Back Setup
by Rick Kogucki, April 2004
Introduction:
This tool sets up control objects for a character's back, including the
advanced twist controls in maya version 6 that allow twisting controls
from both ends of a spline ik.
This information is intended to help you work with or customize the
back setup bonus tool. It explains what the script does, what its
assumptions are and other ways you can tweak the setup.
How to use
it;
You need to have a skeleton already made with a root joint and spine
joints. Select the first spine joint next to the root and the last
spine joint. Select the options you want and press the Rig Back button.
If you are using your own control objects, you must load them into the
option box before selecting the joints and pressing the Rig Back
button. Once the rig is created you can animate the control objects
(default poly cubes or custom objects) or clusters to control your back
motion. This tool integrates well with the foot setup tool in bonus
tools.
Assumptions
and Limitations:
The script assumes that your character has a vertical spine and faces
forwards on the +Z axis. The script will flip the involved joints axes
so that Y points in -Z
and X points down the bone. These orientations are the most common in
rigging. No choice is given here, so beware if your
direction of your axes differs. It does not work well with bound
surfaces as they will twist if the joint axis is reset by the script.
You may be able to compensate for this by rotating the joint axis after
the fact, but better to create the setup before binding or have the
joint axes point the same way as the script would . The script isn't
made to redo an existing setup, so you need to either undo, or delete
the control objects, clusters, spline ik handle and curve. You can keep
custom control objects, but you will need to delete the constraints
from the root if you chose "Use Bottom for Root" in the options.
What the
tool actually builds:
It creates a curve and the
spline ik handle between the selected joints and then creates a cluster
for each cv on the curve. It builds polygon cubes for controls and uses
them for the start and end controls of the spline ik, or uses objects
you load in the options window. It takes advantage of the new
advanced twist controls for spline ik in Maya
version 6, allowing twist controls at either end of the
spline ik, and adjusts the settings best for a spine.You can see the
control objects assigned to the ik handle in the Attribute Editor under
IKSolver> Advanced Twist Controls.
There are many choices in how you can use this feature, but the script
settings should be useful for a spine.
The clusters are parented to the top or bottom spine controls. Moving a
control gives a gross change in the shape of the spine. The clusters
can be used for finer control. Use Bottom Control Object for Root
allows you to use the bottom control object as a point and orient
constraint for the root/pelvis.
More on
Using the tool:
To use your own objects as the top and bottom control for the spine,
you need to turn on the flag and then select the appropriate object and
hit the Get button. Alternatively, you can type in the name of the
object and hit Enter. The script won't move your objects, so place them
where you want them to be first. Fewer clusters may mean less detail in
spine shape, but its also less to animate and tends to a smoother
looking spine. Once you hit the Rig Back button and maya creates the
rig, the Advanced Twist Controls (Attribute Editor for spline ik
handle, under IK Solver Attributes) shows the objects listed as the
World Up Objects. These are effectively aim constraints for the spline
ik. You can adjust the distribution of twisting down the joints with
the Twist Value Type. You can use a ramp to distribute the twisting
down the joint chain and animate it if you want to change the
distribution without moving shoulders and pelvis.. If you aren't
creating a biped spine you might want to experiment with the various
type of twist controls offered.
You can tweak the script or adjust the advanced twist controls
after using the tool to modify the controls. Start from the default
settings. For instance, on a quadruped you probably want the
joint Y axes pointing in world +Y, and pointing close to -Z (default)
will cause flipping if you move the top control in Y. To make the
handle compatible for a horizontal spine as in a quadruped, change the
Up Vectors for the Advanced Twist Controls to 0,1,0 and 0,1,0. This
will stop the flipping and allow you control over a horizontal spine.
You might find that the setup can be good for the long neck of an
animal if you skip using the bottom control for the root.
You may not want to move the whole back with the cube control objects
but rather have a control for the whole torso which would be a parent
for the back controls. You could re-parent the clusters to a torso
control and animate the clusters separately leaving only the
first and last cluster parented to the control objects. Or you could
animate them while still parented to the control objects These
choices really get down to how you want to animate. If you do use a
torso control, you'll need to use the bottom control as a constraint
for the root (Use Bottom Control Object for Root in options, or
constrain it yourself). You need to remember too, that the back
controls are only a part of a character rig and may need to be
constrained or parented to higher level controls to stay with the
charcter as it moves..
.