< Back to Blog

Inverse Kinematics in Game Character Animation

Character animation has come a long way in the last decade. Blocky and unrealistic movements have become a thing of the past thanks to the consistent advancement of technology. “System Requirements” used to be something any PC gamer would have to look at before buying a game, but now most computers have decent graphics handling capabilities.

While the increase in resources and capabilities was a boon to all gamers, game designers and animators celebrated the advancements as well. For quite a long time computer resources were one of the primary inhibitors in preventing realistic movement and animation within game characters. Sliding feet, clunky turns, and movements even a yoga master would cringe at plagued the gaming industry for years.

But as more advanced GPUs became a standard, the ability to provide higher quality animation became a reality. New systems were designed to help create a much more realistic and fluid movement style and animators welcomed the introduction of inverse kinematics.

What is Inverse Kinematics?

To understand Inverse Kinematics (IK), it’s best to know what came before it. Forward Kinematics was one of the first widely adopted animation systems and it worked through parent-child-sibling hierarchies. That means if you were animating your character’s arm, the shoulder would be seen as the parent to the upper arm, and the forearm and elbow would be a child to the upper arm.

 

With Forward Kinematics, in order to move the character’s hand using that basic hierarchy you’d first have to rotate the upper arm, then forearm, and then the hand. While this type of system works fine for basic animation, it became very apparent that this system was severely limited when it came to more advanced and realistic animation.

That’s where Inverse Kinematics came into play. Inverse Kinematics (IK) is a family of techniques, tools, and properties that has helped animators create detailed and lifelike movements. The technique allows the animator to mimic very complex movements of an object’s joints easily and efficiently.  

With the obvious limitations of Forward Kinematics, IK was developed to eliminate many of the issues that had plagued animators for so long. Originally used in animated films, IK utilizes a mesh skeleton that imitates the human body. It’s made up of a rigid structure that utilizes tendons at each joint. The IK technique lets you represent the ability of an object to move each of its joints without having to implement any additional geometry or polygons. 

Rather than using the parent-child-sibling hierarchy, IK uses an inverted tree hierarchy. The inverted tree hierarchy utilizes a “root” at the center of gravity that then connects to “branches” or other extremities. This system flips Forward Kinematics on its head and solves things from the bottom up, hence the name “Inverse Kinematics”. 

Keeping in mind our first arm movement example, if the hand is animated to move, IK will move the hand first and then the forearm and upper arm will follow suit. Compared to the traditionally used Forward Kinematics, IK takes all of the complicated pieces and computational work of posing a character out of character animation.

The Benefits of Inverse Kinematics

When it comes to animating skeletal objects, Inverse Kinematics provides one of the most realistic systems by mimicking the skeleton and tendons within the human body. The IK technique is far more straightforward and time-efficient than dealing with the top-down rotations needed in Forward Kinematics.

The unique benefit of this technique is that it allows designers to build characters that look great without having to rely on animating every object to create that effect. Animating using IK techniques allows characters to move naturally. In fact, one of the biggest advantages to IK is the use of tendons and joints within the skeletal frame. This provides realistic movements for the animator to take advantage of. 

With the use of the skeletal frame, Inverse Kinematics provides a much more fluid and realistic movement system. Solving movement from the bottom up allows the movement of extremities to seem much smoother, and there is much less to worry about since the other connections simply follow suit and settle into place. The use of a skeleton and tendons provides a much more natural control structure that mimics movement more naturally. There is much less rigidity to your characters and this allows for more in-depth animation and control. 

Not only does IK give you more precision, it’s also generally easier to animate these natural movements. For example, imagine that you want to animate a person waving their hand. If you are using Forward Kinematics, you would need to rotate the 3D object’s shoulder, upper arm, elbow, hand, and more to give it a natural feel. However, with IK, you would only need to rotate the object’s hand, and the rest of the arm would rotate as needed. Instead of mentally calculating how far you should rotate each and every part of the arm, IK simply performs these calculations for you when you rotate the hand to the desired position, and the rest of the arm just falls into place.   

When it comes to “skinning” your character, IK is equally useful because it allows you to attach the geometry directly to the skeletal structure. For segmented models, you can simply attach the limbs and parts of the character to the nearest bone or joint. The structure will then act as a “parent” to the skin or “flesh”. If you are using a single-mesh polygonal then you can simply “skin” the character over the entire skeleton.

Applications of Inverse Kinematics

Inverse kinematics offers a wide range of applications in the real world, which has helped to establish it as an efficient method of animation.

One example is in TV animation, where inverse kinematics has been the favored method of many animators for a long time. For many years, numerous animated cartoon shows, including Scooby-Doo, often used this method in conjunction with Forward Kinematics to make character movement realistic and natural-looking.

 

Another popular use of inverse kinematics is in the field of robotic assembly, where inverse kinematics computations are frequently required. In the manufacturing industry, robotic arms are often used on production lines. An operator would most likely want to use a robot arm to find and pick up a particular product part without the process of moving each joint of the robot arm independently. In such a fast-paced environment, such calculations would waste a lot of time and therefore cost the company extra money.

There are certainly cases that using Forward Kinematics would be highly beneficial. For example, Forward Kinematics may be more useful and look more natural if you are animating an object where the precise position of every part of the model isn’t critical to the animation. 

On the other hand, Inverse Kinematics provides a considerable advantage over Forward Kinematics when it comes to animating things with a skeletal structure and the need for precision. So, if you’re looking for a system that can provide excellent fluidity and realistic movement, start with Inverse Kinematics and see how your characters can be brought to life with ease.

Get started with PixCap today.

Get Beta