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Cloth data is used both for hair and actual cloth, which makes things
really difficult. The face number was used for distinguishing the two
types (no faces == hair mesh), but the extra hair data necessary for
hair sim is generated by particles and not available for edge-only cloth
meshes. This really needs to be sanitized ...
Conflicts:
source/blender/physics/intern/BPH_mass_spring.cpp
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approach does not work very well.
Using a cross section estimate still causes large oscillations due to
varying hair force based on angles. It also requires a sensible hair
thickness value (particle radius) which is difficult to control and
visualize at this point.
The new model is based purely on per-vertex forces, which seems to be
much more stable. It's also somewhat justified by the fact that each
hair vertex represents a certain mass.
Conflicts:
source/blender/physics/intern/BPH_mass_spring.cpp
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The previous calculation was modulated with the angle between the wind
direction and the segments, which leads to very oscillating behavior.
Now the formula includes an estimate for the geometric cross section
of a hair segment based on the incident angle and the hair thickness
(currently just the particle size). This gives a more stable behavior
and more realistic response to wind.
Conflicts:
source/blender/blenkernel/intern/particle_system.c
source/blender/physics/intern/BPH_mass_spring.cpp
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A stupid hack is needed here, changing the way the factor is applied to
angular bending springs. In cloth sim the bending factor of individual
springs is applied as a mix value between the bending stiffness and a
max value, but this max value isn't even used in hair sim so that
approach becomes useless.
Conflicts:
source/blender/physics/intern/BPH_mass_spring.cpp
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This helps to create some variation in a hair system, which can
otherwise become very uniform and boring. It's yet another confusing
setting in a system that should have been nodified, but only option for
now (broken windows ...)
Conflicts:
source/blender/blenkernel/intern/particle_system.c
source/blender/physics/intern/BPH_mass_spring.cpp
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to support multiple hash identifiers.
Using explicit hashing functions for every sim debug call defeats the
purpose of having a quick feedback system. Now this can be done simply
by passing an arbitrary number of hash inputs (integers) at the end of
the function calls, which are then combined by a system of variadic
macros (based on the ELEM feature). Up to 8 identifiers are supported
currently, but more could be added easily if needed.
Conflicts:
source/blender/blenkernel/intern/particle_system.c
source/blender/physics/intern/BPH_mass_spring.cpp
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density factor (already included in grid velocity).
Conflicts:
source/blender/physics/intern/BPH_mass_spring.cpp
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needs to be substracted instead.
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other values.
Conflicts:
source/blender/physics/intern/BPH_mass_spring.cpp
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The Eigen solver for hair grids uses an extra margin of 1, which means
greater stride values.
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zero vectors, leading to bad density values.
Conflicts:
source/blender/physics/intern/BPH_mass_spring.cpp
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Conflicts:
source/blender/physics/intern/BPH_mass_spring.cpp
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solver input and output.
This uses the central difference method (instead of combined forward/
backward difference), which makes it easier to correctly account for
grid borders.
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samples.
This is just an intermediate method to make sure the density is valid.
Eventually the closest-point method should be used, but for testing
the poisson solver this is easier to debug.
Conflicts:
source/blender/physics/intern/BPH_mass_spring.cpp
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to ensure we always have one layer of empty cells around the fluid.
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This allows setting a target density which the fluid simulation will
take into account as an additional term in the pressure Poisson
equation. Based on two papers
"Detail Preserving Continuum Simulation of Straight Hair" (McAdams et al. 2009)
and
"Two-way Coupled SPH and Particle Level Set Fluid Simulation" (Losasso et al. 2008)
Currently the target pressure is specified directly, but it will be
a lot more convenient to define this in terms of a geometric value such
as "number of hairs per area" (combined with hair "thickness").
Conflicts:
source/blender/physics/intern/BPH_mass_spring.cpp
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Conflicts:
source/blender/physics/intern/BPH_mass_spring.cpp
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with the Lagrangian hair model.
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on the grid.
This uses the Eigen conjugate-gradient solver to solve the implicit
Poisson equation for the pressure Laplacian:
div(grad(p)) = div(v)
As described in "Detail Preserving Continuum Simulation of Straight Hair"
(McAdams, Selle, 2009).
Conflicts:
source/blender/physics/intern/BPH_mass_spring.cpp
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These make passing data between Eigen vector and matrix types and plain
C arrays more convenient.
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Conflicts:
source/blender/physics/intern/BPH_mass_spring.cpp
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This is a bit more awkward for artists to use, but necessary for
a stable solution of the hair continuum calculation. The grid size is
defined by the user, the extent of the grid is then calculated based on
the hair geometry. A hard upper limit prevents bad memory allocation
in case too small values are entered.
Conflicts:
source/blender/physics/intern/BPH_mass_spring.cpp
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This is an artifact of earlier attempts to implement velocity smoothing,
but doesn't work anyway.
Conflicts:
source/blender/physics/intern/BPH_mass_spring.cpp
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hair vertex data.
Conflicts:
source/blender/physics/intern/BPH_mass_spring.cpp
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This is a leftover from previous approach of hair collisions (with
insufficient results). The hair volumetrics actually implements
"collision" with solid objects as well, but uses a Neumann boundary
condition on the main grid for this purpose.
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This is based on the paper
"Detail Preserving Continuum Simulation of Straight Hair"
(McAdams, Selle, Ward, 2009)
The main difference is that hair line segments are used rather than only
rasterizing velocity at the vertices. This gives a much better coverage
of the hair volume grid, otherwise gaps can be produced at smaller grid
cell sizes and the distribution is uneven along the hair curve.
The algorithm for rasterizing is a variation of Bresenham's algorithm
extended onto 3D grids.
Conflicts:
source/blender/physics/intern/BPH_mass_spring.cpp
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Conflicts:
source/blender/physics/intern/BPH_mass_spring.cpp
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This was introducing energy into the system for any factor beyond 0.5
and caused major instability.
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solver step, instead of using the previous step's velocities.
Conflicts:
source/blender/physics/intern/BPH_mass_spring.cpp
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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)
Conflicts:
source/blender/physics/intern/BPH_mass_spring.cpp
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Conflicts:
source/blender/blenkernel/intern/key.c
source/blender/blenkernel/intern/particle_system.c
source/blender/makesrna/intern/rna_particle.c
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solver variants.
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Also added a couple of utility wrapper functions for Eigen types to make
interfacing with plain float arrays and blenlib math easier.
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new mass-spring solver API.
Conflicts:
source/blender/physics/intern/implicit_eigen.cpp
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This is not necessary: the implicit solver data can keep track instead
of how many off-diagonal matrix blocks are in use (provided the
allocation limit is calculated correctly). Every time a spring is
created it then simply increments this counter and uses the block index
locally - no need to store this persistently.
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redundant calculation for counterforces.
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The bend damping factor was hardcoded to the same value as the stiffness.
Now it has its own factor in the settings and button in hair dynamics.
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