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Comment by codehero | Hacker News Reader

Sell me: I am a crusty old C++ programmer who started numerical and geometric programming on an SGI machine at the turn of the century. I have seen many academic and commercial 3D libraries come, go or perform poorly (vcollide, RAPID, CGAL and many proprietary ones).

It looks interesting but what makes this library any better than what's come before? Why should I learn Rust to use it or really for any computational geometry problem?

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I'm not sure. I certainly would never use a third party collision library because I think collisions are simple enough, but on the larger subject of whether Rust is suitable for game programming; I think so, but cannot confirm. There are a lot of abstractions that require pointer dereference, which is somewhat likely to give you a performance drain, and those abstractions are I guess somewhat harder to avoid than in C++, but not terribly so. Both languages give you the ability to do "data directed design" if you really really want to. My preference for Rust stems from my preference for interfaces, or traits as they are called in Rust. If you want to use features like these then Rust is a no brainer. I have yet to see the true impact on performance these abstractions have however.

What abstractions are you referring to? If you're using traits and writing your functions with generics there shouldn't be any overhead, other than the increased number of instructions, but this is no different than C++ generics. There are trait objects, but these come up somewhat less often then you would think, and have the same performance as using abstract classes/polymorphism in C++. But maybe you're referring to something else?

I am referring to trait objects, and I use those pretty extensively (although I limit my usage because I am making games and want to avoid a pointer dereference). For libraries you might not need them as much, but I definitely find their convenience indespensible for application side programming.

Personally, I've found that most cases you can avoid using trait objects by just using generics, but it obviously depends on your use case. You especially need trait objects to have things like heterogeneous arrays. Trait objects are pretty much the same as C++ classes with virtual methods, with one tweak in the representation. A C++ object is referred to by a pointer to (vtable_ptr, class_fields). A trait object "&Foo" is a fat pointer (vtable_ptr, ptr_to_fields). So a trait object reference is two pointers, but doesn't have a pointer stuck to the front of all of its structs. Method invocation requires you to dereference the vtable, get the function pointer, and call that, passing the data pointer. In C++ you would dereference your pointer, dereference the vtable, then pass the pointer to the function. If anything, I think the Rust method may be faster because you avoid the double de-reference, but it's probably so small as to be unmeasurable in all but pathological uses.

Is this strictly for building shippable software or any time you need collision?

Maybe I am just a web programmer who is really bad at math and programming but this seems like a crazy thing to rewrite for many projects. Even if it only takes a day or an afternoon that is time you could have spent proving if your idea works.

...you just copy your code.

> I certainly would never use a third party collision library because I think collisions are simple enough

Ehhh, I haven't seen a really good, super robust solution for mesh vs. mesh. Deciding whether two meshes collide is simple enough, but robustly pulling out associated information like overlap volumes, penetration depths, etc gets pretty hairy pretty quickly - you start running into similar problems to mesh booleans.

The reason why is that mesh vs mesh essentially should never be used in video games. The accuracy in hitbox bounds and ease of use (making a model mesh your collision mesh) you get from mesh collisions are completely and utterly overshadowed by the performance and lack of accuracy in determining the time of impact. At 60 fps it's really hard to tell the difference in collision between a typical dodecahedron and a sphere. Just use a sphere! (Or, more likely, some aggregate of AABBs, spheres and capsules)

Oh yea, for a game I would almost certainly decompose into some sort of union of easily handled convex shapes. There are definitely applications -- robotics, engineering, hell, even film -- where more accurate mesh-mesh is needed, however, and most solutions are super domain specific and ad hoc.

Haha, I keep forgetting that there are other applications for collision detection than just video games :)

> The reason why is that mesh vs mesh essentially should never be used in video games

Or, alternatively, the reason why mesh vs mesh is never used in video games is that it hasn't been solved well yet and we have to stick to simplified collision geometry and the resulting glitches we get in games?

Mesh vs mesh collisions will never be faster than primitive collisions. Also, simplified collision geometries do not produce glitches in the same way mesh vs mesh collisions do. At most you may have a collision between two primitives where the models don't actually seem to be touching. But this can easily be circumvented. In fact, I would argue that a lot of the physics glitches we've seen are probably the result of using mesh colliders when a simplified geometry would have sufficed.

Check the DOOM 3 source code, they have a pretty good mesh-vs-mesh solver.

i think it's the other way around - if you're using rust, now you have a collision library you can use without having to go back to c++.

I call your bluff on CGAL. What's wrong with a geometry library that can handle exact arithmetic?

Ever used OpenSCAD? Try doing CSG operations with CGAL on complex STL surfaces. Then take those same surfaces with vtk, use their boolean operations. Faster but with significantly more errors.

Not sure what the conclusion is supposed to be here.

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