For effects like smoke or water, a fluid simulation and rendering approach is needed. There are currently two popular methods for this:
Simulate the fluid on the CPU and send the result as particles to the GPU for rendering as billboards. This is often called a particle system. The technique has been around since the dawn of computer graphics.
Simulate the fluid on the GPU and render the result into textures. This will then be rendered by doing volume ray casting (or ray marching) on the GPU. This technique is new and rather unexplored, and there are few real-life implementations. The result can be very realistic but slow.
Technique one burdens both CPU, bandwidth and GPU. Although in modern solutions, it’s the bandwidth that’s the bottleneck. The GPU based technique only burdens the GPU ( but a lot ).
The movie shows the GPU method of fluid simulation and rendering. More info about this particular implementation in the two last links.
Building an Advanced Particle System
Building a Million Particle System
Real-Time Simulation and Rendering of 3D Fluids
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