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Show HN: BepuPhysics – 3D real-time physics simulation library in C# (github.com)
108 points by rossnordby 29 days ago | hide | past | web | favorite | 25 comments

This is very impressive! But honestly I am here to comment about the humor in the demo video. The section towards the end starting from the shrinkwrapped newts genuinely made me laugh.

I'm glad someone respects newt rights.

Came to say the same thing, fun story, great demo!

Are there any other demos/games built with the library yet?

I know some people are fiddling with it, but I'm not aware of any public projects using v2 yet. It's pretty darn early still- convex hulls got added only a few weeks ago.

I did build it for a reason, though- over the coming weeks I'll be diving into actually making stuff with it. That'll be a nice change of pace.

V2 has been a looooong time coming, kudos to the bepu team. For those wondering why C#, one word; monogame. (And previously, xna)

Thanks! XNA and its descendants have definitely had a big impact.

(I remember early on sketching out the development plan for v2 and coming up with like 9 months of work... Things escalated.)

Great (and humorous) video demo, Could this be made cross-platform using Mono or .NET Core?

Yup! The library itself is .NET Standard 2.0, so it'll run anywhere with a supporting runtime. The only issue is that some older JITs don't understand the SIMD intrinsics and will either barf or run 100x slower.

Should not one use GPGPU for physics to get the best performance? Does not seem like your library does that.

Also, do you have a performance comparison with other physics engines?

I feel a little gross about headlining aggressive benchmarks with other engines, especially since most of them are free and open source just like bepuphysics. Plus, I learned a lot from projects like bullet, box2d and friends. I'd recommend potential users benchmark their use cases for themselves- those are the most trustworthy benchmarks anyway.

Regarding GPGPU, you're correct that bepuphysics v2 is strictly CPU-side, and also that I could likely make a GPGPU version faster. I wrote a blog post about why I chose to go with CPU-only: https://www.bepuentertainment.com/blog/2019/1/16/-but-gpus-a...

The short version is comparative advantage. I have other things to use the GPU on, and the GPU is even better at those things. And driver bugs.

Also, as I've mentioned elsewhere, while I have not done rigorous benchmarking, a casual evaluation showed that bepuphysics v2 on the CPU compares well with physX 4.1 running on a GPU of similar cost. Collision detection heavy scenes tend to favor GPU physX, while solver-heavy scenes tend to favor bepuphysics v2.

(Except for AMD's Threadripper CPUs unclear reasons. I suspect inter-CCX communication kills effective memory bandwidth in the solver. A 1700x is about as fast as a 2950x, and modern Intel cpus with full rate AVX2 can be much faster. Zen 2 should change things significantly.)

Agree with almost everything.

> if you want to run physics serverside, it’ll probably need to run on things that aren’t windows

On the other hand, you’re in control of hardware. For some kinds of projects, CUDA saves lots of time compared to DirectCompute or OpenCL: better libraries (these hand-optimized BLAS, FFT, etc.), better runtime i.e. simpler CPU-GPU interop, and better language, CUDA is relatively modern C++, with templates and constexpr. Unlike graphics, CUDA runs fine on Linux.

I’m not an expert in CUDA but I’ve completed a couple of projects where my clients needed to compute some GPGPU stuff on their servers, or on other people’s servers they’ve rented.

Yup- if you have control over the entire deployment, more reasonable options open up.

For my purposes, CUDA isn't really an option due to needing to run simulations on both servers and unknown clients (ideally with a single codebase), but for other services that never leave the datacenter I'd definitely consider it.

Well done! love the intro vid.


Good stuff, just a question, why c#?

That's a good question. It almost wasn't; there was a period post-WinRT, pre-"microsoft open sources everything" where investing a huge amount of effort in the ecosystem didn't seem like it would make sense. I did consider a more traditional (for the games industry) path of C++, but even simple things like the effort required to keep build times reasonable were... not appealing. And modules kept getting kicked back. Even back then, I was considering rust as a preferred alternative.

And then everything went open source, some fruits of Midori started getting rolled in, and a focus on performance appeared. Stack that on top of the fact that C# has some pretty darn good tools, and I was comfortable jumping in more deeply. Notably, I did not just start making bepuphysics v2 at that point. It's sort of the tip of the iceberg- bepuphysics v2 is C# because all the other private stuff I've built is C#, and it's nice to minimize the developmental and runtime overheads between projects when possible.

Not op, but outside of C++, C# is probably the widest deployed language (on the CPU) in the game industry by virtue of the Unity engine.

To make things a little stranger, it's worth mentioning that I'm not sure if bepuphysics v2 works well with Unity right now. Attempts in the last several months by users trying to upgrade from v1 to v2 had... less than ideal results under that runtime.

The library is .NET Standard 2.0 compliant, though, so in theory it should work. It may require some extra work on the runtime side of things. As time goes on, I'd hope that mono/CoreRT or whatever the AOT story ends up being/CoreCLR converge and Unity can embrace them for all their target platforms. (Burst is also an option, but that would require a massive rework in bepuphysics that I can't really afford to do given that my projects aren't on Unity.)

Erm, why not? Seriously question.

What makes "pure" a notable or (presumably) desirable quality? If bits of it were in C++, and that made it faster, how would that make it less useful?

I included that for a couple of reasons: 1) It's unusual. Historically, C# is more of a businessy thing, with a bit of game stuff on the side, mainly in the form of scripting. Extremely high performance C# code is even stranger. 2) It's more easily portable than a mixed library. It's definitely not impossible to make/use a cross platform mixed library (they're all over the place), but depending only on one toolchain is just less complicated.

So not a big deal, just a notable quirk.

> 1) It's unusual. Historically, C# is more of a businessy thing, with a bit of game stuff on the side, mainly in the form of scripting. Extremely high performance C# code is even stranger.

Unity is C#, no? I remember reading that they are moving most of the traditionally C++ 'backend' engine code to C# now, too.

Edit: Found the article: https://lucasmeijer.com/posts/cpp_unity/

Unity is in the process of moving more of their codebase into C#, backed by some in-house compiler work, yes. And they did indeed use C# extensively before then for game scripting, even if not exactly 'pure' or high performance in the same sense.

The work in Unity's Burst/HPC# is very recent, though, and the CoreCLR/language features I rely on in bepuphysics are still pretty obscure. I wouldn't expect most people outside of the weird niche I live in to know about the ecosystem's progress.

In other words, I was just trying to get an unaware visitor to raise an eyebrow.

We took that distracting bit out of the title above.

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