Describes: building a hierarchy, Axis alignment, Inverse kinemat ics dragging.
Inverse kinematics is a powerful positioning tool that allows to move linked structures just like in real world. Experienced animators tend to consider Inverse kinematics too robotic to provide realistic character animation, yet it is an useful tool to be mastered.
First, we need to build a hierarchy composed of a root, some links and an end-effector.
Add a cube, select it, and press (s) to enter scale mode. Then press (z) to disable scaling along the Z axis. Stretch the cube on the XY axes, so that it becomes taller and thinner when seen in Top View.
Figure 1
Enter point mode, and move all points -no need to select them- upward in Top View until the bottom of the cube aligns with the axis' center like in figure 1.
Now go back to group mode and with Modify> Refine> Normalize your object's axes will be properly rescaled. This is very important since the Z axis length determines where the joint between an object and its children lies.
Copy and paste the cube a few times, displacing it along Z axis as you go until you are happy with a stack of cubes, each one -except the first- with the axis center exactly where the Z axis of the father ends. Finally, add an axis and move it at the end of the chain, with its center where the last Z axis ends. Figure 2 shows the relationship between objects.
Figure 2
Now select the first cube, the root, and the next, a link, using Amiga-G to group them. Deselect with the space bar, and go on selecting the second and third, grouping them and repeating the procedure. End grouping the last object with the lone axis, the end effector.
Now select the root and save the hierarchy to disk, then select the end effector. Go in Move Mode pressing (m), press (k) to activate Inverse kinematics, and drag from the right view. The Inverse kinematics chain will react as expected, forcing the link elements to follow the end effector, while the root stays in place unaffected.
Figure 3
As shown in figure 3, a ghost image of the original object is displayed during the IK dragging. This is an helpful reference aid, because if the end effector is moved near the root, the entire chain's orientation suddenly changes.