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Recreating Our Galaxy in a Supercomputer

caltech.edu

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Re: Recreating Our Galaxy in a Supercomputer

#61

Post by lutorm »

privong wrote: post #37
dr_zoidberg wrote: post #31

Earlier quoted context omitted.

Some simulations also use Smoothed Particle Hydrodinamics (see here[0] for a simplified example), like the Earth-Theia impact Moon simulations[1]. However I'm not in the field and can't really tell how much this kind of code has been used lately, but you did remind me of it when you talk about this simulations being monte carlo sampling of stellar mass distribution, because SPH has some concepts like that "inside" th…

SPH has been fairly common in galaxy simulations over the past 20 years or so. But there's some glaring issues with "classic" SPH formulations, to do with fluid instabilities. As a result, lots of people are transitioning to moving mesh codes. I put a few links in my other comment ( https://news.ycombinator.com/item?id=12461336 ). I'm pretty sure the simulation in the press release is also using a moving mesh for the…

No, this sim is done using GIZMO (a derivative of the GADGET-2 code) which uses a hybrid of SPH and AMR methods: http://www.tapir.caltech.edu/~phopkins/Site/GIZMO.html

Re: Recreating Our Galaxy in a Supercomputer

#62

Post by lutorm »

a_throwaway3838 wrote: post #57
VikingCoder wrote: post #55

Earlier quoted context omitted.

"answered questions" Yes. Like, "Given everything we understand, how could it be that in the physical world, there aren't more dwarf galaxies?" And there was no answer to that question, until now. Now they come up with a new model, and run it, and they end up with a number of dwarf galaxies that much more closely matches the physical world. No one had done that before, so we didn't know how it could be. This has answ…

Saying "it has been answered" is a strong definitive statement, though. It's rather arrogant to say "Yes, thanks to this model, we absolutely know ." Are we absolutely certain that this model is correct? Are we absolutely certain that what we can extrapolate from this model accurately applies to the physical world? Granted, yes, questions were answered... I guess. But saying that "The simulation solves a decades-old…

Some people like to say things like that, but it's more accurate to say "this proves our current model is consistent with data". I only skimmed the article (and haven't read the actual paper) but it focused on surrounding dwarf galaxies which have traditionally been a problem with these simulations. I'm sure there are still plenty of ways in which these sims do not match our actual universe.

Re: Recreating Our Galaxy in a Supercomputer

#64

Post by sillysaurus3 »

tizzdogg wrote: post #54
sillysaurus3 wrote: post #24

Earlier quoted context omitted.

We can come up with hundreds of simplified lighting models. Have you tried the one you mentioned? Does it correspond to photographs, video? That's the hard work, and it's why I listed none. Here's something specific: What did you mean by "multiply"? You cannot "multiply" colors. Not unless you concede that your model has nothing whatsoever to do with physical reality. And at that point, why not use a photo of nature…

I work in film vfx, and if you think we dont compare our rendered images to real photographs and video constantly, then you dont really know how modern graphics are produced. We shoot reference for everything, and use things like gonioreflectometers to sample real world BSDFs. Yes, lighting models are always necessarily simplified from the real world physics. But the thing is that 99% of the time that doesnt matter,…

The main point was that film isn't interested in exact photorealism. As you said, it doesn't matter, because the simplified models are good enough. Therefore it's unlikely that the film industry will be the first to produce a fully computer-generated video that will be indistinguishable from a camcorder capture.

The reason most of the CG you see in TV or movies looks very good is because they take place within real video. We're not looking at a completely CG scene -- it's mixed with video from the real world. And that's a perfectly valid technique, but my comment was talking about 100% CG.

A secondary point re: the film industry is that artists must necessarily retain control of the art pipeline in order to create scenes that advance the plot. That requires the art pipeline to be flexible. The more flexible your art pipeline, the more productive your studio is. Yet that flexibility is precisely opposite to realism. Obviously, the more realistic a purely CG scene looks, the less flexibility you get, otherwise it wouldn't appear real; hence the argument that the vfx industry won't be the ones to produce the elusive fully-CG fully-realistic video. (It doesn't make financial sense for them to do so, if nothing else.)

Re: Recreating Our Galaxy in a Supercomputer

#65

Post by semi-extrinsic »

dekhn wrote: post #58
semi-extrinsic wrote: post #51

Earlier quoted context omitted.

Fourier methods are good, but only when your potential is decaying slowly. If it decays fast, like van der Waals interaction which are plentiful in MD, you just have to do the plain n-body approx.

In MD, it's a combination of VDW and electrostatic. Typically VDW is just a cutoff (VDW forces beyond 8A are neglible), and electrostatic is handled by particle-mesh ewald. I worked on DNA and RNA, which have strong electrostatic interactions and my thesis work showed PME was critical for stability.

This was what I was trying to say. Actually, this assumes atomistic MD where you have electrostatics. I've done some work with coarse-grained MD, where you've averaged out the electrostatics and you just have VDW.

Re: Recreating Our Galaxy in a Supercomputer

#67

Post by sillysaurus3 »

hypertexthero wrote: post #40
sillysaurus3 wrote: post #12

Earlier quoted context omitted.

There's pretty much no way to avoid writing a large comment in response to this. So, with apologies... It was my focus for a long time to achieve perfectly photorealistic rendering. I come from a gamedev background, and I've been fascinated since around age ten about how to get a computer to paint pictures. By 17 I was writing game engines mostly equivalent to quake, and used this portfolio to get into the industry.…

The recommendation to get a camera is a very good one. A camera used for an extended period of time is a very good teacher, and not only of seeing. To go even further, learn to draw. Use your hands and other senses instead of only thinking. Read Drawing on the Right Side of the Brain by Betty Edwards and The Hand by Frank R. Wilson. I'm curious what your favorite games are, visually, @sillysaurus3? A couple of mine a…

I'm curious what your favorite games are, visually, @sillysaurus3?

I've been considering an answer to your question for the past couple of days. As cheesy as it sounds, I suppose my favorite isn't necessarily a game, but a video:

https://www.youtube.com/watch?v=ekLEDoitnNg

This was what I grew up with, and what helped inspire me to pursue graphics programming. I used to watch it on loop dozens of times, being amazed at how graphics got better with each game.

That was back in the day when you had to download codecs for DivX, and find these videos on Kazaa... Ah, memories. :)

A couple of mine are Far Cry 2 and [Elite: Dangerous][1] Horizons. I also love the sound in both of these which is to me a seemingly inseparable element to great screen work.

Good choices. Absolutely agreed re: sound. Sound and animation are both vital (but often overlooked) elements of a good story, with all the focus on graphics nowadays.

Re: Recreating Our Galaxy in a Supercomputer

#68

Post by tizzdogg »

sillysaurus3 wrote: post #64
tizzdogg wrote: post #54

Earlier quoted context omitted.

I work in film vfx, and if you think we dont compare our rendered images to real photographs and video constantly, then you dont really know how modern graphics are produced. We shoot reference for everything, and use things like gonioreflectometers to sample real world BSDFs. Yes, lighting models are always necessarily simplified from the real world physics. But the thing is that 99% of the time that doesnt matter,…

The main point was that film isn't interested in exact photorealism. As you said, it doesn't matter, because the simplified models are good enough. Therefore it's unlikely that the film industry will be the first to produce a fully computer-generated video that will be indistinguishable from a camcorder capture. The reason most of the CG you see in TV or movies looks very good is because they take place within real v…

Again, I would wager that you have seen many, many shots on TV and in movies which were fully, 100% CG already. You just didn't notice because they really were indistinguishable. We've definitely already crossed that boundary in many areas, and the main things which still stand out is just bad work.

Re: Recreating Our Galaxy in a Supercomputer

#69

Post by privong »

lutorm wrote: post #61
privong wrote: post #37

Earlier quoted context omitted.

SPH has been fairly common in galaxy simulations over the past 20 years or so. But there's some glaring issues with "classic" SPH formulations, to do with fluid instabilities. As a result, lots of people are transitioning to moving mesh codes. I put a few links in my other comment ( https://news.ycombinator.com/item?id=12461336 ). I'm pretty sure the simulation in the press release is also using a moving mesh for the…

No, this sim is done using GIZMO (a derivative of the GADGET-2 code) which uses a hybrid of SPH and AMR methods: http://www.tapir.caltech.edu/~phopkins/Site/GIZMO.html

GIZMO is the name of the code suite, not the numerical technique. I believe GIZMO contains multiple hydro solvers, including various formulations of SPH, moving mesh, and meshless techniques. But you're right. that the simulation in question wasn't a moving mesh, it was a meshless technique.

Re: Recreating Our Galaxy in a Supercomputer

#70

Post by privong »

lutorm wrote: post #61
privong wrote: post #37

Earlier quoted context omitted.

SPH has been fairly common in galaxy simulations over the past 20 years or so. But there's some glaring issues with "classic" SPH formulations, to do with fluid instabilities. As a result, lots of people are transitioning to moving mesh codes. I put a few links in my other comment ( https://news.ycombinator.com/item?id=12461336 ). I'm pretty sure the simulation in the press release is also using a moving mesh for the…

No, this sim is done using GIZMO (a derivative of the GADGET-2 code) which uses a hybrid of SPH and AMR methods: http://www.tapir.caltech.edu/~phopkins/Site/GIZMO.html

>> No, this sim is done using GIZMO

> GIZMO is the name of the code suite, not the numerical technique.

Oops, misread that. You already know it's the code suite, not the technique. :)

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