Clouds are a ubiquitous, awe-inspiring, and ever-changing feature of the outdoors. They are an integral factor in Earth's weather systems, and a strong indicator of weather patterns and changes. Their effects and indications are important to flight and flight training. Clouds are an important component of the visual simulation of any outdoor scene, but the complexity of cloud formation, dynamics, and light interaction makes cloud simulation and rendering difficult in real time. In an interactive flight simulation, users would like to fly in and around realistic, volumetric clouds, and to see other aircraft convincingly pass within and behind them. Ideally, simulated clouds would grow and disperse as real clouds do, and move in response to wind and other forces. Simulated clouds should be realistically illuminated by direct sunlight, internal scattering, and reflections from the sky and the earth below. Previous real-time techniques have not provided users with such experiences.
I present a physically-based, visually-realistic cloud simulation suitable for interactive applications such as flight simulators. Clouds in the system are modeled using partial differential equations describing fluid motion, thermodynamic processes, buoyant forces, and water phase transitions. I also simulate the interaction of clouds with light, including self-shadowing and multiple forward light scattering. I implement both simulations--dynamic and radiometric--entirely on programmable floating point graphics hardware. The speed and parallelism of graphics hardware enables simulation of cloud dynamics in real time. I exploit the relatively slow evolution of clouds in calm skies to enable interactive visualization of the simulation. The work required to simulate a single time step is automatically spread over many frames while the user views the results of the previous time step. I use dynamically-generated impostors to accelerate cloud rendering. These techniques enable incorporation of realistic, simulated clouds into real applications without sacrificing interactivity.
Beyond clouds, I also present general techniques for using graphics hardware to simulate dynamic phenomena ranging from fluid dynamics to chemical reaction-diffusion. I demonstrate that these simulations can be executed faster on graphics hardware than on traditional CPUs.
Mark J. Harris. Real-Time Cloud Simulation and Rendering. University of North Carolina Technical Report #TR03-040. 2003.