Houdini 16.5 Python Scripting HOM Cookbook

HOM cookbook Object transforms and parenting

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Overview

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

Getting and setting transforms

  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)
    

Create a shelf tool

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.

Generalize the shelf tool

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)
    

Reparenting the Selected 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)
    

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