Simulating Sparkling Liquid Pouring Into A Glass With Houdini

A key element in creating and rendering small-scale liquid simulations in Houdini is understanding how nested dielectrics work. Nested dielectrics address the issue of having one geometry within another that requires distinct physical rendering properties.

For instance, as Sergey Kharitonov explained, when simulating sparkling liquid pouring into a glass, you'll need to assign different index of refraction values, as well as potentially different roughness and dispersion values for the glass, liquid, and bubbles. A common problem in such simulations is the liquid geometry intersecting with the glass, resulting in unrealistic outcomes where the liquid appears to penetrate the glass.

Rather than relying on complex modeling techniques, such as using Boolean operations to cut away intersecting areas or scaling down the liquid to create a gap (which can be inaccurate and time-consuming), nested dielectrics offer a simpler solution. By assigning priorities to different shaders, only the shader with the highest priority is rendered in overlapping areas. This functions similarly to a lightweight Boolean operation at render time: higher-priority shaders override lower-priority ones, effectively carving out overlapping geometry volumes.

"You can assign dielectric priorities using the Render Geometry Settings node or the Karma Material Properties node. Lower numerical values mean higher priority. For instance, in this case, I set the glass (index of refraction 1.5) geometry priority to 0 (highest), the bubbles (index of refraction 1.0) to 5, and the liquid (index of refraction -1.33) to 10 (lowest). This setup ensures that the glass and bubbles correctly override the liquid where they intersect", commented Sergey.

Take a look at an earlier example of his small-scale fluid simulation featuring foam-liquid interaction and bubble behavior:

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