6 ms·

As a person who did a PhD in CFD, I must admit I never encountered the vorticity confinement method and curl-noise turbulence. I guess you learn something new e

As a person who did a PhD in CFD, I must admit I never encountered the vorticity confinement method and curl-noise turbulence. I guess you learn something new every day! Also, in industrial CFD, where the Reynolds numbers are higher you'd never want something like counteracting artificial dissipation of the numerical method by trying to applying noise. In fact, quite often people want artificial dissipation to stabilize high Re simulations! Guess the requirements in computer graphics are more inclined towards making something that looks right instead of getting the physics right.

ChuckMcM 3y ago
> ... the requirements in computer graphics are more inclined towards making something that looks right instead of getting the physics right. That is exactly correct. That said, as something of a physics nerd (it was a big part of the EE curriculum in school) people often chuckled at me in the arcade pointing out things which were violating various principles of physics :-). And one of the fun things was listening to a translated interview with the physicist who worked on Mario World at Nintendo who stressed that while the physics of Mario's world were not the same as "real" physics, they were consistent and had rules just like real physics did, and how that was important for players to understand what they could and could not do in the game (and how they might solve a puzzle in the game).
- hyperthesis 3y ago
  Yes, consistency. Similar in scifi/fantasy, where an absurd conceit is believable if internally consistent, i.e. self-consistent. (I also call it principled) Too much ad hoc prevents suspension of disbelief. Reactions are an important part of this, it's not just how good an actor is, but how other character react to them. If they are ignored, it's like that character doesn't exist, has no effect, is inconsequential, doesn't matter... is not real. In a fluid simulation, when water does not react to the shoreline, rocks in the water, columns supporting a pier etc, it is like the water (or the obstacles) don't exist. In this sense, the water (or the obstacles) "aren't real". For example, the water in Sea of Thieves looks magnificent... but because it doesn't interact (it's procedural), it doesn't feel real. It isn't real. A further problem here with water is that it's difficult to make a continuous fluid that is consistent and isn't realistic, because the basic principles of fluid are so very very simple: conservation of mass, conservation of momentum. [ Of course, you can still vary density, gravity. There are other effects beyond these: viscosity, surface tension. Also, compressible fluids, different reynolds numbers, adiabatic effects, behaviour at non-everyday temperatures, pressures, scales, velocities, accelerations etc. ] There are also simplifications, like depth-averaging (Shallow Water Equations aka St Venant), but again, it's hard to vary it and yet remain self-consistent. Cellular methods - similar to conway's game of life but for fluid - maybe are an exception, because thet are self-consistent - but pretty far from "water", because not lack momentum (and aren't continuous). The final issue is error: simulations are never perfectly self-consistent anyway, because they must discretize the continuous, which introduces error. In engineering, you can reduce this error with a finer grid, until it too small to matter for your specific application. For computer graphics, it only needs to be perceivably" self-consistent - and perhaps pixel size is one measure of this, for the appearance* of consistency, in area/volume, acceleration, velocity, displacement (...though, we can perceive sub-pixel displacement, e.g. an aliased line.)
  - hyperthesis 3y ago
    An imperceptible inconsistency can become perceptible if it is cumulative, such as mass not being conserved very slightly, but over time the amount of water changes significantly - one ad hoc solution is to redistribute the missing water (or remove surplus) everywhere, to compensate.
jwoq9118 3y ago
Was the PhD worth it in your opinion?
- cherryteastain 3y ago
  From a purely economic standpoint, difficult to say. I was able to build skills that are very in demand in certain technical areas, and breaking into these areas is notoriously difficult otherwise. On the other hand, I earned peanuts for many years. It'll probably take some time for it to pay off. That said, never do a PhD for economic reasons alone. It's a period in your life where you are given an opportunity to build up any idea you would like. I enjoyed that aspect very much and hence do not regret it one bit. On the other hand, I also found out that academia sucks so I now work in a completely different field. Unfortunately it's very difficult to find out whether you'll like academia short of doing a PhD, so you should always go into it with a plan B in the back of your head.
  - mckn1ght 3y ago
    I feel this comment. I did a masters with a thesis option because I was not hurting for money with TA and side business income, so figured I could take the extra year (it was an accelerated masters). Loved being able to work in that heady material, but disliked some parts of the academic environment. Was glad I could see it with less time and stress than a PhD. Even so, I still never say never for a PhD, but it’d have to be a perfect confluence of conditions.
  - godelski 3y ago
    > On the other hand, I also found out that academia sucks I also found that I don't want to be a good academic, but a good researcher. Most importantly, that these are two very different things.
- jgeada 3y ago
  Hard to say whether economically a PhD always makes sense, but it certainly can open doors that are otherwise firmly closed.
smcameron 3y ago
I think the curl noise paper is from 2007: https://www.cs.ubc.ca/~rbridson/docs/bridson-siggraph2007-curlnoise.pdf https://www.cs.ubc.ca/~rbridson/docs/bridson-siggraph2007-cu... I've used the basic idea from that paper to make a surprisingly decent program to create gas-giant planet textures: https://github.com/smcameron/gaseous-giganticus https://github.com/smcameron/gaseous-giganticus
- shahar2k 3y ago
  reminds me of this - https://www.taron.de/forum/viewtopic.php?f=4&t=4 https://www.taron.de/forum/viewtopic.php?f=4&t=4 it's a painting program where the paint can be moved around with a similar fluid simulation
- dahart 3y ago
  Hey that paper references me. ;) I published basic curl noise a few years before that in a Siggraph course with Joe Kniss. Bridson’s very cool paper makes curl noise much more controllable by adding the ability to insert and design boundary conditions, in order words, you can “paint” the noise field and put objects into the noise field and have particles advect around them. Mine and Joe’s version was a turbulence field based on simply taking the curl of a noise field because curl has the property of being incompressible. Thought about it after watching some effects breakdown on X-men’s Nightcrawler teleport effect and they talked about using a fluid simulation IIRC to get the nice subtleties that incompressible “divergence-free” flows give you. BTW I don’t remember exactly how they calculated their turbulence, I have a memory of it being more complicated than curl noise, but maybe they came up with the idea, or maybe it predates X-men too; it’s a very simple idea based on known math, fun and effective for fake fluid simulation. We liked to call it “curly noise” when we first did it, and I used it on some shots and shared it with other effects animators at DreamWorks at the time. Actually the very first name we used was “curl of noise” because the implementation was literally curl(noise(x)), but curly noise sounded better/cuter. Curly noise is neat because it’s static and analytic, so you can do a fluid-like looking animation sequence with every frame independently. You don’t need to simulate frame 99 in order to render frame 100, you can send all your frames to the farm to render independently. On the other hand, one thing that’s funny about curly noise is that it’s way more expensive to evaluate at a point in space than a voxel grid fluid update step, at least when using Perlin noise which is what I started with. (Curly noise was cheaper than using PDI’s (Nick Foster’s) production fluid solver at the time, but I think the Stam semi-Lagrangian advection thing started making it’s way around and generally changed things soon after that.) BTW gaseous giganticus looks really neat! I browsed the simplex noise code for a minute and it looks gnarly, maybe more expensive than Perlin even?
Sharlin 3y ago
> Guess the requirements in computer graphics are more inclined towards making something that looks right instead of getting the physics right. The first rule of real-time computer graphics has essentially always been "Cheat as much as you can get away with (and usually, even if you can't)." Also, it doesn't even have to look right, it just has to look cool! =)
- FirmwareBurner 3y ago
  Why not cheat? I'm not looking for realism in games, I'm looking for escapism and to have fun.
  - digging 3y ago
    I don't think this is a great argument because everybody is looking for some level of realism in games, but you may want less than many others. Without any, you'd have no intuitive behaviors and the controls would make no sense. I'm not saying this just to be pedantic - my point is that some people do want some games to have very high levels of realism.
    - someletters 3y ago
      Some of the best games I've played in my life were the text based and pixel art games in MS DOS. Your imagination then had to render or enhance the visuals, and it was pretty cool to come up with the pictures in your head.
      - mewpmewp2 3y ago
        I mean yes, but, ultimately we want to be able to simulate ourselves to the truest to be able to understand our origins, which are likely simulated as well, right.
      - digging 3y ago
        I realize the thread started about graphics, but I didn't only mean realistic graphics when I referred to realism, because the comment I was replying to just said "realism in games". I do expect there's some degree of realism in your favorite text-based or pixel-art games as well. As you say, it's clear that a game doesn't need photorealistic graphics to be good, because most games do not.
kqr 3y ago
Maybe you're not the right person to ask, but I'll go anyway: I would like to learn the basics of CFD not because I expect to do much CFD in life, but because I believe the stuff I would have to learn in order to be able to understand CFD are very useful in other domains. The problem is my analysis is very weak. My knowledge about linear algebra, differential equations, numerical methods, and so on is approximately limited at the level of an introductory university course. What would you suggest as a good start? I like reading, but I also like practical exercises. The books I tried to read before to get into CFD were lacking in practical exercises, and when I tried to invent my own exercises, I didn't manage to adapt them to my current level of knowledge, so they were either too hard or too easy. (Consequently, they didn't advance my understanding much.)
- wiz21c 3y ago
  You'll be able to understand the equations I guess. The hard part is the numerical analysis: how do you prove your computations will: 1/ reach a solution (badly managed computations will diverge and never reach any solution) 2/ reach a solution that is close to reality ? For me that's the hard part (which I still don't get). You could start with Saint Venant equations, although they look complicated they're actually within reach. But you'll have to understand the physics behind first (conservation of mass, of quantity of movement, etc.)
  - kqr 3y ago
    Understand the equations, yes. However, I'm sufficiently out of practise that it takes me a lot of effort to. So I guess you could say I'm not fluent to be able to grasp the meaning of these things as fast as I think I would need to in order to properly understand them.
  - cherryteastain 3y ago
    Regarding your two questions, some terms you could look up are: Courant number, von Neumann stability analysis, Kolmogorov length and time scales With respect to 2, the standard industry practice is a mesh convergence study and comparing your solver's output to experimental data. Sadly, especially with Reynolds-averaged Navier Stokes, there is no guarantee you'll get a physically correct solution.
dagw 3y ago
This is unfortunately a discussion I've had to have many times. "Why does your CFD software take hours to complete a simulation when Unreal Engine can do it in seconds" has been asked more than once.
physicsguy 3y ago
I went to a talk by a guy who worked in this space in the film industry and said one of the funniest questions he ever got asked was "Can you make the water look more splashy"
nautilius 3y ago
Too bad they don't reference the actual inventor of vorticity confinement, Dr. John Steinhoff of the University of Tennessee Space Institute: https://en.wikipedia.org/wiki/John_Steinhoff https://en.wikipedia.org/wiki/John_Steinhoff https://en.wikipedia.org/wiki/Vorticity_confinement https://en.wikipedia.org/wiki/Vorticity_confinement And some papers: https://www.researchgate.net/publication/239547604_Modification_of_the_Euler_equations_for_vorticity_confinement''_Application_to_the_computation_of_interacting_vortex_rings https://www.researchgate.net/publication/239547604_Modificat... https://www.researchgate.net/publication/265066926_Computation_of_High_Reynolds_Number_Flows_Using_Vorticity_Confinement_I_Formulation https://www.researchgate.net/publication/265066926_Computati...

ChuckMcM 3y ago

> ... the requirements in computer graphics are more inclined towards making something that looks right instead of getting the physics right. That is exactly correct. That said, as something of a physics nerd (it was a big part of the EE curriculum in school) people often chuckled at me in the arcade pointing out things which were violating various principles of physics :-). And one of the fun things was listening to a translated interview with the physicist who worked on Mario World at Nintendo who stressed that while the physics of Mario's world were not the same as "real" physics, they were consistent and had rules just like real physics did, and how that was important for players to understand what they could and could not do in the game (and how they might solve a puzzle in the game).

hyperthesis 3y ago
Yes, consistency. Similar in scifi/fantasy, where an absurd conceit is believable if internally consistent, i.e. self-consistent. (I also call it principled) Too much ad hoc prevents suspension of disbelief. Reactions are an important part of this, it's not just how good an actor is, but how other character react to them. If they are ignored, it's like that character doesn't exist, has no effect, is inconsequential, doesn't matter... is not real. In a fluid simulation, when water does not react to the shoreline, rocks in the water, columns supporting a pier etc, it is like the water (or the obstacles) don't exist. In this sense, the water (or the obstacles) "aren't real". For example, the water in Sea of Thieves looks magnificent... but because it doesn't interact (it's procedural), it doesn't feel real. It isn't real. A further problem here with water is that it's difficult to make a continuous fluid that is consistent and isn't realistic, because the basic principles of fluid are so very very simple: conservation of mass, conservation of momentum. [ Of course, you can still vary density, gravity. There are other effects beyond these: viscosity, surface tension. Also, compressible fluids, different reynolds numbers, adiabatic effects, behaviour at non-everyday temperatures, pressures, scales, velocities, accelerations etc. ] There are also simplifications, like depth-averaging (Shallow Water Equations aka St Venant), but again, it's hard to vary it and yet remain self-consistent. Cellular methods - similar to conway's game of life but for fluid - maybe are an exception, because thet are self-consistent - but pretty far from "water", because not lack momentum (and aren't continuous). The final issue is error: simulations are never perfectly self-consistent anyway, because they must discretize the continuous, which introduces error. In engineering, you can reduce this error with a finer grid, until it too small to matter for your specific application. For computer graphics, it only needs to be perceivably" self-consistent - and perhaps pixel size is one measure of this, for the appearance* of consistency, in area/volume, acceleration, velocity, displacement (...though, we can perceive sub-pixel displacement, e.g. an aliased line.)
- hyperthesis 3y ago
  An imperceptible inconsistency can become perceptible if it is cumulative, such as mass not being conserved very slightly, but over time the amount of water changes significantly - one ad hoc solution is to redistribute the missing water (or remove surplus) everywhere, to compensate.

jwoq9118 3y ago

Was the PhD worth it in your opinion?

cherryteastain 3y ago
From a purely economic standpoint, difficult to say. I was able to build skills that are very in demand in certain technical areas, and breaking into these areas is notoriously difficult otherwise. On the other hand, I earned peanuts for many years. It'll probably take some time for it to pay off. That said, never do a PhD for economic reasons alone. It's a period in your life where you are given an opportunity to build up any idea you would like. I enjoyed that aspect very much and hence do not regret it one bit. On the other hand, I also found out that academia sucks so I now work in a completely different field. Unfortunately it's very difficult to find out whether you'll like academia short of doing a PhD, so you should always go into it with a plan B in the back of your head.
- mckn1ght 3y ago
  I feel this comment. I did a masters with a thesis option because I was not hurting for money with TA and side business income, so figured I could take the extra year (it was an accelerated masters). Loved being able to work in that heady material, but disliked some parts of the academic environment. Was glad I could see it with less time and stress than a PhD. Even so, I still never say never for a PhD, but it’d have to be a perfect confluence of conditions.
- godelski 3y ago
  > On the other hand, I also found out that academia sucks I also found that I don't want to be a good academic, but a good researcher. Most importantly, that these are two very different things.
jgeada 3y ago
Hard to say whether economically a PhD always makes sense, but it certainly can open doors that are otherwise firmly closed.

smcameron 3y ago

I think the curl noise paper is from 2007: https://www.cs.ubc.ca/~rbridson/docs/bridson-siggraph2007-curlnoise.pdf https://www.cs.ubc.ca/~rbridson/docs/bridson-siggraph2007-cu... I've used the basic idea from that paper to make a surprisingly decent program to create gas-giant planet textures: https://github.com/smcameron/gaseous-giganticus https://github.com/smcameron/gaseous-giganticus

shahar2k 3y ago
reminds me of this - https://www.taron.de/forum/viewtopic.php?f=4&t=4 https://www.taron.de/forum/viewtopic.php?f=4&t=4 it's a painting program where the paint can be moved around with a similar fluid simulation
dahart 3y ago
Hey that paper references me. ;) I published basic curl noise a few years before that in a Siggraph course with Joe Kniss. Bridson’s very cool paper makes curl noise much more controllable by adding the ability to insert and design boundary conditions, in order words, you can “paint” the noise field and put objects into the noise field and have particles advect around them. Mine and Joe’s version was a turbulence field based on simply taking the curl of a noise field because curl has the property of being incompressible. Thought about it after watching some effects breakdown on X-men’s Nightcrawler teleport effect and they talked about using a fluid simulation IIRC to get the nice subtleties that incompressible “divergence-free” flows give you. BTW I don’t remember exactly how they calculated their turbulence, I have a memory of it being more complicated than curl noise, but maybe they came up with the idea, or maybe it predates X-men too; it’s a very simple idea based on known math, fun and effective for fake fluid simulation. We liked to call it “curly noise” when we first did it, and I used it on some shots and shared it with other effects animators at DreamWorks at the time. Actually the very first name we used was “curl of noise” because the implementation was literally curl(noise(x)), but curly noise sounded better/cuter. Curly noise is neat because it’s static and analytic, so you can do a fluid-like looking animation sequence with every frame independently. You don’t need to simulate frame 99 in order to render frame 100, you can send all your frames to the farm to render independently. On the other hand, one thing that’s funny about curly noise is that it’s way more expensive to evaluate at a point in space than a voxel grid fluid update step, at least when using Perlin noise which is what I started with. (Curly noise was cheaper than using PDI’s (Nick Foster’s) production fluid solver at the time, but I think the Stam semi-Lagrangian advection thing started making it’s way around and generally changed things soon after that.) BTW gaseous giganticus looks really neat! I browsed the simplex noise code for a minute and it looks gnarly, maybe more expensive than Perlin even?

Sharlin 3y ago

> Guess the requirements in computer graphics are more inclined towards making something that looks right instead of getting the physics right. The first rule of real-time computer graphics has essentially always been "Cheat as much as you can get away with (and usually, even if you can't)." Also, it doesn't even have to look right, it just has to look cool! =)

FirmwareBurner 3y ago
Why not cheat? I'm not looking for realism in games, I'm looking for escapism and to have fun.
- digging 3y ago
  I don't think this is a great argument because everybody is looking for some level of realism in games, but you may want less than many others. Without any, you'd have no intuitive behaviors and the controls would make no sense. I'm not saying this just to be pedantic - my point is that some people do want some games to have very high levels of realism.
  - someletters 3y ago
    Some of the best games I've played in my life were the text based and pixel art games in MS DOS. Your imagination then had to render or enhance the visuals, and it was pretty cool to come up with the pictures in your head.
    - mewpmewp2 3y ago
      I mean yes, but, ultimately we want to be able to simulate ourselves to the truest to be able to understand our origins, which are likely simulated as well, right.
    - digging 3y ago
      I realize the thread started about graphics, but I didn't only mean realistic graphics when I referred to realism, because the comment I was replying to just said "realism in games". I do expect there's some degree of realism in your favorite text-based or pixel-art games as well. As you say, it's clear that a game doesn't need photorealistic graphics to be good, because most games do not.

kqr 3y ago

Maybe you're not the right person to ask, but I'll go anyway: I would like to learn the basics of CFD not because I expect to do much CFD in life, but because I believe the stuff I would have to learn in order to be able to understand CFD are very useful in other domains. The problem is my analysis is very weak. My knowledge about linear algebra, differential equations, numerical methods, and so on is approximately limited at the level of an introductory university course. What would you suggest as a good start? I like reading, but I also like practical exercises. The books I tried to read before to get into CFD were lacking in practical exercises, and when I tried to invent my own exercises, I didn't manage to adapt them to my current level of knowledge, so they were either too hard or too easy. (Consequently, they didn't advance my understanding much.)

wiz21c 3y ago
You'll be able to understand the equations I guess. The hard part is the numerical analysis: how do you prove your computations will: 1/ reach a solution (badly managed computations will diverge and never reach any solution) 2/ reach a solution that is close to reality ? For me that's the hard part (which I still don't get). You could start with Saint Venant equations, although they look complicated they're actually within reach. But you'll have to understand the physics behind first (conservation of mass, of quantity of movement, etc.)
- kqr 3y ago
  Understand the equations, yes. However, I'm sufficiently out of practise that it takes me a lot of effort to. So I guess you could say I'm not fluent to be able to grasp the meaning of these things as fast as I think I would need to in order to properly understand them.
- cherryteastain 3y ago
  Regarding your two questions, some terms you could look up are: Courant number, von Neumann stability analysis, Kolmogorov length and time scales With respect to 2, the standard industry practice is a mesh convergence study and comparing your solver's output to experimental data. Sadly, especially with Reynolds-averaged Navier Stokes, there is no guarantee you'll get a physically correct solution.

dagw 3y ago

This is unfortunately a discussion I've had to have many times. "Why does your CFD software take hours to complete a simulation when Unreal Engine can do it in seconds" has been asked more than once.

physicsguy 3y ago

I went to a talk by a guy who worked in this space in the film industry and said one of the funniest questions he ever got asked was "Can you make the water look more splashy"

nautilius 3y ago

Too bad they don't reference the actual inventor of vorticity confinement, Dr. John Steinhoff of the University of Tennessee Space Institute: https://en.wikipedia.org/wiki/John_Steinhoff https://en.wikipedia.org/wiki/John_Steinhoff https://en.wikipedia.org/wiki/Vorticity_confinement https://en.wikipedia.org/wiki/Vorticity_confinement And some papers: https://www.researchgate.net/publication/239547604_Modification_of_the_Euler_equations_for_vorticity_confinement''_Application_to_the_computation_of_interacting_vortex_rings https://www.researchgate.net/publication/239547604_Modificat... https://www.researchgate.net/publication/265066926_Computation_of_High_Reynolds_Number_Flows_Using_Vorticity_Confinement_I_Formulation https://www.researchgate.net/publication/265066926_Computati...