When to Use the Inverse Kinematics Tool
Toon Boom Digital Pro provides different tools with which to animate cut-out characters and trajectories. The two main tools used to animate characters are the Transform and the Inverse Kinematics tools.
In this article, we will compare the usage of these tools and explain the Inverse Kinematics philosophy.
 The Tools
First, let us see what these two main tools are.
The Transform Tool
The Transform tool is also known as Forward Kinematics. This is the main tool to use for cut-out animation. The Transform tool rotates, scales, moves and skews all the elements selected as one global element.
Forward Kinematics means that the element will be animated from the parent element down to the last child as a single piece. In other words, if the shoulder is animated, the arm, forearm and hand will follow as a complete arm drawing.
The Inverse Kinematics Tool
The Inverse Kinematics tool (IK) is an assistance tool. This tool will help you to achieve complex motions such as sitting down or knee bending. The Inverse Kinematics tool will move and rotate every selected element as a chain.
Inverse Kinematics means that the element will be animated from the child element up to the parent element. In other words, if the hand is animated, the forearm, arm and shoulder will follow into a mostly natural move, rotate and bend in reaction to the hand motion.
Inverse Kinematics in Digital Pro
In most 3D animation software, Inverse Kinematics is implemented as a skeleton system. Digital Pro uses the rigging hierarchy already created in the Network and Timeline view. So, there is no need to place bones in the character. Inverse Kinematics in Digital Pro is simply using the connections between the parts’ pivot.
IK Tool and Rigging
The IK tool cannot be used on every type of cut-out character. It will mainly depend on the way the pieces and parts are attached one to the other. In other words, it depends on the rigging type.
There are several ways to rig a puppet. These are the three main rigging techniques:
- The Open Rig technique is the simplest.
- There are no connections between the parts.
- They are free to move, rotate and scale independently one from the other.
- The Inverse Kinematics tool cannot be used on this type of rigging.
- The Hierarchy Rig technique is the most complex.
- All the parts are connected one to the other.
- As they move, rotate and scale, they will influence all the other parts.
- The Inverse Kinematics tool can be used on this type of rigging.
- The Mix Rig technique is the best of the Open and Hierarchy rig.
- Some parts are independent such as the torso.
- Some parts are setup into a hierarchy such as the arms and legs
- The Inverse Kinematics tool can be partially used on this type of rigging.
For more information on the rigging techniques, refer to the Digital Pro Cut-out Animation User Guide on page
226 to 289. You can also watch the «Rigging» chapter of the Cut-Out Animation video tutorial.
Animating with the Tools
Puppets are animated by using a combination of Transform and Inverse Kinematics tools.
You will mainly use the Transform tool (Forward Kinematics) as it has all the main motions; rotate, scale, skew, move and select. You can also control the exact position of the pieces while using the Transform tool. As well, the Transform tool creates a temporary global pivot on the selection that can be moved around for the animation purpose.
To complete complex motions, such as bending the knees while keeping the feet on the ground, you will use Inverse Kinematics tool as an assistance tool. By adding IK Constraints (Nails), it is possible to lock the feet or any other parts on the spot and move the rest of the body, which will react to the constraints.
The Inverse Kinematics Philosophy
It is important to keep in mind that the Inverse Kinematics tool is an assistance tool. You won’t use it to animate all the time, not because some parts are connected to a hierarchy, but simply because it is another way to move a character.
Also, the Inverse Kinematics tool can be used to do the character’s posing faster. Posing is a crucial part in the animation process. The IK tool can be used to pose the character more naturally, depending on your own preferences.
An IK Nail can be added anywhere on the character to animate only a little part of the hierarchy chain, such as an arm or a portion or the arm.
For more information on Inverse Kinematics tool and its usage, refer to the Digital Pro Cut-out Animation User Guide on
page 395 to 404. You can also watch the «Animation» chapter of the Cut-Out Animation video tutorial.
Tips and Tricks
Here are some tips and tricks to help you to use the Inverse Kinematics tool.
- IK on Mix Rigging
The Inverse Kinematics tool can be used on a full puppet’s body even if it is connected as a mix rigging. For
example, you can add IK Nails on the puppet’s feet, select the full body master peg and move the character as if
it was connected in a full hierarchy rig.
This technique works on most of the mix rigging.
- IK Constraints
It is important to remember that you can add and remove IK Constraints (Nails, Hold Orientation, etc.) at anytime during the animation. They simply hold a certain part on the spot while you animate the rest of the body. Because they are not constricting one part to another object, you can add or remove them without affecting any of the animation you have already done.
- Useful Shortcuts
- While using the IK tool, you can hold down the [Ctrl] key to select any part.
- You can hold down the [Alt] key to rotate the selected part without affecting the IK chain.
- You can press the [Shift] key and click in a part’s pivot to add or remove an IK Nail.
- You can press the [Shift] key and click with the middle mouse button in a part’s pivot to add or remove and Hold Orientation constraint.
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