Rigidbody Physics

[RiscadoA]

Checklist

• Current Path:

1. Transition to TGS Soft solver
2. Implement restitution and friction (might make sense to implement physics materials at this point unlike what I initially thought)
3. Implement torque and angular velocity in components and add them to integration systems
4. Implement contact points on collision
5. Implement solving for collisions with rotation

• Detour
  Upon discovering that XPBD is patented by NVIDIA, it may be a good idea to change our solver at this stage. In Erin Catto comparison of multiple solvers in Solver2D, TGS Soft seems to be a good bet. We should look more into it before doing anything else.

  • Convert to TGS Impulse based solving #1269

• Constraints

  • Switch to constraint-based collision solving #1015
  • Implement Spring Constraint #1237
  • Implement Rod Constraint (Fixed distance) #1238
  • Implement Max distance Constraint (Rope) #1239
  • Implement Min distance Constraint #1240
  • Create constraint sample to show off the constraints #1241
  • Add hinge joint

• Rigidbody Physics v0.1

  • Expand PhysicsForce and PhysicsImpulse components to handle rotation #865
  • Handle body rotation on integration #1242
  • Add ability to calculate contact point for collision and store it for each entity #1243
  • Add friction on constraint solving #1244
  • Easier way to activate/disable gravity for bodies #1245
  • Add trigger areas #1246
  • Add rotation on constraint solving #1272
  • Implement restitution #1275
  • Make all magic numbers in the solver easily configurable #1281

• Stability and Optimization

  • Look into stability of the simulation (particularly collisions since they are still kinda trying to be crazy) #1247
  • Implement Contact Cashing #1355
  • Slop/Warm starting #1248
  • Sleeping bodies #1249
  • Stress test the physics #1250
  • Check correctness of epsilon value for friction (1e-6)

• Rigidbody Physics v0.2

  • Implement non uniform mass distribution for bodies #1251
  • Allow disabling rotation on 1 or more axis #1252
  • Implement Static Bodies #1253
  • Different physics materials #1254
  • Divide physics material friction into static and dynamic
  • Look into how we can make fully elastic collision (even with bounciness at 1 bodies will loose some energy, this may require messing with damping on integration)

• Docs

  • Improve documentation with use examples (and tags).

• Other known issues

  • Applying impulses without delay on physicsApplyForcesTag does not work (essentially on first frame)
  • Decide how to represent infinite mass (and inertia) for bodies.
  • Currently the bodies rotate if falling straight into the ground, which shouldn't happen, warm-starting and contact cashing could solve this, but it's yet to be seen.
  • Look into rapiers way of applying friction in 3D.
  • Currently bodies behavior still looks a bit weird, we should look into this, particularly, soft constraint parameters

Problem

Currently our physics engine completely disregards rotation.

References

Overall solver stuff:
High level description of the solver:
https://box2d.org/posts/2024/02/solver2d/
Solver2D implementation:
https://github.com/erincatto/solver2d/blob/main/src/solve_tgs_soft.c
Box2D v3.0 implementation, only got restitution from here at this point:
https://github.com/erincatto/box2c/blob/1d7d1cf14722a06f6a7b1c3b0850e70bcbec8558/src/contact_solver.c#L347
Iterative Dynamic Paper, not the easiest to read, can't remember exactly what I got from here:
https://box2d.org/files/ErinCatto_IterativeDynamics_GDC2005.pdf
Sequential Impulses, very relevant:
https://box2d.org/files/ErinCatto_SequentialImpulses_GDC2006.pdf
Soft Constraints, also very relevant:
https://box2d.org/files/ErinCatto_SoftConstraints_GDC2011.pdf
Formulas for collision response, where I got the separation impulse formula without the rotations:
https://research.ncl.ac.uk/game/mastersdegree/gametechnologies/physicstutorials/5collisionresponse/Physics%20-%20Collision%20Response.pdf
Allen Chou post on the matter:
https://allenchou.net/2013/12/game-physics-resolution-contact-constraints/

Only Friction:
https://www.gamedev.net/forums/topic/613945-sequential-impulse-bias-velocities/
https://pybullet.org/Bullet/phpBB3/viewtopic.php?t=8658
https://www.gamedev.net/forums/topic/673822-contact-tangents/ (what I meantioned on github)
https://gamedev.stackexchange.com/questions/179606/how-to-calculate-plane-corner-vertices-from-plane-origin-point-and-plane-normal (find tangents for friction)

Angular Motion:
https://research.ncl.ac.uk/game/mastersdegree/gametechnologies/physicstutorials/3angularmotion/Physics%20-%20Angular%20Motion.pdf

Manifolds and collision points:
https://research.ncl.ac.uk/game/mastersdegree/gametechnologies/previousinformation/physics5collisionmanifolds/2017%20Tutorial%205%20-%20Collision%20Manifolds.pdf
https://research.ncl.ac.uk/game/mastersdegree/gametechnologies/physicstutorials/4collisiondetection/Physics%20-%20Collision%20Detection.pdf
Also used the structures to hold the date in Box2D/Avian for reference
Code: https://research.ncl.ac.uk/game/mastersdegree/gametechnologies/previousinformation/csc8503coderepository/

Collision Response:
https://research.ncl.ac.uk/game/mastersdegree/gametechnologies/physicstutorials/5collisionresponse/Physics%20-%20Collision%20Response.pdf
Box2D/Avian/Box2c