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1 Tutorials LightWave 3D Simple IK Chains em Qua Jan 26, 2011 3:17 am


by Joel B Fant and William "Proton" Vaughan

Part I: Preparation

Hi folks, welcome to a tutorial
on simple IK chains. I'll be assuming that you
have worked with LightWave enough to know how
to get around fairly well. We'll be using a scene
and object supplied by William "Proton" Vaughan
for this exercise. The following is a link to
the source files in ZIP format.

Inverse Kinematics is goal-driven.
You set the hierarchy chain (bones or objects)
to try to meet a goal object. You set the items
in the hierarchy to follow the goal, and the
solution will try to place that item's pivot
as close as it can to the goal object's pivot.
You can also set multiple items in the hierarchy
to follow different goals.

Ok, let's get at this. Open
up Layout if you haven't done so already, and
clear the scene if it is not. Add the ik_proton.lwo
object into the scene. The object already contains
skelegons, so use Convert Skelegons into Bones
and we'll have some bones to setup for an IK
chain. If you're unfamiliar with it, Convert
Skelegons is found under the Items tab > Add > Bones

Now we have 12 bones to adjust
settings for. Start with the bone named Base:
select it, and open its Item Properties panel.
Set the Bone Weight Map to the map named "Base." You
won't need to mess with any other settings. Continue
up/down the hierarchy, and set the weight maps
for bones Rope01 through Rope09 to the map named "Rope," and
set the weight map for the Rock Bone to the map
named "Rock." I bet you already figured
that out, though.

If you'll notice, there is a
small bone called Tip Bone at the end of the
chain, and it's outside of the object itself.
For this bone, turn Bone Active off. It will
not serve to deform the object at all. Since
IK will try to match the pivot point of the goal-seeking
item to that of the goal itself, we do not want
the large Rock Bone trying to match the base
of the rock to the goal, but rather to match
the top of the rock to the goal, hence the sole
purpose of Tip Bone.

Let's go ahead and create a
goal object before we do anything else. Create
a null and name it Goal. Now move it 2.5 meters
up the Y axis at Frame 0. This way, when we fully
activate IK and the IK chain snaps toward the
goal, it will stay in the same position.

Soon we'll start setting up
the IK chain of the bones. I'm going to do this
somewhat backwards, to show why certain settings
do what they do. Select the object named Goal.
It should already have a keyframe at 0. Go to
frame 30, and move Goal to about 1 meter on the
Y axis, and 1.5 meters on the X axis. Create
a keyframe here. Now, also go to frame 60, and
create a keyframe with Goal at 0 on the Y axis,
0 on the X, and about 1.5 meters on the Z axis.
This will be a simple test animation for seeing
if IK is working properly or not. Return the
timeslider to frame 0.Part II: Setting up IK

Ready to set the goal-seeker?
Select Tip Bone, and open its Motion Options
panel. For Goal Object, select the object named
Goal from the drop-down. Move the timeslider
forward and then back, and you should see a dotted
line appear between the pivot of Tip Bone and
the pivot of Goal. Scrub through the animation,
and you'll see nothing happens. That's normal
so far.

Select the next bone in the
hierarchy, Rock Bone. In the Motion Options panel,
go to the tab marked Controllers and Limits.
You'll notice that each rotational controller
is still set to Key Frames. Change the first
one, Heading Controller, to Inverse Kinematics.
Scrub through the animation again. Still nothing

Go back to Tip Bone, and select
the IK and Modifiers tab of the Motion Options
panel. Turn on Full-time IK, and scrub through
the animation again. This time the rock should
look like its trying to follow the goal, but
only on Rock Bone's Heading axis, making the
rock bend backwards.

Go back to the Rock Bone's Controllers
and Limits settings, and change the other two
controllers to Inverse Kinematics as well. Now
the rock will lean to the side, and sortof roll
around to the back.

Now set all the other bones
to have Inverse Kinematics as their Heading,
Pitch, and Bank Controllers. Don't do this to
Base Bone. Leave those Controllers set to Key
Frames. If you set them to Inverse Kinematics,
that bone will also bend around, thus bending
the metal anchor the rock is tied to, and we
don't want that anchor to move. Also, don't mess
with Tip Bone's settings, as changing its controllers
will have no effect. Now scrubbing through the
animation, you'll see everything bending properly
to follow the goal.

The scene file ik_proton.lws
is an example animation by William Vaughan using
this same setup, if you'd like to look at that
and see how one might use this setup.

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