This adds transformations for each hair from world to "root space".
Currently positions and velocities are simply transformed for the solver
data and inverse-transformed when copying the results back to the cloth
data. This way the hair movement becomes independent from the movement
of the emitter object. Eventually the "fictitious" forces originating
from emitter movement can be added back in a controlled way.
http://en.wikipedia.org/wiki/Fictitious_force
Ignoring these fictitious forces or scaling their effect is physically
correct, because in the absence of external forces the hair will always
return to rest position in this root frame.
External forces currently are not yet transformed into the root space.
mode.
Eigen can become very slow in debug mode, which is a bit of a problem.
It relies heavily on compiler optimizations to remove function calls
etc. More optimizations may be desirable, possibly putting the implicit
solver into its own little library and enabling optimizations in debug
mode there could help.
This is not nice at all, but for some reason (possibly time scale) the
old force values are much too high and cause the solver to become
unstable. These will be revisited later anyway, so for now such scaling
should be fine.
Note that goal springs currently are really bad ... They have a factor
on hairs that "fades" goal influence from the root to the tip. The last
point on the hair is completely free, which makes the goal springs
pretty much useless on their own without supporting bend stiffness.
Can only assume this was added to compensate unphysical behavior of
goal springs when using uniform weight, but it's a poor replacement for
true localized bending forces ...
a custom built solver.
The old cloth solver is broken unfortunately. Eigen is a designated
linear algebra library and very likely their implementation is a lot
better (can't compare until it's implemented though).
Only basic gravity is active atm, spring forces, external force fields,
damping and volumetric friction have to be added back by converting
the data into the Eigen format.
