Posted June 25, 2019

Graham Collier - Senior Generalist and FX TD at RealtimeUK - goes behind the scenes of their work for Rebellion's Zombie Army 4

My favourite part of the day: exploding zombies in Houdini :D

On previous trailers I used Houdini's Finite Element Methods (FEM) to fracture and create soft body simulations of exploding chunks of flesh.

This time I wanted to test the new Vellum tools in Houdini 17.5 especially the Vellum Tetrahedral Fiber Constraints. After a few tests I found that I could get similar results as FEM but with much faster simulation times. This was really useful as we had multiple zombie meshes to simulate and we needed to create fast iterations.

To control the fracturing of the Vellum soft bodies I had to set up groups containing areas of impact and impact velocities. I used these groups to add Pin to Target constraints to constrain the mesh to the incoming animation. Within the DOP simulation I then used Vellum Constraint Properties to remove the Pin constraints on the impact frames. I also used Geometry Wrangle nodes to adjust Break Thresholds and add in the Custom Velocities. This enabled me to direct several impacts on the zombies and gave me enough control with the fracturing. The simulation was very fast which enabled me to create several iterations before I was happy with the result.

Previously I used Houdini POP networks for blood simulations, but this time we wanted to go further with Fluid Implicit Particles(FLIP). These fluid simulations were emitted from the internal fractured surfaces. I used a Sign Distance Field (SDF) with a small offset to check if the internal surface was enclosed or open. If open I then scattered points onto the surface and set velocities based on the Normal's and movement. This data was then used as a FLIP source. The fluid simulation itself made use of Viscosity, Collision Velocity Blending and Surface Tension which all really helped control the flow of the blood.

Finally these simulations were all exported as Alembic caches and rendered in 3DS-Max with VRay.

Houdini users can download this cut down Vellum setup showing a single impact simulation using Tetrahedral Fiber Constraints.


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