Fluid Simulation for Video Games (part 1 of 10)

http://software.intel.com/en-us/articles/fluid-simulation-for-video-games-part-1/

by spacemanaki • 15 years ago

90 16 15 years ago

software.intel.com

http://http.developer.nvidia.com/GPUGems/gpugems_ch38.html

^This is another great tutorial for real-time fluid simulation. If anyone's interested, I adapted this method to simulate fire as well. (demo here: http://www.youtube.com/watch?v=MIi62cwjqMA)

6ren4• 15 years ago

The algorithms are embarrassingly parallel, but he doesn't use the GPU because it's busy rendering. I wonder if intel's increasingly powerful integrated graphics (usually supplanted with a video card) could be used for this?

> In the endeavor to achieve real-time fluid motion, some other fluid simulations exploit general-purpose computing on graphics processing unit (GPGPU). However clever, such approaches do not help with current gaming hardware, because in video games, the GPU tends to be busy with rendering and has no time left over for simulation. http://software.intel.com/en-us/articles/fluid-simulation-fo...

Also: pretty videos http://www.youtube.com/mijagourlay#p/u/4/G9E8xEjGzk0 (download http://software.intel.com/file/23546/)

palish4• 15 years ago

GPGPU is incredibly overkill for simulating a water surface.

Create two 256x256 RGB textures, call them foo and bar.

The R channel will store current height of wave. The G channel will store the previous height of wave (allowing you to derive velocity).

  ... each frame ...
  render_to(bar)
  sample_from(foo)
  /* insert fancy partial differential algorithm */
  foo,bar = bar,foo

In other words, you're "ping pong"-ing between two textures.

Now if you set an arbitrary texel to, say, 1.0 (maximum), that texel will propogate outwards as if it were a ripple.

Bonus: the result is a seamless tilable texture.

Look up "toyshop rain" for the exact details. But I used this method to augment an ingame lake surface's normal map (thunderstorm effect --- rippling raindrops propagating across the lake). This was in 2007, running on 2007 hardware. GPGPU need not apply.

6ren4• 15 years ago

"toyshop rain" http://www.youtube.com/watch?v=LtxvpS5AYHQ (but youtube video content seems to be down at the moment, at least here in Australia anyway)

BTW: The article (10 parts!) deals with other fluid effects besides surfaces, the first few paragraphs even distinguish between them and gases.

daeken• 15 years ago

My first two demos (Waveride and Armitage by Straylight if you search on Pouet) used a 3-array water surface simulation, cycling through them like you mentioned. The first used an implementation on the CPU with a 128x128 grid, the second did 5 separate 128x128 surfaces on the GPU (render to texture). The second ran all 5 about 10x faster than running a single one on the CPU, and that was with effectively no optimization on the GPU side. With some tweaking, I would've been able to do 10000^2 grids cheaply; the biggest bottleneck was that I moved the textures to and from the GPU for each frame, which I could eliminate with some use of vertex shaders and abuses therein.

Anyway, saying that GPGPU is overkill here is pretty silly -- sure, you can do some water surface simulation on the CPU, but you hit the wall very, very quickly if you're doing anything but that.

lutorm• 15 years ago

Are fluid simulations for video games different from "actual" fluid simulations? The article just seemed like a discussion about fluid dynamics, but when it says things like "... for games", I always think it's about making something look realistic without actually solving the real equations.

5hoom• 15 years ago

I think that most fluid simulations for games would be pretty far from physically accurate just for the sake of speed.

Doing a full simulation at any sort of resolution would totally kill your framerate, and you've got a lot of other stuff going on as well.

A relevant quote from Dwarf Fortress developer ToadyOne (regarding the fluid system in DF): "The only thing impressive about it is probably that it runs at all while everything else is going on".

w-ll• 15 years ago

I felt the same about the title. Games today have no need for realistic fluids, and for that reason the trick is they only render the surface.

cooldeal• 15 years ago

Yes, actual fluid dynamics are extremely slow and not needed just for the visuals.

Read this seminal paper for the details:

http://www.dgp.toronto.edu/people/stam/reality/Research/pdf/...

Sample quote:

We believe that a better alternative is to use the physics of fluid flows which have been developed since the time of Euler, Navier and Stokes (from the 1750’s to the 1850’s). These developments have led to the so-called Navier-Stokes Equations, a precise mathematical model for most fluid flows occurring in Nature. These equations, however, only admit analytical solutions in very simple cases. No progress was therefore made until the 1950’s when researchers started to use computers and develop numerical algorithms to solve the equations. In general, these algorithms strive for accuracy and are fairly complex and time consuming. This is because the applications that require these solvers have to be physically accurate. It is obviously crucial that the stresses and drag on an airplane or a bridge are calculated precisely.

In computer graphics and in games on the other hand what matters most is that the simulations both look convincing and are fast. In addition it is important that the solvers aren’t too complex so that they can be implemented on standard PCs, game consoles or PDAs. In this paper we present a set of algorithms which meet these requirements. To achieve these goals we depart from conventional wisdom in computational physics and develop algorithms custom tailored for creating visual effects. Unlike physically accurate solvers which have strict bounds on their time steps, our algorithms are stable, and never “blow up.”

felipemnoa2• 15 years ago

Thanks. This is a very useful info.

Fluid Simulation for Video Games (part 1 of 10)

Related Stories