[ODE] constrain to bézier curve

[Daniel Holth]

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Aha!

All one has to do is, for each time step, constrain the object to a line
tangent to the curve at the closest point on that curve. Eventually do
the same for the object's orientation, but that is not quite as important.

And then, pull the object towards that closest point to correct the
inevitable error. I'm not yet sure how this is supposed to work; I could
set the object to the closest point, create the slider joint, move it
back, and let ODE's error correction do the rest, or could try to do it
with connected slider joints where a slider joint perpendicular to the
line had limits and forces pulling the object back into the curve.

I've got the first part working with slider joints. No, that is not my
final bézier closest-point code. My example code is Python code at
http://dingoskidneys.com/~dholth/bezier/bezier.zip

- - Daniel Holth
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