Something You Need to Know About 3D Binding

Before embarking on the journey of 3D binding, it is essential to comprehend the rationale behind this process. 3D binding is a pivotal phase in the production of animations, which involves connecting the vertices of a model to a skeletal structure, thereby enabling a more natural and efficient animation creation process. In the following sections, we will delve into the concepts of skeletons, rigging, skinning, as well as Forward Kinematics (FK) and Inverse Kinematics (IK), and illustrate their applications in 3D animation with practical examples.

1. Skeleton: In the realm of 3D animation, models are typically composed of thousands, if not millions, of vertices. Without the binding process, manipulating these vertices directly to achieve animation would be an incredibly arduous task, accompanied by a monumental workload. To streamline this process, our engineers have sought inspiration from the natural world. Much like how living organisms move through the synergistic action of bones, muscles, and skin, our 3D models also require such “skeletons” to facilitate their movement. Engineers design “skeletons” within 3D software to assist the models in their motion. During animation production, animators only need to adjust the position and orientation of the skeleton, and the model will respond with corresponding movements, significantly simplifying the workflow. The illustration presents a simple human 3D model skeleton, comprised of the spine, limbs, and other major bones, which are interconnected through joints to form a structure capable of flexible movement.
2. Rigging: Should you create bones without interconnecting them, they would fail to function in unison. At this juncture, the rigging process becomes particularly crucial. Rigging is the procedure of linking individual bones into a cohesive structure, allowing joints to control the segments of the skeleton they are intended to govern. In the absence of rigging, the leg bones might not connect properly with the torso, resulting in a character’s upper body moving forward while the legs remain anchored to the spot. Through rigging, we can ensure that the movement of the legs propels the entire body, rendering the running or walking motions of animals more realistic.
3. Skinning: Skinning is the process of tightly integrating the skeleton with the model, ensuring that the model’s vertices deform correctly in response to the skeleton’s movement. The central task of this process is to allocate the control weights of the skeleton over the vertices, determining which bones influence each vertex and to what extent. Should your character be clad in armor, without precise skinning, when your character swings an arm to strike an enemy, the armor on the arm might incorrectly adhere to the body instead of moving with the arm. Through accurate skinning, we can ensure that the armor deforms and moves correctly in tandem with the character’s actions.
4. FK and IK: Now, let us discuss the two driving methods in animation—FK and IK. FK (Forward Kinematics) and IK (Inverse Kinematics) represent two distinct animation driving methodologies. FK refers to the movement pattern in which higher-level bones drive the motion of lower-level bones within the binding relationship.

As depicted in the figure, we have established a simple arm skeleton, and now we need to maneuver the hand to grasp the cube located in the upper right corner. In FK mode, we would need to sequentially move and rotate bones 1, 2, 3, and 4, enabling 4 to ultimately come into contact with the cube. Such an operation is exceedingly cumbersome and challenging to ensure the fluidity of motion.

We can create an FK control handle (Create IK Handle), unifying bones 1 and 4, along with the bones in between, into a single entity. By directly adjusting the position of 4, we can cause the other bones to move naturally.

Simply placing the position of 4 at the location of the cube will cause the other parts to move in concert, presenting a normal kinematic posture without the need for additional adjustments. Consequently, we can efficiently and conveniently achieve the desired animation.

When producing a walking animation, the movement of the legs drives the movement of the feet, exemplifying an application of FK. Conversely, IK is characterized by the motion pattern in which lower-level bones drive and influence the movement of higher-level bones. When creating an animation of a character reaching for an item placed high up, we typically employ IK to control the position of the hand, allowing the arm and shoulder to naturally follow the hand’s movement, resulting in a smoother and more natural animation. However, IK is not always superior to FK; animators will select the appropriate mode based on the specific requirements of the animation. For instance, when crafting an animation of a character forcefully pushing an object forward, FK might be necessary to better control the bending and stretching of the body. Therefore, during the binding process, we usually create both FK and IK modes to switch between as needed during subsequent animation production.

Additionally, it is important to note that the binding process involves the generation and calculation of numerous parent-child relationships. Therefore, before you begin the binding steps, ensure that your 3D model is clean and precise, free from broken faces and unnecessary vertices, among other issues. It is recommended that you frequently utilize the Delete By Type and Freeze Transformations features to simplify your model data. However, once you have entered the binding process, it is best not to casually delete history data or caches, as doing so may result in the removal of your binding operations or the invalidation of parent-child relationships.

In summary, 3D binding serves as the bridge connecting models and animations, enabling animators to create vivid character movements more efficiently and accurately. Through the judicious application of skeletons, rigging, skinning, and both FK and IK, animators can craft a wide array of animation works, ranging from simple movements to complex scenes. As technology advances, 3D binding technology continues to evolve, providing animators with an ever-growing arsenal of tools and possibilities, propelling the development of animation art forward. Now, go ahead and try adding a skeleton to your 3D model and perform rigging and skinning! Believe that you can certainly bring it to “life”!