Let's first look at bones. Bones objects are like a family, You
start with a parent, then have children, grandchildren, etc.. Any bones
object can have any number of children but will have only one parent.
There are two ways to manipulate bones objects, directly (there
probably is a better word) or inversely.
When You move an object directly, You're moving from a parent's
point of view. Any manipulation to You affects all of Your children,
grandchildren, etc.. In Imagine You do this by picking in "Pick Group"
mode any bones axis and manipulating it. This will also move, rotate,
etc.. any bones objects that are down the family tree from it.
When You work inversely (Inverse Kinematics) You are working
from the child point of view and affecting all of the way up the family
tree. In Imagine You do this by clicking on the "Constrain" menu item.
For example, take a human-like object.
In direct mode click on the thigh "bone". Rotate it and the
shin bone and foot bone move with it.
In inverse or constrain mode click on the foot bone. Rotate it
and all of the other bones in the body move. This is because You are
affecting up the family tree, and parents always affect down the tree to
the other children.
This is where "freeze" comes in.
First click on the hips bone ?:) and click on "freeze" menu
item. Check all of the axis in world. This freezes the hips from
moving. Now go back, click on "constrain", move the foot bone. The
shin bone and thigh bone move with the foot bone but nothing else. You
can also use "freeze" to only limit movement to certain axis. In the
above example You could keep the shin bone to move only as a knee would
bend. Some programs even allow You to limit movement in an axis to a
certain number of degrees or range of movement.
As it seems to me, direct mode is useful to pose a figure or
object in an unusual pose or where placement is more on looks then on
exactness. Inverse mode is useful to position a piece of an object in a
certain place.
For example, use direct mode to position a cheerleader's arms.
Use inverse mode to place a person's foot up on a step.
Make sure You watch what mode You are in! Direct mode use Pick
Group mode and inverse mode uses Pick Object mode.
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