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Interpolation
Realsoft 3D key framer supports two interpolation methods for key framing rotations.
Angle Space
Quaternion space
Angle space interpolation simply interpolates recorded rotation angles. The drawback of this method is that the two dimensional angle space has two singularity points. Any path passing near these points typically cause problems.
Quaternion interpolation solves this problem by using four dimensional angle space for interpolation. This is an advanced method and results very nice rotations. However, the problem is that the resulting animation curves are somewhat difficult to control.
Angle System
Skeletons support two kind of angle systems:
HPB - Heading Pitching Banking
HPT - Heading Pitching Twisting
HPB is the standard HPB angle system (stands for Heading, Pitching and Banking). The first two angles define polar coordinates for the bone. The third angle 'Banking' is defined so that as long as the 'x' axis of the bone lays on the ground plane, banking is said to be zero. This makes the method particularly suitable for controlling cameras. With zero banking, the x axes of the subsequent joints lay on the common plane.
HPT is a new angle system developed by Realsoft.
The difference to the standard HPB system is that the third angle is defined in such a way that it minimizes the surface tension of the attached body rather than trying to keep the bones horizontally aligned.
Think about a striped flexible plastic pipe. Bend it to any direction and the stripes along the body tell you intuitively whether there are twisting or not. If the stripes appear spiral like, there is twisting involved. If stripes along the body follow straight lines, the twisting is zero.
In other words, when there is no real twist on the body attached to the skeleton, the third angle 'Twist' is said to be zero. What could be a more natural way of defining bone angles!
This system has several advantages over the standard HPB angle system. First, you typically have to edit only two bone angles: heading and pitching. You need to touch twisting angle only when you really need to get some real skin twisting introduced. In the standard angle systems, the banking angle has nothing to do with real twisting. In animations, you have to edit the banking angle all the time to eliminate unwanted twisting from the body. Another advantage is that I.K. can now operate based on heading and pitching angles only. This makes the I.K. tool much more useful.
The third and significant advantage of the new HPT angle system is that it helps to eliminate the biggest problem with angular spaces. In any angle space, there are two so called singularity points - the poles. When you animate, say, a camera over these poles, you'll get sudden jump at the north pole. The new HPT system eliminates this problem completely. In this respect, HPT angles are almost as good as Quaternion angles, and easier to handle in the curve editor.
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