Month: August 2012

Michael B. Nielsen, Andreas Soderstrom, Robert Bridson

Computer animated ocean waves for feature films are typically carefully choreographed to match the vision of the director and to support the telling of the story. The rough shape of these waves is established in the previsualization (previs) stage, where artists use a variety of modeling tools with fast feedback to obtain the desired look. This poses a challenge to the effects artists who must subsequently match the locked-down look of the previs waves with high-quality simulated or synthesized waves, adding the detail necessary for the final shot. We propose a set of automated techniques for synthesizing Fourier-based ocean waves that match a previs input, allowing
artists to quickly enhance the input wave animation with additional higher frequency detail that moves consistently with the coarse waves, tweak the wave shapes to flatten troughs and sharpen peaks if desired (as is characteristic of deep water waves), and compute a physically reasonable velocity field of the water analytically. These properties are demonstrated with several examples, including a previs scene from a visual effects production environment.

Synthesizing Waves from Animated Height Fields

Karthik Raveendran, Nils Thuerey, Chris Wojtan, Greg Turk

We present an approach for artist-directed animation of liquids using multiple levels of control over the simulation, ranging from the overall tracking of desired shapes to highly detailed secondary effects such as dripping streams, separating sheets of fluid, surface waves and ripples. The first portion of our technique is a volume preserving morph that allows the animator to produce a plausible fluid-like motion from a sparse set of control meshes. By rasterizing the resulting control meshes onto the simulation grid, the mesh velocities act as boundary conditions during the projection step of the fluid simulation. We can then blend this motion together with uncontrolled fluid velocities to achieve a more relaxed control over the fluid that captures natural inertial effects. Our method can produce highly detailed liquid surfaces with control over sub-grid details by using a mesh-based surface tracker on top of a coarse grid-based fluid simulation. We can create ripples and waves on the fluid surface attracting the surface mesh to the control mesh with spring-like forces and also by running a wave simulation over the surface mesh. Our video results demonstrate how our control scheme can be used to create animated characters and shapes that are made of water.

Controlling Liquids Using Meshes

Alfred Barnat, Nancy S. Pollard

Smoke is one of the core phenomena which fluid simulation techniques in computer graphics have attempted to capture. It is both well understood mathematically and important in lending realism to computer generated effects. In an attempt to overcome the diffusion inherent to Eulerian grid-based simulators, a technique has recently been developed which represents velocity using a sparse set of vortex filaments. This has the advantage of providing an easily understandable and controllable model for fluid velocity, but is computationally expensive because each filament affects the fluid velocity over an unbounded region of the simulation space. We present an alternative to existing techniques which merge adjacent filament rings, instead allowing filaments to form arbitrary structures,and we develop a new set of reconnection criteria to take advantage of this filament graph. To complement this technique, we also introduce a method for smoke surface tracking and rendering designed to minimize the number of sample points without introducing excessive diffusion or blurring. Though this representation lends itself to straightforward real-time rendering, we also present a method which renders the thin sheets and curls of smoke as diffuse volumes using any GPU capable of supporting geometry shaders.

Smoke Sheets for Graph-Structured Vortex Filaments