CAP 202 - Computer Animation II

Lesson 11 - Hierarchies


This lesson discusses linking objects so the motion of one object determines the motion of another, in a parent child relationship. Objectives important to this lesson:

  1. Creating hierarchies
  2. Using tools to create hierarchies
  3. When to use specific tools
  4. Transform or Modify: choosing
  5. Helpers
  6. Animation rigs

The lesson begins with an explanation about objects that are dependent on one another.

The example of a door is a good one. A door that is mounted on hinges cannot move unless the hinges move as well. The movement of the door and the movement of the hinges are dependent on one another.

In the case of 3DS Max, we must decide on a hierarchical order for these objects. The text makes it easier by starting with a doorframe object, which is the parent of the hinge object. The hinge rotates with respect to the doorframe. The door can be made a child of the hinge object, since it rotates with respect to the hinge. (If it seems strange to think of a doorframe as a parent, think of the doorframes that moved on conveyors in Monsters, Inc.)

This order becomes important in 3DS Max: a child is linked to a parent. The movement of the child is dependent on the movement (or location) of the parent. The child inherits characteristics like movement from the parent.

The text reminds us that we have already learned to place objects in groups. Groups are not useful for movement in the way that hierarchies are useful.

  • When objects are linked in a hierarchy, any of those objects can be selected and animated as needed, without changing the hierarchy.
  • To animate an object that is in a group, the group must be selected and opened, the object must be selected and animated, and the group must be closed again.
Exercise Notes and Questions

Work through the exercises and turn in your answers to all questions below as part of the homework for this assignment.

Exercise 1: This exercise illustrates linking objects in a hierarchy. It uses a button we have not clicked before, the Select and Link button.
Question 1: Where did you find the Select and Link button?

Exercise 2: Exercise 2 walks you through creating a hierarchy with the Schematic View tool. Near the end of exercise 2, you are told to look at the page and continue linking objects as shown. The diagram is impossible to read. This is the series I believe you should follow:

  1. Link the clamps to 360_head_pivot
  2. Link 360_head_pivot to Vertical_head_pivot
  3. Link Vertical_head_pivot to Horizontale_head_pivot
  4. Link Horizontale_head_pivot to Head_holding
  5. Link Head_holding to 3rd_pivot
  6. Link 3rd_pivot to 2nd_arm
  7. Link 2nd_arm to 2nd_pivot
  8. Link 2nd_pivot to 1st_arm
  9. Link 1st_arm to 1st_pivot
  10. Link 1st_pivot to Base_holding
  11. Link Base_holding to Base

Exercise 3: This exercise has little value. We will skip it.

There is a brief lesson before exercise 4. It introduces two terms: Forward Kinematics (FK) and Inverse Kinematics (IK).

  • Forward Kinematics - the process by which a child object inherits motion from its parent object
  • Inverse Kinematics - the process by which we examine the motion of a child object to determine the motion of a parent object

Exercise 4: This exercise does not explain the process of IK, but it has you use a file that has IK imposed on a (very basic) model of a human leg. Open the file and understand that moving the back section of the foot moves the calf and the thigh as well.

The text continues by describing features of the Hierarchy panel. Three buttons allow us to control three kinds of things:

  • Pivot - the pivot point of each object is important to rotation, scaling, and inverse kinematics
  • IK - this button leads to several rollouts related to the inverse kinematics of an object
  • Link Info - this button leads to the Locks rollout which is used to restrict movement, and to the Inherit rollout which is used to control what a child inherits from its parent.

Question 2: Where are the three buttons, on the hierarchy panel?

Exercise 5: This exercise examines hierarchy information about a railroad engine wheel. They are linked in a hierarchy, but there is an error in it.

Question 3: In step 8, you are told to turn off Y axis rotation for the Pivot pin of the wheel assembly. Are we telling it NOT to inherit rotation on the Y axis from its parent, or not to pass Y axis rotation TO its parent?