超纯水系统error2:角色动力学设置

(2010-01-15 19:12:32)转载
标签： 杂谈
利用MAYA模拟massive系统的角色动力学，具有动画的角色系统基础上添加角色动力学，使其具有人性的关节反应，跟碰撞物交互，例如：跑步动画中穿越丛林，碰撞到树木或者物体的时候会自动作出全身反应，更具有真实性，全身为刚体碰撞模拟，并且保证带有正确的骨骼旋转方向和肌腱拉伸系统，保证肌肉的弹性
因为家里的机器太慢，playBlast就死掉MAYA了。迟些我换台机子playBlast,有空的话把教程翻译为中文
下面是在ILM工业光魔上下的原理教程，利用刚体+弹簧模拟物理的秤杆原理来控制骨骼，有看不明白的可以留言问我或者感兴趣的一起研究
I WHY USE DYNAMICS FOR CHARACTER ANIMATION?
Situations in which this technique is most viable is with actions that are nonacting
driven. Most common are where the performance is derived from
external forces affecting the character and where broad motion is needed.
These situations are sometime difficult to keyframe but when driven by
physics can be completed convincingly and quickly.
This methodology is ONLY intended as an addendum to existing animation
tools and rigs.
II BIO-DYNAMICS
In investigating motion dynamics of animals we have found that their anatomy
is designed to perform movements that are within a range of motion. That
motion is executed on axes of rotation. How many axes of rotation a
particular movement needs can be described in two major categories/joint
types – Hinges and Ball Joints.
The most limited range of movement occurs on only one axis – this describes
the functionality of a Hinge joint. The knee for example operates around a
pivot with a given axis of rotation running through it.
The Ball Joint allows for movement across three axes of rotation and usually
places limits on one or more axis in varying amounts. For example, your
shoulder and your wrist can be considered ball joint connections in that you
have three degrees of freedom in movement. However, the range of
movement between the two is different. There is less freedom in the wrist.
If we replicate this behavior between rigid bodies we can create a system of
movement. Depending on how we arrange this system we can create
dependencies that operate according to anatomical principles.
Maya provides three dynamic constraints that combined together provide the
necessary elements for creating our joint types.
PINS
A Pin constraint links two rigid bodies at a specified position. It works
as if the two objects are connected by a metal pin with a ball joint
between its ends.
HINGES
A Hinge constraint constrains rigid bodies along a specified axis.
SPRINGS
A Spring constraint simulates an elastic cord that has a stiffness and a
damping ability to it.
The purpose of the dynamics rig is to connect rigid bodies using a
combination of the constraint types listed above. When the dynamic
constraints are put together into a structure a parent/child dependency is
created between rigid body segments. This creates a forward kinematic
relationship between the rigid bodies so that when simulated, they reflect
forward kinematic movement (i.e. the parent’s movement affects the child’s
location) including the proper limitations in range of movement. The
limitations will be indicative of the structure of the constraint system, which
will either reflect a hinge or a ball joint.
The pin and hinge constraints provide the range of movement but also as the
pivot for movement. So, we can accurately place these constraints at the true
pivot locations of real bones. These two constraints provide the difference
types of motion (single axis or multi-axis). It should be noted that parenting
dynamic constraints and rigid bodies may cause instability in the solver. The
general rules are that pins cannot be parented while hinges and springs can.
There is an inherent problem in 3D in that the hinges and pins provide for a
very large range of motion. This range needs to be reduced by a counter
force applied to the rigid bodies. This counter force replicates the natural
limitations provided by real tendons and muscles. The spring constraint will
be our counter force.
Another problem arises in 3D in that springs are created between the centers
of mass of rigid bodies. In this scenario there is no existing countering force
to another rigid constraint running between the centers of mass. To
counteract or limit a range of movement we need some leverage. A spring
cannot provide torque against movement without being leveraged some
distance away from the centers of mass.
So for either a Hinge joint or a Ball Joint, our hinge and pin constraints will run
between the centers of mass of the rigid bodies and our spring constraints will
be offset. The mathematical reasoning behind offsetting the spring
constraints is further illustrated below.
 
Figure 2.1 – Explanation of offset spring
Torque, the force that makes objects spin can be represented with a vector.
To find the vector torque T we calculate the cross product of the force applied
to the rigid bodies, in this simple case g, the gravity, and the vector r running
from the hinge to the center of mass of the rigid body. If this is the only force
on the system (g) the torque (T) causes the rigid bodies to spin around the
hinge. To counter balance that force and be able to control the system we
need another torque force T2 applied in the opposite direction. To create T2
we use the force of a spring (F) offset by a distance (b).
The longer the vector b is, the longer and stronger the T2 vector is going to be
and vice versa. This is why we cannot have the spring directly through the
center of mass of the two rigid bodies. The vector b would be 0, the cross
product would also be 0 and consequently the torque T2 would be 0 causing
no amount of opposing force.
In order to offset the spring location (our explanation of the counter force
above) we must create other rigid bodies to host the spring constraint. The
spring must be “connected” to the rigid bodies as well. For our spring holders
we will use two cubes turned into rigid bodies with the spring between them.
For each connection/joint between rigid bodies we must decide which body
will act like a parent and which like a child. As mentioned above the goal is to
create a forward kinematic dependency. This is accomplished by how the
“spring cubes” are setup. By turning one spring cube active and the other
passive the force of the spring will affect the active spring cube. If the inactive
spring cube can “inherit” its transforms from the object we want to be the rigid
“parent”, the transforms of the rigid “parent” will ultimately drive the location of
the active spring cube. If the passive spring cube is parented underneath the
rigid parent its position will not update properly when the animation rig drives
the position of the rigid bodies. To bypass this problem we point constrain the
passive cube to a locator that is a child of the rigid parent.
As the parent moves, the inactive spring cube follows causing the spring to
change length driving the placement of the active cube. The active spring
cube is “attached” via hinge constraints (the rotation torque hinges in Figure
2.2) to the rigid “child” helping to drive its transformations. Two hinges at 90
degrees of one another produces two forces that cancel each other out thus
creating a dynamic parent relationship. This is how the force of the spring is
transferred to the rigid child.
So, the combination of the hinge or pin constraint (providing a pivot and range
of motion) and the leveraged/offset spring (torque) help dictate the
parent/child relationship between the rigid bodies.
The existence of the spring cubes is only for offsetting our spring counter
force. They should not contribute in any way to the simulation except
providing a counter force and helping drive the transformations of the rigid
child. Therefore, they should not have any relevant mass, friction, damping,
bounciness or collsion. The script that generates them takes care of setting
those attributes for you.
Anatomy of a Hinge Joint
Below is the layout of a Hinge Joint. There is a hinge constraint created
between the two rigid bodies and a single offset spring.
 
Figure 2.2 – The Hinge Joint Layout
Anatomy of a Ball Joint
Figure 2.3 represents the layout of a Ball Joint. There are two offset springs
and a pin connecting the two rigid bodies.
 
Figure 2.3 – Ball Joint Layout
It is important to not that the further away you place the offset springs the
more leverage you create between the spring and the hinge/pin constraint.
The torque will be provided to help offset or counter the amount of rotation the
hinge/pin wants to do between the rigid bodies helping to stabilize the system.
In addition, for Ball Joints another method of stabilization is to create two Ball
Joint systems for one connection between rigid bodies. For example,
between the hips and the waist and the waist and the chest it is wise to put
two Ball Joints at each position. When the character bends or leans in any
direction there is a spring helping to balance the amount of movement. This
dual Ball Joint configuration also helps in that the placement of the spring
cubes can be much closer to the rigid bodies instead of further away if only on
Ball Joint existed. This can help in visually deciphering what is occurring in
the rig.
原文来源：http://blog.sina.com.cn/s/blog_60b3e1ab0100h3zn.html