From cgwiki


The previous section treated usd as a cache format. You're probably aware of Solaris/Lops, the USD editing mode within Houdini. Working in Lops lets you focus on another feature of USD, hierarchies and assembly.

Loading a usd in lops

  1. In the network editor, tab in a 'file' node
  2. Point it to the pig.usd you made earlier. Careful, the file parameter is about 2/3 the way down the parameter pane, not at the top where you'd expect. Don't be fooled!
  3. There's the pig, same as before. Big deal.
  4. Look at the Scene Graph Tree. This is a tree view/outliner style view of your usd file. At the moment you probably see nothing.
  5. Expand it out. You'll see there's a little hierarchy, very similar to the maya style thing of a transform and shape. In this case its /usdexport1/mesh_0

So far, so dull. Note that the name of the node is 'sublayer', which is a USD term for loading files from disk. It's the default opalias used by Houdini when you type 'file' cos it's a simple file loader, but there's a more interesting alternative, a reference.

  1. Tab in a reference node
  2. Paste in the path, display the node. There's the pig. Note that in the scene graph tree the 'transform' is called reference1, and it's green.
  3. This is kiiiindasorta like a reference in maya. You ever try lots of references in maya to the same file? No? Well, lets try it here.
  4. Duplicate the reference node, append it below the first. Display it. You can see in the scene graph tree there's 2 reference nodes now.
  5. Duplicate those 2, append them. Now there's 4 references. Great, buuuut there's no change in the viewport. Makes sense I guess, in that they're all in the same place. What if you wanna move stuff around to see them all?
  6. Select reference2 in the scene graph tree, move your cursor to the viewport, hit 't'. That'll bring up the translate tool, you can move that pig somewhere else. Select reference3, move it, select reference4, move it.
  7. Take a look at what's happened in the node editor and the parameter pane. Lops has appended an edit node, and the parameter has a list of nodes that have changed. You can turn on a toggle to see the modified translate values.

So this is kindasorta like a edit sop, but its also kindasorta a procedural edit node; you can move stuff, and it's recording your changes on a node.