Blender Git Loki

Removed arbitrary factor 2.0 on volumetric hair friction factor.

This was introducing energy into the system for any factor beyond 0.5
and caused major instability.

Fill the hair volume grid with the _new_ velocities from the first
solver step, instead of using the previous step's velocities.

Merge branch 'master' into viewport_experiments

Debug drawing feature to visualize the hair continuum grid.

Perform grid-based velocity smoothing for hair outside of the implicit
solver step.

Calculating forces and jacobians from linearly interpolated grid values
is problematic due to discontinuities at the grid boundaries. The new
approach of modifying velocities after the backward euler solver step
was suggested in a newer paper

"Detail Preserving Continuum Simulation of Straight Hair"
(McAdams, Selle 2009)

Merge branch 'master' into gooseberry

Experimental color wire-frame (for testing in the studio)

Fix for warnings/errors

Merge branch 'master' into hair_immediate_fixes

Removed unused variable.

Reorganizing hair dynamics UI layout a bit to make settings slightly
more intuitive.

Also removed a couple of unused or useless features from the UI:
* collider friction is unused and replaced in favor of true collision
* spring damping refers to structural springs (stretch), which is
not noticable in hair due to extreme stiffness atm.
* pressure factors are not sure since this feature is too unstable atm.

Unified the main Eigen solver function a bit for constrained/unconstrained
solver variants.

Moved render simplification function for particle distribution into the
distribution code.

Disabled the hair preview feature in the code, due to limited usefulness.

Interpolating hairs in a meaningful way is a quite difficult problem.
It works ok-ish for child hairs as long as they don't move, but with
motion the direction of interpolated hairs can become quite nonsensical
quickly.

Probably a better way to create simulation previews is to limit
simulation to a particular well-defined area (like render preview
borders), but run it in full detail in that area. The error from
omitting some interaction should still be much less than orientation
errors from interpolation.

Updating Eigen implicit dynamics solver implementation to adhere to the
new mass-spring solver API.

Removed unused line.

New hair editing feature "Shape Cut", for cutting hair based on a mesh
shape instead of a brush tool.

The brush cutting tool for hair, while useful, is not very accurate and
often requires rotating the model constantly to get the right trimming
on every side. This makes adjustments to a hair shape a very tedious
process.

On the other hand, making proxy meshes for hair shapes is a common
workflow. The new operator allows using such rough meshes as boundaries
for hair. All hairs that are outside the shape mesh are removed, while
those cutting it at some length are shortened accordingly.

The operator can be accessed in the particle edit mode toolbar via the
"Shape Cut" button. The "Shape Object" must be set first and stays
selected as a tool setting for repeatedly applying the shape.

Switched back to blender implicit solver for merge.

Moved particle code for distributions into own file to make hacking
easier.

This code is badly broken and needs to be replaced, but at least having
a workable code structure might help with quick hacks to fix the worst
cases.

Ported the remaining implicit solver functions for Eigen.

Also added a couple of utility wrapper functions for Eigen types to make
interfacing with plain float arrays and blenlib math easier.