Very cool! What does the changing color of the fluid indicate, particle velocity?
Yes, particle velocity! I believe this clip is after I implemented metaballs too, so the particles blend together to make it feel more fluid-like.
Readit Newsvedosity commented on Fast real time fluid simulator based on MPM algorithm kotsoft.github.io/... · Posted by u/andrewla
vedosity commented on Fast real time fluid simulator based on MPM algorithm kotsoft.github.io/... · Posted by u/andrewla
Hi, I made this demo. This is actually all from the paper Particle-based Viscoelastic Fluid Simulation (Simon Clavet, Philippe Beaudoin, and Pierre Poulin). It is an SPH type (particle-particle interactions), not MPM (particle-grid-particle). I do a lot of MPM nowadays, and I have a multi-grid thing to help with incompressibility, but this was me re-implementing the first fluid sim paper I ever implemented.
I did the implementation in JS to help other people reading the paper, and tried to keep everything as similar as possible to the pseudocode from just this single paper. Maybe it would be cool to integrate an additional grid on which incompressibility is enforced better but I didn't want to make the source confusing.
It is also a little difficult to do density ratios with just what is shown in the paper, here the masses are set to (1, .8, .6, .4). This is what causes the lightest particles to get launched so violently in the air. Probably would be useful to integrate some ideas from the paper Density Contrast SPH Interfaces (Barbara Solenthaler, Renato Pajarola).
I started revisiting SPH because I have some new ideas to combine it with an MPM/FLIP grid for closeups. I'm trying to do a multi-scale MPM simulation that can handle better surface tension droplets in closeups while also doing extremely large scale scenes. You can see some larger scale parallel sims on my YouTube: https://www.youtube.com/c/GrantKot
Hey this was also the first fluid sim paper I ever implemented! I haven't done any grid-based simulations (except maybe the classic "Real-Time Fluid Dynamics for Games" by Jos Stam), but I've been able to greatly improve compressibility by using "Position-Based Fluids" by Miles Macklin. Someday I'll dive into WebGPU and post a JS version on HN...
Short clip of my implementation here: https://imgur.com/a/2IARiBq
I like the different fluids in your sim though, I may try that myself.
My guess is, given state of the charging connector still attached but ripped off (https://news.ycombinator.com/item?id=38914210):
The entire thing fell out of plane (phone + charging cable plugged), then landed in a tree, where the charging cable got tangled in branches and that's when phone broke out of it and fell in grass.
So phone was able to release kinetic energy in 2 big events (+a few branches hit maybe), not a direct splash on the ground.
Wonder if somehow they can analyze the accelerometer data of the phone and figure out if that correlates with that scenario.
In skydiving, it isn't unheard of for someone to have their phone fall out of their pocket and survive impact without the screen shattering. That's from ~13000ft, but once the phone reaches terminal velocity, it's all the same impact force regardless of altitude.
I think dirt/grass is just a lot softer than the things we usually drop our phones over, like concrete or tile.
vedosity commented on Challenging projects every programmer should try (2019) austinhenley.com/blog/cha... · Posted by u/azhenley
Sebastian Lague recently did a video on simulating fluids, which may be interesting. As always, he takes a "from scratch" approach to it.
https://youtu.be/rSKMYc1CQHE?si=pXdsHlQSCpw8nY8m
The GitHub repository also contains links to some of the research papers used to implement the simulation.
I recently went this route. I didn’t want to set up or use Unity so I wrote my own 2D fluid simulator based on some of the same papers using Metal compute shaders (though I’d love to try again using webgpu). Sebastian’s video is great and the implementation is good. But this was a great (and fun) opportunity to look for ways to improve on it.
For starters, the way he’s doing the spatial lookup has poor cache performance, each neighbor lookup is another scattered read. Instead of rearranging an array of indices when doing the sort, just rearrange the particle values themselves. That way you're doing sequential reads for each grid cell you look for neighbors in, instead of a series of scattered reads. The performance improvement I got was about 2x, which was pretty impressive for such a simple change.
The sorting algorithm used isn’t the fastest, counting sort had much better performance for me and was simpler for me to conceptualize. It involves doing a prefix sum though, which is easy to do sequentially on the CPU but more of a challenge if you want to try keeping it on the GPU. "Fast Fixed-Radius Nearest Neighbors: Interactive Million-Particle Fluids", by Hoetzlein et al [0].
Or, if you want to keep using bitonic sort, you can take advantage of threadgroup memory to act as a sort of workspace during bitonic merge steps that are working on small enough chunks of memory. The threadgroup memory is located on the GPU die, so it has better read/write performance.
I ended up converting his pure SPH implementation to use PBF ("Position Based Fluids", Macklin et al, [1]), which is still SPH-based but maintains constant density using a density constraint solver instead of a pressure force. It seems to squeeze more stability out of each “iteration” (for SPH that’s breaking up a single frame into multiple substeps, but with PBF you can also run more iterations of the constraint solver). It’s also a whole lot less “bouncy”. One note: I had to multiply the position updates by a stiffness factor (about 0.1 in my case) to get stability, the paper doesn’t talk about this so maybe I’m doing something wrong.
The PBF paper talks about doing vorticity confinement. It’s implemented exactly as stated in the paper but I struggled for a bit to realize I could still do this in 2D. You just have to recognize that while the first cross product produces the signed magnitude of a vector pointing out of the screen, the second cross product will produce a 2D vector in the same plane as the screen. So there’s no funny business in 2D like I had originally thought. Though, you can skip vorticity confinement, the changes aren't very significant.
There’s a better (maybe a bit more expensive) method of doing surface tension/avoiding particle clustering. It behaves a lot more like fluids in real life do and avoids the “tendril-y” behavior he mentions in the video. "Versatile surface tension and adhesion for SPH fluids" by Akinci et al [2].
One of the comments on Sebastian's video mentions that doing density kernel corrections using Shepard interpolation should improve the fluid surface. I searched and found this method in a bunch of papers, including "Consistent Shepard Interpolation for SPH-Based Fluid Animation" by Reinhardt et al, [3] (I never implemented the full solution that paper proposes, though). There's kernel corrections, and then there's kernel gradient corrections, which I never got working. With the kernel corrections alone, the surface of the fluid seems to "bunch up" less when it moves, and it was pretty simple to implement. Otherwise, the surface looks a bit like a slinky or crinkling paper with particles being pushed out from the surface boundary.
I found [0] and [1] on my own but I found [2] through a thesis, "Real-time Interactive Simulation of Diverse Particle-Based Fluids" by Niall Tessier-Lavigne [4]. I also use the 2nd order integration step formula from that paper. It has some other excellent ideas that are worth trying.
Many years ago I used a paper (that is in fact one referenced by Sebastian’s video) and some C sample code I found to write an SPH simulator in OpenCL. I had been wanting to write one again but this time get a real understanding of the underlying mathematics now that I have some more tools under my belt. I owe it to Sebastian that I finally started on my implementation and I understand SPH a lot more now.
[0]: https://on-demand.gputechconf.com/gtc/2014/presentations/S41...
[1]: https://mmacklin.com/pbf_sig_preprint.pdf
[2]: https://citeseerx.ist.psu.edu/document?repid=rep1&type=pdf&d...
[3]: https://www.hdm-stuttgart.de/hochschule/forschung/forschungs...
[4]: https://project-archive.inf.ed.ac.uk/ug4/20181074/ug4_proj.p...
vedosity commented on Signtime.apple: One-on-one sign language interpreting by Apple signtime.apple/applecare/... · Posted by u/popcalc
Ok guys, please actually explain instead of just pouncing. Why wouldn't text chat be more appropriate in this situation? I understand that ASL could be much more quick/fluent in a person to person interaction, plus it doesn't need any technical means of writing, whether pen and paper or a computer. But isn't technical support a rare use case where typing speed is not a big bottleneck and where also writing things out can help precision and cut down on having to repeat yourself? Given that resources are always limited, wouldn't it be better to invest in accessibility where that has the most impact rather than a corner case where other workarounds are available?
Written english is often only a second language for Deaf and hard-of-hearing people. Sign languages have their own idioms, culture, and identity; ASL isn’t signed English.
I previously worked at a video relay service company (VRS in the US is a service paid for by the TRS fund through the FCC, and allows Deaf and hard-of-hearing people to make phone calls through video-chat with a ASL interpreter). In written English-based interactions with Deaf and hard-of-hearing colleagues, there is often a communication barrier as there often is with anyone speaking a second language.
In my personal experience working on tickets written by D/deaf colleagues, while sometimes we could communicate by whiteboard or text-based chat, it was indispensable to have the option to discuss the ticket with someone who could interpret present.
vedosity commented on Passage by 1Password: Add passkey support to your app or website blog.1password.com/passag... · Posted by u/donutshop
Passkey folks - can someone explain to me how this makes things more secure?
For example, let’s take Apple. Apple Passkey support is great - I can store a passkey and it syncs through iCloud to all my devices. So my phone can login, my macbook can login. Neat.
But thinking about this a little more, Passkeys on iPhones are secured with FaceID, so to login I have to use FaceID or other biometrics. But on iPhone you can skip the FaceID check if you know the device passcode as fallback. So now, if someone has access to my iPhone and knows my passcode, they have access to ALL my accounts that have Passkeys stored on my iCloud.
Previously, even if an attacker had access to my iPhone, they still wouldn’t be able to login because they don’t know my password. And 1Password itself uses a separate unique password and can’t be skipped with device passphrase.
I’m honestly surprised that we can’t lockdown passkeys on iOS with a separate password/key that’s used to encrypt those. It just seems like I’m giving up on security by switching to passkeys, away from randomly generated passwords. IMHO it should be: FaceID, and if you can’t use biometrics, you HAVE to specify that unique passkey-only encryption phrase to unlock it. Not device passcode.
If not that, then I would have expected passkeys to be factor 1 authentication to directly replace passwords, and then have something else as second factor, such as TOTP/yubikey/SMS auth. But current implementations on any website I’ve seen so far treat Passkey as “ok you’re in”, while going the password route usually triggers a second-factor check.
The passcode fallback isn't available for passwords, just for unlocking the device. In fact, IIRC I don't think you can set up iCloud Keychain without FaceID being enabled...
vedosity commented on The Geomagnetic Storm Continues swpc.noaa.gov/news/geomag... · Posted by u/uticus
> Published: Monday, April 24, 2023 17:26 UTC
The forecast was updated at 1:00PM EDT so I believe it ends Tuesday at 8:00AM EDT
There's a conflicting forecast here[0], that says it's decaying. The graphic for the "decaying" forecast says it's a forecast for 10am EDT. The forecast linked in OP says it's for 2am EDT, which is before the publish date for that forecast. So something's weird with the publish date listed on the forecast, I'm not sure I would trust that date.
Seems like it's decaying, the decaying forecast is linked on their homepage and other space weather sites are forecasting it decaying as well.
[0]: https://www.swpc.noaa.gov/news/geomagnetic-storm-continues-d...
I’ve been looking around the iOS App Store for exactly that app, for playing multiplayer card games with one dealer while on a train etc. Haven’t found the right thing yet, but the App Store is sooo spammy especially when it comes to card game apps. Does anyone know of such a thing?
(As far as I know, the p2p requirements mean that on iOS at least, this cannot be implemented as a web app. Again, would love to learn that I’m wrong.)
One might be able to do this with peer to peer networking through WebRTC. You still need a way to send each phone an SDP offer/answer but maybe this could be done with QR codes.
vedosity commented on Apple’s use of Swift and SwiftUI in iOS 15 blog.timac.org/2021/1219-... · Posted by u/ingve
> an app I developed, Clatters, where I use Swift, SwiftUI, Objective-C and C, the most appropriate programming language being used to solve each specific problem.
Slightly OT, but when might one choose to write part of a new app in Objective-C instead of Swift? From my understanding, Swift can call any ObjC code.
At my employer we use Objective-C++ to interface with some shared C++ code. Swift can use C directly but it can be useful to wrap a C library in an Obj-C interface to be easier to use in Swift.
vedosity commented on Pilot Says Too Many Jumpers Outside Led to King Air Spin avweb.com/aviation-news/p... · Posted by u/graderjs
Amazing footage. I’m sure the video distorts perspective but it looks like they were very lucky the plane didn’t hit a skydiver after spinning.
I know very little about aviation but this seems like a pretty foolish and avoidable incident. I would have thought there would be a procedure for how many people can leave or hang from the back of the plane as it would very obviously affect how it flies. I can’t imagine that sort of manoeuvre is good for the airframe?
At my local drop zone one of our pilots posted this video as a reminder of why we let the pilot know when we have large groups leaving the plane. Large groups outside like this can starve the elevator of airflow, and the sudden change in center of gravity is also a factor.
This seems like a weakness of a king air as a jump plane. The pilot has to throttle down the left engine so jumpers aren’t thrown into the tail on exit by the prop wash. I’m happy with our single-engine super caravan with its wide door so not so many people have to be outside to do big groups like this.