Jacob Bock returned with a new Houdini experiment simulating watercolor effects using Copernicus. This project is based on a 1999 paper titled Simple Cellular Automaton-based Simulation of Ink Behaviour and Its Application to Suibokuga-like 3D Rendering of Trees by Qing Zhang, Youetsu Sato, Jun-ya Takahashi, Kazunobu Muraoka, and Norishige Chiba.

The paper splits the simulation into a water layer, a wet/dry ink layer, paper height, and paper absorption. Water spreads across the surface, carries pigment through advection, and dries over time. Jacob used these ideas, ported the simulation to OpenCL, and extended it with physically based color mixing using the Kubelka-Munk model, which he previously wrote about in a blog post we featured last year.

He also mentioned that in the future, he would like to experiment with custom methods using motion vectors and diffusion to achieve similar results.

In the 1930s, Paul Kubelka and Franz Munk published A Contribution to the Optics of Paint Layers, which introduced what is now known as the Kubelka-Munk model, a method for physically based color mixing, especially for paints and translucent materials. Jakob explained how to use this model in Houdini's VEX language, which can be useful for fluid sims like FLIP, POP, or Vellum, and could be improved by using real pigment data.

He also released Settle, an HDA that automatically finds plausible rest positions for any asset without needing a simulation:

Houdini 22 is coming soon, with more details expected during the online Keynote on June 22:

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