Flash inverse kinematics tutorial

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1 Introduction

Learning goals
  • Insert bones in shapes
  • Connect symbols with bones
  • Create a motion animation with bones
  • Attach envelope points to bones
Flash level
  • Flash CS6 or better
Moving on
Level and target population
  • Beginners
  • This is a first draft. I will have to complete things, add some more screen captures, rewrite some stuff, and add more examples, etc.
Alternative Version
Flash CS4 inverse kinematics tutorial

The bones tool is an inverse kinematics (IK) tool with which one can create armatures for shapes or connected symbols instances. These armatures connect objects or parts of shape in a hierarchical tree. These parts can be called bones or limbs. "Outer" (or child) limbs that are moved also will move "inner" (or parent) limbs. E.g. In a human avatar, if you pull a finger, the hand will move too and the hand in turn will move the lower arm, etc. So in contrast to "forward kinematic animation", where each movement for each component must be planned, only the starting and ending locations of a limb are necessary to get a basic animation going.

A group of several symbols with IK bones attached. Copyright: Adobe License: Creative Commons by-nc-sa
A shape with an IK armature added. Copyright: Adobe License: Creative Commons by-nc-sa

“Inverse kinematics is the process of determining the parameters of a jointed flexible object (a kinematic chain) in order to achieve a desired pose. Inverse kinematics is a type of motion planning. Inverse kinematics are also relevant to game programming and 3D animation, where a common use is making sure game characters connect physically to the world, such as feet landing firmly on top of terrain.” (Wikipedia, retrieved 27 November 2008 ).

“Inverse kinematic animation (IKA) refers to a process utilized in 3D computer graphic animation, to calculate the required articulation of a series of limbs or joints, such that the end of the limb ends up in a particular location. In contrast to forward kinematic animation, where each movement for each component must be planned, only the starting and ending locations of the limb are necessary.” (Inverse kinematic animation, retrieved 17:52, 28 November 2008 (UTC)).

“The characters in a game have skeletons. Similar to our own skeleton, this is a hidden series of objects that connect with and move in relation to each other. Using a technique called parenting, a target object (the child) is assigned to another object (the parent). Every time the parent object moves, the child object will follow according to the attributes assigned to it. A complete hierarchy can be created with objects that have children and parents [...] Once the skeleton is created and all of the parenting controls put in place, the character is animated. Probably the most popular method of character animation relies on inverse kinematics. This technique moves the child object to where the animator wants it, causing the parent object and all other attached objects to follow.” (How stuff works, retrieved 27 November 2008).

Since Adobe CS4, Flash fully integrates inverse kinematics modelling. “Inverse kinematics (IK) is a method for animating an object or set of objects in relation to each other using an articulated structure of bones. Bones allow symbol instances and shape objects to move in complex and naturalistic ways with a minimum of design effort. For example, inverse kinematics lets you create character animation, such as arms, legs, and facial expressions much more easily.
You can add bones to separate symbol instances or to the interior of a single shape. When one bone moves, the other connected bones move in relation to the bone that initiated the movement. When animating using inverse kinematics you need only specify the start and end positions of objects. Inverse kinematics lets you create natural motion much more easily.”
(Using inverse kinematics, retrieved 27 November 2008).

Simple demo

The following application shows the difference between an IK structure (armature) made with red symbols and an another made with a blue shape.

Symbol vs. shape IK structures

Load the symbol-vs-shape-armature.html application and drag the tips (or other parts of the shapes around. Make sure that you have a Flash 10 player installed. Else, it will not work. Notice: The armatures have been defined as type "Runtime", meaning that we let the user do the animation (manipulate the IK structure).



  • An armature or IK structure or bones structure defines how "bones" connect either symbols or various areas of a shape into an articulated whole.
  • Nodes refer to symbols connected by a bone to a "parent" symbol
  • Bones or limbs means connectors of either symbols or elements of the bones structure of a shape.
  • Ik shape refers to a shape that contains an armature

2 A simple armature animation with symbol instances

Armatures can be used either for animation, or for end-user manipulations. In this section we shall explain how to create an animation using a set of connected symbols.

2.1 Creation of a simple IK animation with symbol instances

Have a look at the symbols-ik-armature-intro example first.

A simple IK animation with symbol instances

If you want to play along, you can start with the following source file. It only includes the symbols, the IK armature has to be created:

A connected balls and ovals example

Step 1 - create the symbols that you plan to connect with a bones structure

  • Create a new layer
  • Create a movie clip symbol, draw a red ball and right-click convert to symbol. Select "Movie clip".
  • Then adjust the instance on stage (e.g. fix its color and size or add a bevel filter)
  • Then create copies of this instance. The fastest way is to hold down CTRL-ALT and drag.
  • Repeat this with a rounded rectangle.

In your library you now should have two Movie clip symbols, e.g. Red ball and Red rectangle

Step 2 - move the symbols into an initial position

  • You could try to reproduce our example, i.e. plan to create a simple "chain animation"
  • Make sure that all the symbols you need are on the stage in the same layer. You won't be able to add additional symbols once you start defining the armature, i.e. the bones structure that will connect these symbols.
  • You could rotate objects (either with the transform panel of the Free Transform tool).
Symbols that a ready for assembling into an IK armature

Step 3 - create the armature

Again, make sure that all the symbols you need are on the stage and in the same layer.

The armature must have a hierarchical structure. The "parent" will be attached to the stage, its children to the parent, and the great children to the children, etc. In our case, this means that we start with the rectangle that should be attached to the floor (i.e. the right-most rectangle).

  • Select the bone tool Bone tool.png and click on the first symbol. The first one you select will be the parent shape, i.e. it will attach the whole armature to the scene. Again, in our case we start with the rectangle to the right.

Now let's connect this first symbol to the next symbol:

  • From from the inside of this first symbol, start dragging with the bone tool, stop somewhere inside the next symbol, i.e. release the mouse where you want your bone to end. If you release the mouse outside of a symbol nothing will happen. If you are succesful, you can see a bone as shown in the next picture.
Connecting the first two elements (right-to-left)
  • You may have to zoom in (200-400%). Hit "F4" if you need more drawing space and hit "F4" to display the panels again.
  • Repeat this step until all of the symbols are linked. Make sure to link these in the right order.

You will notice that Flash created an armature layer (called "Armature_1" or similar). You can't edit objects that are part of the armature). They have been copied to the armature layer and did become ikNodes. You may change the name of that layer if you like, and of course, you can edit the graphics of a symbol (double click).

Connected armature

A note on terminology: Each bone will get a name like "ikBoneName1" (that you can change) and each connected symbol will get a name like "ikNode1" (that you also can change). So in an IK armature we got bones and nodes. The latter are made of your symbols.

Now you can play a bit, e.g. drag the last child in the IK structure around and see if the joints are in the right place. If not, adjust (see below)

2.2 Adjustments

Adjust center points
  • If the start of a bone is in the wrong place, you can move the rotation point with the free transform tool.

You also could turn the shape, move it or adjust its size, so that it would adjust to the new rotation and length of the bone, but that's a bit tricky. You will learn move about bone and node manipulation later, but for starters this it is enough to move bones up and down.

The picture below shows what happens if you move a shape and change its rotation point. The result is not entirely convincing (compare with the previous picture above) ...

Adjusting joints (enter points) with the Free Transform tool in Flash CS4

2.3 Animation

Let's now create an IK animation. Before you start, make sure that the IK structure is what you want. It must be complete. You won't be able to change the structure itself later on. You only can change poses, do some transformations and other more advanced tuning. You cannot add other symbols to the armature. So here is the procedure.

  • Select the selection tool (Selection tool.png) in the tools panel
  • If you only have a single frame in the timeline, you need to extend the armature span. Select the armature layer and drag out frame one (watch for the double arrow cursor to appear)
  • Now move the red playhead on top to a new frame and define a new pose. In any frame you like, you can move all the joints. Each time you do this, it will create a new keyframe with a pose. Keyframes are represented by little lozenges in the armature span in the timeline.
  • You can copy/paste poses: Right-click directly in the timeline on a keyframe (i.e. an existing pose) and copy pose. Then right-click in a different frame and paste pose. Make sure to directly right-click (no click first), else it won't work.

What if you got it all wrong ?

  • Select the armature layer by clicking on it. This selects the whole IK structure
  • Kill the parent bone (in our example it its on the bottom)
  • The select all symbols and copy them back to the initial layer ("plaything" in our case). You could use ctrl-shift-v to "copy in place".
  • Then delete the armature layer and start again.

2.4 Adjustment of the time line

  • Just drag the latest frame out or in. Make sure that you see a horizontal double arrow, i.e. don't click in the last frame. In the latter case you would just move the whole animation chain to the right.
Result and source

3 Adding armatures to shapes

You also can add an armature to a shape as we have seen in the initial demo shown at the start. I recommend the following steps:

Step 1 - Create a new layer with a single shape
  • Draw a shape or a simple drawing in object mode. Make sure to finish the drawing, later you only can adjust its shape.
  • Make that you only have a single shape, else CS4 will sort of freak out, e.g. freeze a bit or create more than one armature layers ....

I suggest the following procedure for drawing:

  • Draw all the parts of the shape in object mode
  • Once you are happy, you should save a copy of your artwork in case you want to come back to it later: CTRL-A and Right-click; Create Symbol.
  • Then break a part the symbol instance on the stage and make a union' of all graphics: CTRL-A, then menu Modify->Combine Objects->Union. This is necessary because you do need a single shape or drawing object.
Step 2 - Add the inverse kinematic structure (bones)
  • Select the bone tool from the tool panel
  • Click inside the shape where you the the armature to be attached to the scene. The parent point will not move.
  • Then hold down the mouse at the same spot and drag out a bone
  • Click at the end point of this new bone and drag again
  • etc.

You now will see an armature layer and you can't edit your objects anymore since they all have been moved to the armature layer.

Step 3 - Add another shape.
  • If you want to create an other IK structure (shape or symbol-based) just create a new layer and start again.
Step 4 - Create the animation and adjust various poses in keyframes
  • Rotate bones as explained abouve. If you turn on the circle close to the joint you will just move the joint without turning the parents.
  • To adjust the shape in poses, see just below. Also consult the tools and objects overview.
Two flowers example

This example is really ugly. I would have to do it again. It seems that using the pencil tool (strokes), for the blue flower was not such a good idea. If you need fat lines, rather use the rectangle tool or the paint tool. Then you could adjust the envelope later on.

Shape rubberman example
Shape-based IK structure of a rubberman

This shape has been drawn with the Paint tool in object mode. Once the drawing was done, we made a union of all its parts (Modify->Combine Objects->Union) and then optimized (Modify->Shape-Optimize) at 100%.

  • The rubberman has many joints as you can see in the picture.
  • The parent bone starts in the hip area.
  • We not only animated the poses but also move the rubberman in different positions in various keyframes.

The result isn't great, but given the time we spent on it (little) it's ok.

Fixing the shape in poses

You sometimes can see that the shape doesn't really follow the bones (I mean these sharp edges that can appear). Such behavior can be fixed:

  • Use the Subselection tool to move shape control to a different place. Firstly, magnify the scene (at least 400%). Then you can move or kill the blue control points or adjust them with the red curve control handlers. Take it slowly and wait to see the result of a manipulation since Flash recomputes the whole interpolations ! Even my DELL M1740 high-end laptop needs to think about it, before it let's me see a deletion, a move or a curve change happen. Also bones may be shown displaced. Click in another keyframe and come back ...
  • Use the bind tool to associate shape controls with bones.
Fixing a curve of the shape in a IK pose with the subselect tool

Unless you are looking for a "snake-like" behavior, it is probably better to use a symbol-based armature as in the next chapter.

4 A stickman avatar

(not done yet ... try it yourself)

Let's now create a stickman avatar that roughly has human proportions. In order to create a "human" stick man we need a few objects to represent body parts. Here are a few rules of thumbs regarding size of body parts:

  • Total body height should be about 6-8 times the size of the head
  • Head to crotch and crotch to feet is about the same size
  • shoulders and hips are about the same (shoulder is smaller for females and bigger for males)
  • Waist (if you have one is midway between shoulders and crotch
  • Arm length: From shoulder to mid-thigh (or shorter)
  • Feet length: About the same as forearm
  • Face is an oval, about 6-8 smaller than the total size. Eyes are roughly in the middle.
Adjust properties

Select a bone and play with the options in the properties panel

  • Speed refers to the rotation speed in relation to the rest (default is 100%)
  • Joint rotation constraints are rotational constraints on the joints of a bone structure.

5 Animation by user action

You can produce a inverse kinematics armature that the user can manipulate. To do so:

  • Only use frame 1 of the Armature layer, i.e. don't do any animation (kill all the other frames in the Armature layer)
  • Select the armature layer
  • In the properties panel, choose Type -> Runtime.
Runtime Armature (no animation), the user can play
  • You now can play the movie (publish it) and play as in the demo that we showed at the beginning of this tutorial

6 Tools and objects overview

This is a sort of short manual for the IK tools an objects

6.1 Tools overview

Below is a list of tools you may use and their function. See further down for "how to".

The Bones tool Bone tool.png
  • Define bones
The Bind tool Bind tool.png
  • Associate envelope points with a given bone. (more later)
The Selection tool Selection tool.png
  • Select bones or associated ikNodes (shapes) for further manipulation, either by dragging, ALT-dragging or via the properties or transform panel.
The Subselection tool Subselection tool.png
  • Allows you to move joints of bones (i.e. make them longer or shorter and rotate) of bones within a shape armature. I.e. this won't work with IK structures that use symbols. To move bones for symbol armatures, use the Free Transform tool.
  • Allows to adjust shapes
The pen tool Pen tool.png
  • As alternative tool to add/remove control points to a shape.
The Free Transform tool Free transform tool.png
  • Allows to move joints (starting points of bones) of an symbol-based armature.
The Transform panel
  • Change size and rotation of both bones and associated ikNodes (shapes).

6.2 Dealing with the armature layer

An armature is defined in a special Armature layer. By clicking on the layer, you then can change some of its properties in the properties panel:

  • Ease: You can define ease-in and ease-out parameters as in other tweening animations
  • Options:
    • Change type: Authortime to animate yourself in the armature layer, runtime to allow the user move the IK structure. In the latter case, you can't have your own animations. I.e. you only can define frame 1.
    • Style: This will display the bone structure in various ways. Use solid for simple stuff and wire to be able to work on finer details.
Flash CS4 armature properties

Flash automatically moves all the object from the drawing layer to the armature layer as soon as you create an IK. Therefore if you want to break the armature, make sure to copy first all your stuff from the armature layer to another layer or much better, before you start adding an armature, simply save your drawings as a symbol in the library (break apart the instance on stage again after you saved).

6.3 Dealing with the whole IK structure

  • To select the whole IK structure or shape, click on it.
  • You then can change its position (e.g. use the arrows)
  • You can scale it using the Transform panel

6.4 Editing Bones

After creating bones (i.e. the connectors), there are many ways of editing them. You can reposition the bones and their associated objects, move a bone within an object, change the length of a bone, delete bones, and edit the objects containing the bones.

Remember that you should edit the bones before you start adding poses !

  • To select a bone, use the select tool. shift-select will select several and double-click will select all bones.
  • To kill a bone, select it and hit delete. This will also kill its children.
  • To edit properties of a selected (or several selected bones) use the parameters panel.

You can define 3 sets of parameters for each bone (or set of bones)

  • Joint translation is disabled by default, i.e. the size of limbs remains the same. To allow stretching of bones edit the child's Joint: X translation and Joint: Y translation parameters. You then may define how far in x/y direction you can stretch the joint, i.e. the start of the bone. Basically, when you move a joint in x/y direction (which normally you can't) the parent bone will grow/or shrink and change its angle.
  • To limit rotation (by default you can rotate in 360 degrees) edit the Joint: rotation parameters. Positive numbers mean "clockwise".

To rotate a bone (turn it), use the selection tool to drag the bone (or the shape). All bones that are in the same branch also will move. But if you select a spot closer to the joint, they will move less.

To rotate a bone without moving the parents, shift-drag (hold down the shift key).

To move a joint, select the joint with the free transform tool. In other words, you simple move the rotation point, i.e. little white circle you should know from old style CW motion animation. You also can play with the transform panel. If you want to stretch the bone of parent A, select child B.

I did not manage to move bone ends with the Subselection tool for symbol-based armatures, only within a shape-based IK.

6.5 IK nodes in symbol-based armatures

Symbol instances connected with bones become ikNodes. I.e. when you click on a shape with the select tool, you will see something like ikNode_3 in the properties panel.

  • In the transform panel you now can change its size (x and y) and also it's rotation. E.g. if you did some wild bone manipulation stuff you can realign a shape with the bone.
  • Alternatively, you also can drag the ikNode (shape) with ALT-drag.

In both cases the bones will adapt, i.e. stretch and rotate.

6.6 Dealing shape-based IK shapes

  • To edit a shape in a shape-based IK structure, use the Subselection tool
  • If you click on the stroke (the border of the shape) you will see control points. You then can drag control points or click on one and adjust the shape with the curve controls that will appear.
  • To add a new control point click somewhere on the stroke.
  • To delete a control point, select it and hit delete

Tip: Magnify a lot (e.g. 400%), it's really hard to get the right point. Also CS4 behaves a bit erratically, i.e. I sometimes have trouble inserting a new control point vs. moving a curve control. Therefore you also can use the pen tool and its sub-tools to add/remove controls.

6.7 Bind bones to shape points

When you move the bone structure to a different pose, you may not be happy with the result, but you can fix that. By default, the control points of a shape are connected to the bone that is nearest to them, but you can change that with the bind tool (that sits underneath the bone tool in the tools panel).

If you click on a bone it will show all associated control points (i.e. parts of the shape that go with a bone). The bone will be red and the associated control points yellow. Points that are associate with just one bone are yellow squares. The ones that connect to more than one bone are yellow triangles.

  • To remove a control point association with a bone, CTRL-click on the yellow.
  • To associate a control point with a bone, SHIFT-click on a red (not associated) control point.
  • To add a control point, use the Subselection tool.

The other way round. If you click on a control point (blue if no bone is selected) it will become red and show the associated bones in yellow.

7 Links

Introductions to inverse kinematics
Adobe documentation
Avatars (theory, technology and design)