I have an animation with balls rolling over an isosurface.  My algorithm
essentially moves the surface tangent point around, and draws a sphere
with center surfacepoint+Normal*radius.  It looks ok with uniform
textures, but I want the balls to roll.

In 1D, it's easy.
     rotate <0, 0, distancerolled.x*360/2/pi/radius>

In 2D, it's not that hard (is this right)?
     rotate <distancerolled.z*360/2/pi/radius, 0,
distancerolled.x*360/2/pi/radius>

I've tried all kinds of tricks for 3D, and nothing looks right. The
problem is with a surface with a y component (rolling hills). If it
drops a great amount in y, it should roll more, if it drops less in y,
it rolls less.

Anyone ever addressed this problem?

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