Building animation tools for fluid-like motions is an important and challenging problem with many applications in computer graphics. The use of physics-based models for fluid flow can greatly assist in creating such tools. Physical models, unlike key frame or procedural based techniques, permit an animator to almost effortlessly create interesting, swirling fluid-like behaviors. Also, the interaction of flows with objects and virtual forces is handled elegantly. Until recently, it was believed that physical fluid models were too expensive to allow real-time interaction. This was largely due to the fact that previous models used unstable schemes to solve the physical equations governing a fluid. In this paper, for the first time, we propose an unconditionally stable model which still produces complex fluid-like flows. As well, our method is very easy to implement. The stability of our model allows us to take larger time steps and therefore achieve faster simulations. We have used our model in conjuction with advecting solid textures to create many fluid-like animations interactively in twoand three-dimensions. CR Categories: I.3.7 [Computer Graphics]: Three-Dimensional Graphics and Realism—Animation

Stable fluids

From the Publisher:
Artificial life, a field that seeks to increase the role of synthesis in the study of biological phenomena, has great potential, both for unlocking the secrets of life and for raising a host of disturbing issues -- scientific and technical as well as philosophical and ethical. This book brings together a series of overview articles that appeared in the first three issues of the groundbreaking journal Artificial Life, along with a new introduction by Christopher Langton, Editor-in-Chief of Artificial Life, founder of the discipline, and Director of the Artificial Life Program at the Santa Fe Institute.

Artificial Life: An Overview

This paper describes a new animation technique for modeling the turbulent rotational motion that occurs when a hot gas interacts with solid objects and the surrounding medium. The method is especially useful for scenes involving swirling steam, rolling or billowing smoke, and gusting wind. It can also model gas motion due to fans and heat convection. The method combines specialized forms of the equations of motion of a hot gas with an efficient method for solving volumetric differential equations at low resolutions. Particular emphasis is given to issues of computational efficiency and ease-of-use of the method by an animator. We present the details of our model, together with examples illustrating its use.

Modeling the motion of a hot, turbulent gas

Animation through the numerical simulation of physics- based graphics models offers unsurpassed realism, but it can be computationally demanding. Likewise, the search for controllers that enable physics-based models to produce desired animations usually entails formidable computational cost. This paper demon- strates the possibility of replacing the numerical simulation and control of dynamic models with a dramatically more efficient al- ternative. In particular, we propose the NeuroAnimator, a novel ap- proach to creating physically realistic animation that exploits neu- ral networks. NeuroAnimators are automatically trained off-line to emulate physical dynamics through the observation of physics- based models in action. Depending on the model, its neural net- work emulator can yield physically realistic animation one or two orders of magnitude faster than conventional numerical simulation. Furthermore, by exploiting the network structure of the NeuroAni- mator, we introduce a fast algorithm for learning controllers that en- ables either physics-based models or their neural network emulators to synthesize motions satisfying prescribed animation goals. We demonstrate NeuroAnimators for a variety of physics-based mod- els.

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NeuroAnimator: fast neural network emulation and control of physics-based models

A new method for the visualization of two-dimensional fluid flow is presented. The method is based on the advection and decay of dye. These processes are simulated by defining each frame of a flow animation as a blend between a warped version of the previous image and a number of background images. For the latter a sequence of filtered white noise images is used: filtered in time and space to remove high frequency components. Because all steps are done using images, the method is named Image Based Flow Visualization (IBFV). With IBFV a wide variety of visualization techniques can be emulated. Flow can be visualized as moving textures with line integral convolution and spot noise. Arrow plots, streamlines, particles, and topological images can be generated by adding extra dye to the image. Unsteady flows, defined on arbitrary meshes, can be handled. IBFV achieves a high performance by using standard features of graphics hardware. Typically fifty frames per second are generated using standard graphics cards on PCs. Finally, IBFV is easy to understand, analyse, and implement.

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Image based flow visualization

Methods based on aerodynamics are developed to simulate and control the motion of objects in fluid flows. To simplify the physics for animation, the problem is broken down into two parts: a fluid flow regime and an object boundary regime. With this simplification one can approximate the realistic behaviour of objects moving in liquids or air. It also enables a simple way of designing and controlling animation sequences: from a set of flow primitives, an animator can design the spatial arrangement of flows, create flows around obstacles and direct flow timing. The approach is fast, simple, and is easily fitted into simulators that model objects governed by classical mechanics. The methods are applied to an animation that involves hundreds of flexible leaves being blown by wind currents.

Animation aerodynamics

We explore ways in which physically-based simulation can generate the motion of objects for computer animation. We model flexible and brittle objects and their interaction with wind fields using classical mechanics. We have enhanced our animation environment with a capability that allows quick visual preview of simulations. This is an essential tool for prototyping motion and previewing scenes before they are converted into high quality images with a raytracer. Complex motion can be both specified and controlled by designing wind fields and by making use of the preview capability. Examples are given of how fields and preview are used to create animated scenes that involve hundreds of objects in wind fields.

Aspects of motion design physically-based animation

The authors present a framework for modeling and simulating objects with physical attributes. Rather than attaching physical properties to geometric shapes, one directly designs and manipulates objects with intrinsic dynamic properties such as stiffness, mass, and angular momentum. Flexible and rigid bodies can interact in the same simulated physical environment, responding to collisions, fluid velocity fields, friction, and gravity. A modeler and simulator organized along these principles has been implemented and used in computer animation. The results obtained with these systems are presented. >

Modelling physical objects for simulation

By combining control mechanisms and physically-based models of articulated, rigid body systems, we hope to simplify the task of generating realistic articulated motion for computer animations. As an example of this approach, we have developed a control system and a physically-realistic model to animate the hovering flight of a hummingbird. The computer model of the bird is based on the physical parameters of hummingbirds reported in the biomechanics literature. The animator specifies the desired location of the bird and the control system manipulates the model so that the aerodynamic forces acting on the wings propel the bird to satisfy that goal.

David Haumann

Papers

Animation aerodynamics

Aspects of motion design physically-based animation

Modelling physical objects for simulation

The control of hovering flight for computer animation