# Posts tagged ‘Theory’

As I’m currently in the process of skinning many meshes for the current game I’m working on here are some rules I’ve learnt on the way:

• Don’t attempt to skin spherical deformation without having bones for deformation or quaternion based skinning methods. Key places where this must happen is the shoulders, thighs, elbows, knees, wrists and ankles.
• You don’t need lots of twist bones, three including the actual bone is enough e.g shoulder *deformer bone, main twist (100% to upper arm with a direction pointing to the deformer bone), 50% twist bone, 0% twist (100% to the upper arm)

*By deformer bone i mean a tweak bone/point, etc  has average deformations between the  shoulder and upper arm. Similar to elbow or wrist deformer bones.

• Don’t model the wrist/hand attached to the sleeve, tuck it inside and treat it as an element. The same applies to the ankle/foot and trouser leg.
• If you don’t see the underside of a mesh, don’t model it, cap it off. A good example of this is a skirt.
• The deformation of the wrist is not the same as the elbow or shoulder; the shoulder can be considered a one dimensional quaternion – in this i mean its twist is dictated by its direction.  The wrist could be considered two dimension as the first quaternions direction dictates the rotation space (one plane) for the second quaternion to ride on.  The wrist bone dictating the direction for the hand to ride on, as oppose to the upper arm bones direction dictating the entire deformation of the shoulder.
I’ll discuss more on the differences of the wrist compared to the shoulder in later posts.

Just a quick theoretical idea – I’ve been thinking that a rotation is basically the reflection or product of a direction indirectly or directly controlled by its spin.  This seems simple to understand in theory but in practice its more complex. Im coming to terms that rotation is really an illusion of a direction or a side product.

Thinking about this more in my notes, we can hypothesize that if a rotation is really just a direction, then we can seperate it into two parts: the direction and the spin and more importantly have direction controlled by anything, namely the tangent of a spline or curve. This seems amazingly simple now, the curviture of a spline dictating the direction of a quaternion about a hemisphere. But detaching yourself to thinking this way is really hard.

In this way we can treat the wrist and spine articulation the same – the spin (or twist in the conventional sence) is just used to dictate the direction of the system, with a spine you’d use the tangent of the curve itself,  with the wrist the hand  is the direction.