Game Development Reference
In-Depth Information
rigid transformations while being computationally competitive and easy to imple-
ment. Its only downside is that it does not directly support the blending of joints
with scaling. That said, scaling can be done in a separate pass and added after
the DQ deformation. The newest version of Maya does exactly this in its DQ
implementation. Readers interested in implimenting DQ skinning in their own
engine (as several studios already have) will find sample source codes available
from Ladislav Kavan's website: http://isg.cs.tcd.ie/projects/DualQuaternions/ .
Spline skinning. Another interesting extension of smooth skinning comes from
the paper titled “Fast Skeletal Animation by Skinned Arc-Spline Based Deforma-
tion” [Forstmann and Ohya 06]. This paper contains what I believe to be the best
solution for character skinning. For whatever reason, it has received very little
attention from industry professionals (that I am aware of). What follows is a brief
description of how it can fix the problems with smooth skinning.
The technique involves the creation of spline curves that run through the skele-
ton, giving it a sense of connectivity. Control points of the spline are placed along
the lengths of the bones. This results in a smooth curve that runs through the
skeleton. After the joints are animated, the curve's control points are updated to
lie along the new bone positions (point positions are implicitly defined as a per-
centage along the length between two joints). New control-point positions are
then used to recompute the curve itself each frame.
13.3.5 Deforming with Splines
Mesh vertices are then attached to a parameterized position, u ( t ), along the length
of the skeleton curve. The parameter u ( t ) can be automatically precomputed as
the closest point on the curve, making attachment automatic (and far easier than
the comparatively laborious task of painting weights).
To compute the deformed vertex positions, a new transformation is computed
on-the-fly at the curve position u ( t ). This transform matrix is positioned at u ( t )
with the x -axis pointing down the curve and the y -axis aligned with the twist of
the parent joint. The z -axis is computed as the cross-product of the other axes:
X.x
X.y
X.z
0
Y.x
Y.y
Y.z
0
( Y
×
X ) .x
( Y
×
X ) .y
( Y
×
X ) .z
0
u ( t ) .x
u ( t ) .y
u ( t ) .z
1
Twisting can be added by taking a percentage of the child joint's twisting
rotation and applying that down the length of the parameterized curve using a
linear or exponential falloff. Scaling can be applied similarly.
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