The way I do it is as follows:
For both representations I compute the values at the 16
points (0,0),(1/3,0),(2/3,0),(1,0),(0,1/3),(1/3,1/3),...(1,1)
as a function of the input parameters. This wil give a 16 by
16 matrix. Then I compute the inverses of these matrices.
conversion from bezier to hermite is then by first applying
the bezier matrix followed by the inverse hermite matrix.
You can do this in one step of course.

Hermite patches use the value at the 4 points, plus the
two first order derivatives in the u and v direction,
plus the second order derivative in the uv direction.
So we have for each corner i: A_i, dA_i/du, dA_i/dv and
d^2A_i/dudv, if this notation makes any sense to you.

If you need more help I will be glad to be of assistance,
(<didactic mode> but you generally understand it better if
you do the computations yourself </didactic mode>).

	Andrel

Sascha Ledinsky wrote:
> I'd like to convert the 16 control-points of a (bicubic) bezier patch 
> into a hermite patch and vice versa.
> I know how to do it for curves, but with patches I don't know how the 4 
> inner bezier-controlpoints can be converted into the "twist vectors" of 
> the hermite form (and vice versa).
> What do the twist vectors actually represent?
> Do you know an equation, or a link to some paper describing this issue?
> 
> Thanks for your help!

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