Adobe Flash CS4 Professional for Windows and Macintosh: Visual QuickStart Guide

Constraining IK Motion

Intro to Inverse Kinematics

383

Constraining IK Motion

You can limit the way IK bones move. You

can allow or prevent a bone from rotating, for

example, to control the way a character can

bend its arms and legs. You can also allow

or prevent movement along a bone’s x-axis

(x-translation) and/or y-axis (y-translation).

 e axes in this case refer not to the x- and

y-axes of the Stage, but to the axes of the

bone instance (see the sidebar “Translating

the Direction of Translation”). Enabling trans-

lation for the parent bone in an armature

allows the entire armature to move around

on the Stage. Enabling translation for a child

bone allows that bone to move closer to its

parent or farther away. Moving a child bone

along its x- and/or y-axis within an armature

in an IK shape can make the shape expand

and contract. Making such movements in an

armature connecting IK nodes can help you

to create mechanisms with moving parts.

 e methods for enabling and constraining

rotation and translation with IK nodes and

IK shapes is similar.  e tasks below work with

IK nodes. Try experimenting with enabling

and constraining the motion of bones within

a shape once you’ve completed the tasks

with nodes.

Translating the

Direction of Translation

Although x- and y-translation deals with

movement along the horizontal and

vertical axes, the axes in question are not

the axes of the Stage.  is can be confus-

ing at  rst. Each bone instance has its

own x-axis and y-axis; the orientation of

the axes depends on a variety of factors,

including the bone’s original orienta-

tion to the Stage, the bone’s original

orientation to its parent bone, whether

the bone is in an IK shape or is connect-

ing IK nodes, and whether or not the

bone has been rotated from its original

position.  e easiest way to tell where

the axes of a bone are, and what direc-

tions they cover, is to select the bone and

look at the range-of-motion icon at its

head (see the sidebar “ e Mysterious

Range-of-Motion Icons”). When transla-

tion is enabled for the selected bone,

the icon includes double-headed arrows

for unconstrained x- and y-translation,

I-beams for constrained translation.  e

arrows (or I-beams) indicate the direc-

tions in which the head point of the bone

can move.

Constraining IK Motion

384

Figure 13.24 Selecting a bone in an armature reveals

its properties in the Property inspector. By default,

joint rotation is enabled. You can manipulate the

selected bone by dragging it with the selection tool;

you can rotate the bone 360 degrees around the

head point. The rest of the armature follows along.

By default, joint translation is disabled. The head of

the bone cannot move from its current x-/y-coordinate.

Figure 13.25 When you select the Constrain check

box in the Joint: Rotation section of the Property

inspector, Flash creates Min and Max values of

–45 and 45 degrees for the selected bone (top).

The range-of-motion icon depicts the motion

available to the bone (bottom).

To constrain rotation at a bone’s head:

1. Create a simple IK armature that uses two

bones to connect three symbol instances

in a horizontal chain.

 is task uses three instances of a symbol

containing a rectangle.

2. Using the selection tool, select the

armature containing the bone you want

to constrain.

3. Click the parent bone, the  rst bone in the

chain, and access the Property inspector.

 e selected bone highlights, and its

properties appear in the inspector

(Figure 13.24).

4. With the parent bone selected, in the

Joint: Rotation section of the Property

inspector, select the Enable check box

(it is selected by default).

When joint rotation is enabled, the

selected bone can rotate through 360

degrees around its head. To restrict

that movement, you must constrain

the rotation.

5. Select the Constrain check box.

Flash assigns default constrain values:

Minimum –45 degrees, Maximum 45

degrees (Figure 13.25).

Selected bone

Chapter 13