3D Toolkit: Skinning & Weight Painting

Skinning & Weight Painting

Skinning is the process of connecting model geometry to a rig: placing the skin atop a rig and allowing for the geometry of a model to deform and move as the rig is manipulated. This allows for the process of weight painting, in which the influence of joints is proportioned out across regions of geometry. There are several methods of determining how joint weight are distributed.

In weight distribution and skinning, there is the concept of different "binds", determining in different terms how the skin of a model moves. This is in relation to the strength and difference between weight distribution. Often, weight distribution is measured on a scale from 0 to 1, with 0 meaning that a joint has no weight on a section of geometry and 1 meaning it has a great degree of control. If, when computing weight differences, the binary values of 0 and 1 are all that is used, the bind is considered a "Hard Bind", being split uniformly between areas with no influence an areas of great influence. This may be used when rigging machines or other hard-surface models which do not bend or deform in a smooth or organic manner.
Alternatively, "Soft Binds" refer to weight distributions in which a variety of decimals are used between the values of 0 and 1, such as 0.25, 0.5 and 0.75. This creates a greater degree of control and deformation across the rig, allowing for smoother motions and more natural deformations. This is particularly useful when creating organic rigs and characters that move in a more smooth manner, with areas of anatomy or geometry that deform as joints move. 

When creating skins and weight painting, it is important to test to see how a rig is deforming the model it is bound to. To test this routinely and accurately, a simplistic animation can be created that cycles continually and moves every joint. This allows for errors in the weight painting to be identified and rectified, clearly pointing out where and how the model is deforming. 

To create a skin bind, layer the model over the rig. Go into the Rigging menu and into the "Skin" Menu. This has all of the settings and tools needed to bind the model to a rig. Then, select the rig and shift-select the model. Press "Bind Skin". This will bind the model to the rig. The weight distribution of this rig can be changed through the weight painting process.

When a Skin Bind is created, the rig turns different colours. This shows which areas of the rig are connected to which others. As can be seen below, the elbow joint - yellow - is connected to the shoulder joint, which is orange. This shows the hierarchy of motion associated with the rig.

Another option seen in the Skin Bind menu is that of the "Max Influences". This determines how many joints can influence a single vertex. The higher this is, the more flexible the geometry will be, and as this lowers the geometry will become more rigid. For this example of a human arm, I set it to 3. This is because ultimately, when creating weight paints for an organic sculpt, a degree of falloff is desired, showing how the geometry is influenced across the entire form of the model. In this, regions close to the joints will be greatly affected and regions further away will be less effected, deforming in a more subtle manner. This effect can be edited and tweaked through the weight painting process. 

When weight painting, there are several methods that allow for weight to be distributed across the form of a model. One of these is through the use of the Component editor, which describes - in numeric terms - how much influence is shared between local joints on any one vertex.

When selecting a vertex, the Component Editor will describe the joints nearby it and list numerically (from 0 to 1) the scale of influence each has on the vertex. The total of these influences is always 1. As such, these numbers can be manually tweaked to change how the falloff occurs across them, giving some joints a greater degree of control and others a lesser degree of influence. Though undoubtably precise, this method is not visual in nature and requires a degree of visualisation to understand how the weight is being distributed.

The Paint Editor visibly shows how much influence is distributed from a joint across the geometry of a model. In this sense, areas that are black have an influence of 0 and areas of pure white have an influence of 1. Areas of muted white and gey cycle through decimal values between these two constraints. When weight painting, you can set the value and the opacity of the "brush" to paint this weight over the mass of the model. In this, the falloff of joint influence can be visually seen across the surface of the model, at the cost of a potential loss in accuracy.

Another method in Maya for weight painting is that of the Interactive Skin Bind, which again shows visually how the weight of a joint is distributed over the surface of a model. This controls almost like a gimbal in Maya, with the weight being able to be moved, scaled and intensified across the surface of the geometry. Green handlebars change the region of influence over the model, red changing the intensity and blue scaling the weight in a more general sense. This, in my mind, is the most straightforward and simple of the weight painting methods but is also the least accurate, and is likely what I would use in sculpts if I wanted to get a quick feel for how a model's geometry may move.