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Posts tagged ‘Twist’

Rules on modeling for games, and deformation.

October 7, 2008

Charles

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.

Double strung quaternions to reinforce rotations.

June 18, 2008

Charles

I don’t know if this is new but I’m calling it double strung quaternions (DSQ). Its a method to reinforce a rotation relative to a transform space. In lamens terms it trys to make rotations true to our way of seeing them in a control. I’ve been looking into ik spine systems and hopefully this maybe a way towards helping out with rotation issues. Here’s a demo using z as its direction.


 (click on the above image to view it the demonstration)

Bugs im noticing are that it uses linear distances to work our the quernions, quatDrivers as I call them (they can infact drive any value with distances) – i get some slight wobblying, but i think this can be cured using a value-space normalisation method like so:

  1. a = 10, b = 20
  2. summation = 30
  3. norm a = .33 norm b = .66
  4. multiplication by the norm gives a = 3.33r and b = 13.66r

This is a derived final value which might yeild better results. The whole system is enclosable and useable across a variety of articulation problems.

Shoulder rotation – the constant rotation and reference.

October 21, 2007

Charles

A simple problem, with your arm out t-pose palm flat (facing down) do this:

  1. Rotate you arm down to the side, then forward (You’ll notice the bicep faces up)
  2. Now go back to your arm out. Rotate it forward (the bicep will face to the right or left now)

We have an oddity here, and I think it’s one of the founding principles of biomechanical rotation, it ‘resolves’ itself. When two rotations meet the union causes another plane of freedom to be introduced – i.e the top of the bicep will twist 45 degrees with your arm going forward and -45 going backwards. Looking at this from a math perspective its spherical rotation. (a quaternion)This spherical rotation similar to a quaternion is what stops the arm from twisting itself off its joint. The muscles are are treating its ball and socket joint as a spherical rotation or in other words a quaternion. Now this may not seem interesting but this is before there’s been any rotation of the elbow.I.e the twist of the upper arms is brought on by the constant rotation of the shoulder  and its resolution or twist,  is brought on by the rotation of the elbow. This is why it can be hard to get a frame of reference for the twist. I’ll see if i can update this with some pics.