BEPUphysics
ConvexBodyProximityQueries.jl
Our great sponsors
BEPUphysics | ConvexBodyProximityQueries.jl | |
---|---|---|
5 | 1 | |
2,126 | 22 | |
4.2% | - | |
8.9 | 1.8 | |
21 days ago | over 2 years ago | |
C# | Julia | |
Apache License 2.0 | GNU General Public License v3.0 or later |
Stars - the number of stars that a project has on GitHub. Growth - month over month growth in stars.
Activity is a relative number indicating how actively a project is being developed. Recent commits have higher weight than older ones.
For example, an activity of 9.0 indicates that a project is amongst the top 10% of the most actively developed projects that we are tracking.
BEPUphysics
-
Current state of 2D game code-first frameworks?
The best pure-C# physics library (hands-down) is bepuphysics2, which unfortunately is mainly a 3D physics library, but could be used for 2D if you wanted to get your hands dirty.
- Physics Engine
-
Open Source C++ Physics Libraries for Dedicated FPS Server?
Bepu Physics is pretty good and is written in really optimized C#, the author's blog post are really interesting to read.
-
GJK: Collision detection algorithm in 2D/3D
The usual approach is some form of sweep to get a time of impact. Once you've got a time of impact, you can either generate contacts, or avoid integrating the involved bodies beyond the time of impact, or do something fancier like adaptively stepping the simulation to ensure no lost time.
If the details don't matter much, it's common to use a simple ray cast from the center at t0 to the center at t1. Works reasonably well for fast moving objects that are at least kinda-sorta rotationally invariant. For two dynamic bodies flying at each other, you can test this "movement ray" of body A against the geometry of body B, and the movement ray of body B against the geometry of body A.
One step up would be to use sphere sweeps. Sphere sweeps tend to be pretty fast; they're often only slightly more complicated than a ray test. Pick a sphere radius such that it mostly fills up the shape and then do the same thing as in the previous ray case.
If you need more detail, you can use a linear sweep. A linear sweep ignores angular velocity but uses the full shape for testing. Notably, you can use a variant of GJK (or MPR, for that matter) for this: http://dtecta.com/papers/jgt04raycast.pdf
If you want to include angular motion, things get trickier. One pretty brute forceish approach is to use conservative advancement based on distance queries. Based on the velocity and shape properties, you can estimate the maximum approaching velocity between two bodies, query the distance between the bodies (using algorithms like GJK or whatever else), and then step forward in time by distance / maximumApproachingVelocity. With appropriately conservative velocity estimates, this guarantees the body will never miss a collision, but it can also cause very high iteration counts in corner cases.
You can move a lot faster if you allow the search to look forward a bit beyond potential impact times, turning it into more of a root finding operation. Something like this: https://box2d.org/files/ErinCatto_ContinuousCollision_GDC201...
I use a combination of speculative contacts and then linear+angular sweeps where needed to avoid ghost collisions. Speculative contacts can handle many forms of high velocity use cases without sweeps- contact generation just has to be able to output reasonable negative depth (separated) contacts. The solver handles the rest. The sweeps use a sorta-kinda rootfinder like the Erin Catto presentation above, backed up by vectorized sampling of distance. A bit more here, though it's mainly written for users of the library: https://github.com/bepu/bepuphysics2/blob/master/Documentati...
ConvexBodyProximityQueries.jl
-
GJK: Collision detection algorithm in 2D/3D
I should be writing a thesis about AI but got inside the collision rabbit hole so I have this fresh.
From the description of the algorithm you are doing I think you are thinking about Lin-Canny or V-Clip, which certainly may have that kind of numerical error problems.
GJK has also numerical problems but they are different. In principle it shouldn't be affected by coplanarity of several faces since you just need the vertex with highest support for a given direction. It could be a problem if you find the support point by hill climbing from vertex to vertex. GJK however does have numerical problems but they are of a different kind related to the degeneracy of the simplices it computes.
But you are so right about the subtetly of the problem: there is a very fine thread between infinite looping and incorrect answers. I have been bitten by this trying to implement geometric algos. There should be a special hell for people that output coplanar faces.
I know one of Bullet Physics/MuJoCo has the GJK, not remember which one. If anyone is curious I know of two Julia implementations:
https://github.com/JuliaRobotics/EnhancedGJK.jl
and my favorite: https://github.com/arlk/ConvexBodyProximityQueries.jl
This latter one is great as you are just required to implement the support function and are ready to go. Julia performance is great if you are concerned about using a dynamic language (i.e: ~2us for collision between two convex bodies of 1000 faces each)
Finally, about the convex hull computation it looks like some kind of solved problem, I mean, O(n log(n)) for 3D. Wrong!!!! QHull in this regard is fantastic as it has several heuristics to solve problems caused by finite precision, not to mention that I think worse case is O(n^2) as it doesn't implement the asymptotically optimal algo (not sure...). If you scale to more dimensions, which could happen even in if 3D because you transformed your problem to a convex hull problem you will be hit with O(n^2), bad news. There are several other libraries (CCD, LRSLib and more) that allow you to use arbitrary precision but you will get something like a 100x penalization for the luxury.
What are some alternatives?
JoltPhysics - A multi core friendly rigid body physics and collision detection library, written in C++, suitable for games and VR applications.
Stride Game Engine - Stride Game Engine (formerly Xenko)
MonoGame - One framework for creating powerful cross-platform games.
Xenko
Nez - Nez is a free 2D focused framework that works with MonoGame and FNA
osu-framework - A game framework written with osu! in mind.
CocosSharp - CocosSharp is a C# implementation of the Cocos2D and Cocos3D APIs that runs on any platform where MonoGame runs.
UnrealCLR - Unreal Engine .NET 6 integration
VelcroPhysics - High performance 2D collision detection system with realistic physics responses.
FNA - FNA - Accuracy-focused XNA4 reimplementation for open platforms
Wave Engine - This repository contains all the official samples of Evergine.
UrhoSharp - Code to integrate with the Urho3D engine