Graphics Reference
In-Depth Information
Wind input
W
3
W
4
W
1
W
2
Figure 1.5.
A single wind input is converted to four wind directions to generate ran-
domized effects and prevent hair clumping.
wind magnitude gets modulated by using the sine function and frame number as
below:
=(sin
2
(frame
wind magnitue
×
×
0
.
05) + 0
.
5)
.
Equation (1.6) shows how wind force is calculated and applied to position
update.
W
1
,
W
2
,
W
3
,and
W
4
are the four wind vectors, and the strand index is
used to add more randomization:
a
= ((strand index)%20)
/
20
,
W
=
a
×
W
1
+(1
−
a
)
×
W
2
+
a
×
W
3
+(1
−
a
)
×
W
4
,
V
=
P
i
−
P
i
+1
,
(1.3)
f
=
−
(
V
×
W
)
×
V,
Δ
t
2
.
P
i
+=
f
×
The number 20 in the first equation is an arbitrary choice,
W
is an interpolated
wind vector by
a
from four wind vectors, and Δ
t
is timestep.
The wind is applied after the shape constraints and is not a part of integration
because shape constraints are too strong and could cancel its effect if it is applied
as an external force during integration. The key point of calculating wind is to
create randomized direction and periodic magnitude. So it is more empirical than
physically correct.
1.7 Authoring Hair Asset
The TressFX demo and Lara Croft's hair assets were created using Autodesk
Maya. (See Figure 1.6.) However, it can be authored using any DCC (digital
