_existing_selection=False,
       quick_select=True,
       prompt="Select new parent node")[-1]
   
   parentAndKeepPos(node_to_reparent, new_parent)