Blender Git Loki

Fix for effector force fields on hair, using the world-to-root
transformation.

Add weak repulsion forces to hair collision response to keep the hair
at a the margin distance ("outer" softbody margin).

This has to be clamped arbitrarily unfortunately, otherwise the
repulsion force can add too much energy into the system. A factor of
4 * restitution impulse seems to give good results for now, this can
be refined later on if necessary.

Added back the velocity smoothing implementation.

This is now also decoupled from the internal solver data. The grid is
created as an opaque structure, filled with vertex or collider data
(todo), and then forces can be calculated by interpolating the grid at
random locations. These forces and derivatives are then fed into the
solver.

Fix for Sintel hair bug.

The hair solver needs sane input to converge within reasonable time
steps. In particular the spring lengths must not be too difference
(factor 0.01..100 or so max, this is comparable to rigid body simulation
of vastly different masses, which is also unstable).

The basic hair system generate strands with equally spaced points, which
is good solver material. However, the hair edit operators, specifically
the cutting tool, can move points along the strands, creating tightly
packed hair points. This puts the solver under enormous stress and
causes the "explosions" observed already during the Sintel project.

The simple solution for now is to exclude very short hairs from the
simulation. Later the cutting tool should be modified such that it
keeps the segments roughly at the same length and throws away vertices
when the hair gets too short (same goes for the extension tool).

The hair system should have a general mechanism for making sure that
situations such as this don't occur. This will have to be a design
consideration for replacements in any future hair system.

Minor fix when gravity code is disabled.

Removed unused code.

Conflicts:
source/blender/physics/intern/implicit_blender.c

Added a calculation function for the fictitious forces introduced by
moving hair root reference frames.

This calculates Euler, Coriolis and Centrifugal forces which result
from describing hair in a moving reference frame.
http://en.wikipedia.org/wiki/Fictitious_force

Fix for own misconception of fictitious forces in the moving hair root
frames.

These forces don't have to be calculated for each individual
contribution. Rather they can be split off and be calculated on top of
the basic force vector rotation (todo).

Added back spring force definitions outside the implicit solver.

There are currently 3 types of springs: basic linear springs, goal
springs toward a fixed global target (not recommended, but works) and
bending springs.

These are agnostic to the specific spring definition in the cloth system
so hair systems can use the same API without converting everything to
cloth first.

Conflicts:
source/blender/physics/intern/implicit_blender.c

Main cloth force calculation function outside of implicit core code.

Still misses spring forces.

Hair volume calculation is now in its own file.

Code is currently disabled until the other main forces are in place.

Moved the unused goal force calculation function to the main mass-spring
source file.

Moved most of the main cloth solver function out of implicit code core.

Force calculation is disabled, will follow shortly.

Moved init/free functions for solver data out of implicit core.

This also initializes spring matrix indices (off-diagonal 3x3 blocks),
which now uses a new API function.

Moved "set_positions" for cloth out of core implicit solver.

API for the solver now has functions for setting of vertex motion state
and the associated root transform data.

Renamed functions to make them explicitly refer to cloth, and split the
create/free functions for solver data off from the cloth solver.

Moved the cloth solver code into a new subfolder/library inside Blender
code.

The implicit solver itself should remain agnostic to the specifics of
the Blender data (cloth vs. hair). This way we could avoid the bloated
data conversion chain from particles/hair to derived mesh to cloth
modifier to implicit solver data and back. Every step in this chain adds
overhead as well as rounding errors and a possibility for bugs, not to
speak of making the code horribly complicated.

The new subfolder is named "physics" since it should be the start of a
somewhat "unified" physics systems combining all the various solvers in
the same place and managing things like synchronized time steps.

Desperate attempt to get stable collisions: Disable restitution and
handle only one collision contact at a time.

Collision still randomly explodes, even with differing results on the
same file. This could indicate a threading issue, possibly also related
to the dependency graph since multiple objects are involved in
collisions.

Revert "Ignore velocity changes when the cloth solver does not converge."

This reverts commit c52b8ae818844965d56714a71255408873275dc1.

Sadly, at this point solver convergence is an exception rather than the
rule... Individual hairs can "explode" easily and thus disable the whole
simulation, which isn't helpful either.

Ignore velocity changes when the cloth solver does not converge.

This helps keep the simulation stable as long as there are only a few
substeps that become too constrained for the solver.

Eventually we need better feedback about these solver results, so that
artists can tweak situations specifically to resolve bad solver results.
This is somewhat similar to the camera tracker, which also can run into
cases that cannot be resolved and have to be fixed manually.