rgreid: added new "Spinning top" demo to compound.html.

[rgreid]

Two cylinders demonstrating gyroscopic precession, as per my email.

[schteppe]

Hi,
Thanks for the demo! Actually, Cannon.js does not have correct physics for gyroscopic forces. These are often omitted in physics engines because they make the simulations less stable. Your demo seem to work quite well anyway, and I'm a bit surprised that it works so well without gyro forces.

Stefan

[rgreid]

Thanks Stefan. I understand the stability issues. Do you have a rough estimate how complex the code changes would be to implement gyroscopic forces? i.e. if I set a summer student working on it.

[schteppe]

Hm. Not sure. The actual force is computed in just a few lines of code, see the implementation in Bullet physics here: http://bulletphysics.org/Bullet/BulletFull/btRigidBody_8cpp_source.html#l00260

What I'm unsure about is whether this math should go into the constraint equations somewhere. In that case, we're talking complicated physics stuff.

[rgreid]

ODE implements gyroscopic forces directly in the main stepping code. It also uses an approximation [1] to make things more stable: https://bitbucket.org/odedevs/ode/src/a2104504c78b79cd9a08a6d9de49260304786015/ode/src/step.cpp?at=default#cl-445

[1] "Stabilizing Gyroscopic Forces in Rigid Multibody Simulations" (Lacoursière 2006)

[schteppe]

If it's nothing more than adding some force, then it should be simple to implement. Testing would take take most time.