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A new method for an old problem

Most recently, I have been working on upgrading our entire vegetation interaction pipeline from the ground up. Our old system required artists to hand place and animate spheres to move vegetation around. Artists did not have a reliable way of visually seeing what the vegetation was doing until render time. This resulted in a massive tax on the Simulation department; most of the time it was just easier to remove any vegetation that was too close to a character.


Old System

  1. Artists needed to manually place spheres anywhere the characters may touch vegetation

  2. Artists would hand animate the spheres to fake physics.


  3. Artists would need to find and manually connect each individual type of vegetation to each sphere in the scene. 


  4. Artists only had control over areas, not individual blades.


  5. Artists could not see their work until render time.

New System

  1. Autogenerates VDBs from the characters for realistic contact with the vegetation.

  2. Physics is now handled by the new Vellum Solver in Houdini.

  3. Script automatically scans the scene for every type of vegetation that may come in contact with a character and auto generates any necessary connections

  4. Artists can now select individual blades in Houdini viewport to kill them.

  5. Artists can now instantly view all of their work in the viewport.


Technical Overview

The Pipeline itself is built in python and utilizes USD and Houdini. It creates a box based on a characters' movement, scans the scene for any render time procedurals that exist inside of that box. It bakes all of those procedurals into USD geometry and stores them in our global repository. The USD geo can then be brought into Houdini where a simulation artist can adjust physical parameters(e.g. bend stiffness, plasticity, gravity, wind) and run a physics simulation where the USD geo collides with the characters in the scene. The sim data is then exported as a USD overlay that is sub-layered on top of the original bake. 



Teamed up with Pixar University to create a three day intro to Houdini course. This course taught the basics of Houdini. On the last day we use the new pipeline and sim a shot. I ran the class for the Simulation department. I additionally created comprehensive documentation on how to use the pipeline as well as a website with easy to find tricks and tips. 


Before my pipeline, a single shot with Vegetation interaction would take about 2 weeks to complete. Afterwards a single artist was able to turn around between 8 to 10 shots per week. It has been used on 100s of shots in 4 feature productions. The Pipeline has saved hours of time, and the results look much more physically accurate. Further improvements are on going. 

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