Houdini USD Example Workflow

Houdini USD Example Workflow


An example of using Houdini in a USD-centric pipeline

Authoring USD Overrides in Houdini

This tutorial shows an example using Houdini to modify the transform of model in a USD scene. The files for this tutorial live in USD/extras/usd/tutorials/Houdini. It is assumed that you have a working knowledge of Houdini.

In summary you will do the following

  1. View the USD Scene
    1. use usdview to determine what you need to import into Houdini.
  2. Import the USD geometry
    1. import all the geometry to modify and anything needed for reference into Houdini.
  3. Houdini Shot work
    1. modifying the imported models position, orientation et al.
  4. Export the chages to a USD Overlay. 
  5. Insert the Overlay back into the USD Scene.


View the USD Scene

Open the USD scene in usdview to identify the geometry you want to modify by using the following command line.

$ usdview  USD/extras/usd/tutorials/Houdini/shot.usda



 

In this case we are interested in the little red house at the origin:

  • First select the prim by clicking in the prim browser on the 'house' row.
  • Next hit 'f' to bring it into frame.

Note that the house appears yellow because we have selected it.

 

 

You can find the name of the prim path of the house in this USD stage(/World/sets/house) by looking at the Prim Name window in the upper left hand corner of usdview. You'll use this information later when we import the house into Houdini.

Imagine that you have received a note that the house should be placed at the top of the highest hill on the terrain.

Import USD geometry into Houdini

It's time to import the little red house into Houdini. 

  1. From the Geometry context in Houdini drop down a USD import SOP.

  2. Enter the name of, or browse to the USD file that contains the stage you’re interested in modifying in the "USD File" parameter.

    1. in this case: USD/extras/usd/tutorials/Houdini/shot.usda

  3. enter the name of USD Prim path you are interested in modifying into the "Prim Path" parameter.

    1. in this case:  /World/sets/house
    2. you can also use the Tree view to visually browse through the USD stage to select  the prim path.

 

 

In this case we also need to set the "Traversal" parameter to Gprims

The above steps sets set the traversal  criteria for importing geometry into HOUDINI

You should now see the little red house in your Houdini  viewport.

On middle mouse click of the usdimport SOP you will notice that there is one primitive, a packed USD. As you will only be altering the transformation of this model you can keep it packed and export what is known as a transform overlay.

Import the terrain for visually reference.

Repeat the steps above to import the terrain model by duplicating the usdimport SOP and replacing the "Prim Path" with  /World/sets/terrain and templaing the new usdimport SOP.

Houdini shot work

Use the transform sop to move the house to the top of the highest hill.

Export the USD Overlay

  1. Create a "USD Output" ROP in the Outputs context of Houdini.
  2. Enter the path to the transform sop you used to move the little red house.
  3. Toggle the "Overlay Existing Geometry" Parameter on.
  4. Enter the path of the new USD file that will contain the overlay. 
    1.  In this case:    USD/extras/usd/tutorials/Houdini/OVR_house.usda
  5. Enter the file path of the USD file that contains the USD prim you wish to override into the "Overlay Reference File"  parameter.
    1. In this case: USD/extras/usd/tutorials/Houdini/shot.usda
  6. As you are only interested in altering the transform of our model we are going to uncheck "Overlay All" and check "Overlay Transforms"

 

 

Hit "Render to Disk"

 

Insert Overlay into USD Scene

  1. Drop down a "USD Layer" ROP into the Outputs context of Houdini.
  2. Set the first "Source" paramter field to    USD/extras/usd/tutorials/Houdini/OVR_house.usda
  3. Hit "Render to Disk" on the USD layer ROP

If everything has gone according to plan, you should now be able to see the house in it's new location at the top of the hill in usdview:


The completed Houdin file can be found here:  USD/extras/usd/tutorials/Houdini/overlay_tutorial.hip

You can see other example of how to use USD in Houdini in the included hip file: USD/extras/usd/tutorials/Houdini/examples.hip

 



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