7 ms·

Fluid Simulation for Video Games (part 1 of 10)

90 points by spacemanaki 15y ago

madamepsychosis 15y ago
http://http.developer.nvidia.com/GPUGems/gpugems_ch38.html 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 http://www.youtube.com/watch?v=MIi62cwjqMA)
6ren 15y 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-for-video-games-part-3/ http://software.intel.com/en-us/articles/fluid-simulation-fo... Also: pretty videos http://www.youtube.com/mijagourlay#p/u/4/G9E8xEjGzk0 http://www.youtube.com/mijagourlay#p/u/4/G9E8xEjGzk0 (download http://software.intel.com/file/23546/ http://software.intel.com/file/23546/)
- palish 15y 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.
  - 6ren 15y ago
    "toyshop rain" http://www.youtube.com/watch?v=LtxvpS5AYHQ 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.
    - palish 15y ago
      Thank you. Probably my favorite tech demo of all time. It gives me goosebumps every time I watch it. Likely watched it at least 30 times by now. Everything about it is beautiful, from the art to the music to the sound effects to the incredible feeling of "being drenched". Natalya Tatarchuk is truly a hero. Watch it all the way through, with headphones, in fullscreen, or don't watch it at all! The experience was so much more intense back in 2007; it was breathtaking. (Nooooo I don't wanna get old! Make it stop!)
      - 6ren 15y ago
        In case you haven't seen it, From Dust uses fluid simulation as a game mechanic. It's a god game, with flowing water that erodes rivers etc, at a constant 30 fps. There's an xbox demo, and a PC demo to come. Here's a video: http://www.youtube.com/watch?v=gSOQGazo7Oo&t=33s http://www.youtube.com/watch?v=gSOQGazo7Oo&t=33s (I think the fluid grid is slightly coaster in the actual game, but water basically flows the same). BTW: I think it's fantastic as a toy, less good as a game. However, from what you're saying, the fluid simulation isn't as expensive as I thought.
        palish 15y ago
        Wow, that's incredible! Thanks for sharing. Sorry, I miscommunicated --- fluid simulation isn't expensive as long as it doesn't affect gameplay. Because then you can do it all on the GPU, and you don't have to transfer it back to the CPU (to perform checks like "is the water touching me?") In this case, From Dust is simulating the fluids on the CPU, then building geometry / textures on the fly and uploading that to the GPU and rendering. It's quite an impressive tech demo, both from a technical and artistic standpoint.
lutorm 15y 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 15y 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 15y 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 15y 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/GDC03.pdf 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.”

90 points by spacemanaki 15y ago

http://http.developer.nvidia.com/GPUGems/gpugems_ch38.html 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 http://www.youtube.com/watch?v=MIi62cwjqMA)

6ren 15y 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-for-video-games-part-3/ http://software.intel.com/en-us/articles/fluid-simulation-fo... Also: pretty videos http://www.youtube.com/mijagourlay#p/u/4/G9E8xEjGzk0 http://www.youtube.com/mijagourlay#p/u/4/G9E8xEjGzk0 (download http://software.intel.com/file/23546/ http://software.intel.com/file/23546/)

palish 15y 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.
- 6ren 15y ago
  "toyshop rain" http://www.youtube.com/watch?v=LtxvpS5AYHQ 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.
  - palish 15y ago
    Thank you. Probably my favorite tech demo of all time. It gives me goosebumps every time I watch it. Likely watched it at least 30 times by now. Everything about it is beautiful, from the art to the music to the sound effects to the incredible feeling of "being drenched". Natalya Tatarchuk is truly a hero. Watch it all the way through, with headphones, in fullscreen, or don't watch it at all! The experience was so much more intense back in 2007; it was breathtaking. (Nooooo I don't wanna get old! Make it stop!)
    - 6ren 15y ago
      In case you haven't seen it, From Dust uses fluid simulation as a game mechanic. It's a god game, with flowing water that erodes rivers etc, at a constant 30 fps. There's an xbox demo, and a PC demo to come. Here's a video: http://www.youtube.com/watch?v=gSOQGazo7Oo&t=33s http://www.youtube.com/watch?v=gSOQGazo7Oo&t=33s (I think the fluid grid is slightly coaster in the actual game, but water basically flows the same). BTW: I think it's fantastic as a toy, less good as a game. However, from what you're saying, the fluid simulation isn't as expensive as I thought.
      - palish 15y ago
        Wow, that's incredible! Thanks for sharing. Sorry, I miscommunicated --- fluid simulation isn't expensive as long as it doesn't affect gameplay. Because then you can do it all on the GPU, and you don't have to transfer it back to the CPU (to perform checks like "is the water touching me?") In this case, From Dust is simulating the fluids on the CPU, then building geometry / textures on the fly and uploading that to the GPU and rendering. It's quite an impressive tech demo, both from a technical and artistic standpoint.

lutorm 15y 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 15y 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 15y 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 15y 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/GDC03.pdf 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.”