Skip to content

yoanlcq/boulette-physiques

master
Switch branches/tags

Name already in use

A tag already exists with the provided branch name. Many Git commands accept both tag and branch names, so creating this branch may cause unexpected behavior. Are you sure you want to create this branch?
Code

Latest commit

 

Git stats

Files

Permalink
Failed to load latest commit information.
Type
Name
Latest commit message
Commit time
 
 
 
 
src
 
 
 
 
 
 
 
 
 
 
 
 

Boulette Physiques

Basic physics using fixed-point or integers. For determinism, stability and consistency across all setups (useful for networked simulations using the lockstep model).
Prerequisites for the test program are SDL2 and SDL2_ttf.

Evolution and known problems -- Please read before reviewing --

I was initially planning to implement this physics engine the way I'm used to : have collision primitives, and write functions to detect collisions between them.
The following (legacy) files show my initial efforts :

  • include/boulette/aabb.hpp
  • include/boulette/box.hpp
  • include/boulette/disk.hpp

But soon I needed to implement velocity and acceleration, so I figured I'd do it the old way :

void Physics::update() {
    //For each object...
    obj.position += obj.velocity;
    obj.velocity += obj.acceleration;
}

I saw no problems with it, until I saw this article from Gaffer on Games and basically learned that what I was doing sucked pretty bad because of the accuracy loss over time (the example shown by the author is really convincing).
I then moved all my efforts into implementing a Verlet Integration-based physics system, which is, right now, this project's main selling point.
It (the algorithm) appears quite good for cloth simulation, and was also used in "Hitman : Codename 47" for ragdolls and other stuff.

The file is include/boulette/verlet.hpp. One can test it using various types, by changing the unit and real typedefs in include/TestVerlet.hpp (this is documented in that file).

There's no render-time simulation-space to screen-space transformation, but it would be a nice feature for TestVerlet to implement (and perhaps it would allow integer-based simulations, since the simulation wouldn't be limited by the window's size).

Known problems

  • Picking vertices with the mouse deforms the edges - this is because the update step's frequency is known to be slower than the display rate. I tried a lot a things to fix this, but none are satisfying (it either caused jitter or made the code more obscure).
  • Sometimes, enabling friction seems to "extend" bodies as if they grew invisibly. I haven't been able to prove why.
  • The test program can get slow pretty easily - I blame it on SDL2's renderer which doesn't allow me to cleanly render all of my objects at the same time. I would probably be wise to do this in proper OpenGL.

On Data-Oriented Design

I also took it as an opportunity to put what I've learned about Data-Oriented Design (DOD) into practice. However, since it looks like OOP's nemesis (while still valuing understandable code), the Verlet System's code is a bit unusual, so I took extra time to document it.

DOD values the understanding of the target hardware and assembly, which is why I concern myself with the layout of the data, the access patterns, and the assembly output (even though it's pointless because SDL2's renderer is one big overhead).
The files in sse2_tests show my experiments with SIMD instructions, and finally helped me understand why the assembly output of my program was not what I expected :

  • First, methods seem to be inlined, which is why GDB's disas won't allow me to inspect most of them. This can be "solved" using GCC's __attribute__((noinline)).
  • The compiler cannot prove that my arrays are aligned on a 16-bytes boundary, so it has to generate some extra instructions to handle potentially misaligned data.
  • The compiler cannot prove that my float/int arrays have an element count that is a multiple of 4 (the SIMD loop could cause a segmentation fault because of this, so extra checks are performed).

About

Boulette physiques. A naïve, but deterministic physics engine.

Resources

License

Stars

Watchers

Forks

Releases

No releases published

Packages

No packages published