The final Hourglass loop
On the left are the Vellum grains, center is the resmeshed surface, right is the export to Unreal. Not that the bottom sim is running backwards, this was just for me so I could copy and paste animation curves onto it easier
The hourglass reset sim. Left is the grains, right is the output to Unreal. The start and the end are deformed over time to match the main pouring sim output
The very first version of the Hourglass sim, almost good enough but needed some refining
Hourglass sand simulation following an animation made by Matt Pierson. The sand was simulated as vellum grains in Houdini in two stages. The first stage was pouring the sand, the second was simulating the flip.
The difficult part of this was blending the starts and end of the sand to make a seamless loop. This meant that the final position of the falling sand was the start of the reset, and the start of the falling sand had to match the end of the reset. On top of that, I wanted to mitigate perf impact so these sims ended up being 3 separate softbody VATs that get swapped in and out in a sequence.
Speed was exported from Houdini as a vertex color and was used to drive the "molten" sand material in engine. The rest of the material was procedurally made in Unreal. It all became a delicate dance but adding all of the effects into a level sequence helped to keep it manageable.
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