Blender Git Commits

Completed the implementation of bent rest shapes for hair.

Basically follows the Pixar approach from "Artistic Simulation of Curly
Hair".

Target calculation for local non-straight rest shapes.

This is more involved than using simple straight bending targets
constructed from the neighboring segments, but necessary for restoring
groomed rest shapes.

The targets are defined by parallel-transporting a coordinate frame
along the hair, which smoothly rotates to avoid sudden twisting (Frenet
frame problem). The rest positions of hair vertices defines the target
vectors relative to the frame. In the deformed motion state the frame
is then recalculated and the targets constructed in world/root space.

Have to include BLI_utildefines before BKE_collision now to have bool
defined.

Clear forces and constraints together at the start of the time step,
easier to verify.

Fix for missing initialization of local cloth modifier inside particle
systems.

Proper implementation of angular bending springs including jacobian
derivatives for stabilization.

The bending forces are based on a simplified torsion model where each
neighboring point of a vertex creates a force toward a local goal. This
can be extended later by defining the goals in a local curve frame, so
that natural hair shapes other than perfectly straight hair are
supported.

Calculating the jacobians for the bending forces analytically proved
quite difficult and doesn't work yet, so the fallback method for now
is a straightforward finite difference method. This works very well and
is not too costly. Even the original paper ("Artistic Simulation of
Curly Hair") suggests this approach.

Unused function declaration.

Basic solver result feedback from the mass-spring (cloth/hair) solver.

This returns a general status (success/no-convergence/other) along with
basic statistics (min/max/average) for the error value and the number
of iterations. It allows some general estimation of the simulation
quality and detection of critical settings that could become a problem.
Better visualization and extended feedback can follow later.

Calculate bending targets based on the direction of previous segments.

This makes the bending a truely local effect. Eventually target
directions should be based in a local coordinate frame that gets
parallel transported along the curve. This will allow non-straight
rest shapes for hairs as well as supporting twist forces. However,
calculating locally transformed spring forces is more complicated.

Simple debug drawing support inside the implicit solver.

Added a damping term for angular bending springs.

Simple initial implementation of angular bending springs.

These are much better suited for creating stiff hair. The previous
bending springs are based on "push" type spring along the hypothenuse
of 3 hair vertices. This sort of spring requires a very large force
in the direction of the spring for any angular effect, and is still
unstable in the equilibrium.

The new bending spring model is based on "target" vectors defined in a
local hair frame, which generates a force perpendicular to the hair
segment. For further details see
"Artistic Simulation of Curly Hair" (Pixar technical memo #12-03a)
or
"A Mass Spring Model for Hair Simulation" (Selle, Lentine, Fedkiw 2008)

Currently the implementation uses a single root frame that is not yet
propagated along the hair, so the resulting rest shape is not very
natural. Also damping and derivatives are still missing.

Replace the dynamic root transform in the implicit solver data with a
single transform matrix.

Dynamic properties of the transformation are only needed during the
setup phase when they should be read from external data (hair system
roots) and generate fictitious forces on each point.

More scons fixes for physics lib.

Fixed scons build files for new physics subfolder.

Disabled the velocity filtering feature for now, since the weighting is
incorrect.

The voxel grid needs better tool support to make it usable, so fixing
the filtering is not high priority right now.

Added basic filtering feature for velocity smoothing.

This is part of the original method from "Volumetric Methods for
Simulation and Rendering of Hair". The current filter is a simple box
filter. Other energy-preserving filters such as gaussian filtering
can be implemented later.

The filter size is currently given as a cell count. This is not ideal,
rather it should use a geometrical length value, but this is too
abstract for proper artistical use. Eventually defining the whole grid
in terms of spatial size might work better (possibly using an external
object).

Made the voxel grid size for hair interaction configurable and increased
the default to 32.

Conflicts:
source/blender/blenloader/intern/versioning_270.c

Reduced the length threshold for disabling short hairs from 0.1 to 0.01.

Fix for rB71271bb, was replacing the force vectors entirely instead of
adding.