[ITK] Definition of Composite Transform

[Floris Berendsen]

Hi all,

I think there is some inconsistency between comments in the code of the
composite transform and its (doxygen) documentation.
The documentation states that a transformation is composed in the following
way:
T_0 o T_1 = T_0(T_1(x)), with T_1 the transform added latest.
https://github.com/InsightSoftwareConsortium/ITK/blob/master/Modules/Core/Transform/include/itkCompositeTransform.h

However, some (very insightful) comments in the code state that:
T is composed of T1(p1,x), T2(p2,x) and T3(p3, x) as: T3(p3, T2(p2, T1(p1,
x0)))
https://github.com/InsightSoftwareConsortium/ITK/blob/master/Modules/Core/Transform/include/itkCompositeTransform.hxx#L620-L659

The confusing part is that the ordering is reversed (and starting from 1
instead of 0). This may also explain the next comment:
// to do: why parameters are listed from N-1 to 1???
https://github.com/InsightSoftwareConsortium/ITK/blob/master/Modules/Core/Transform/include/itkCompositeTransform.hxx#L674

The first definition is consistent with the code as far as I can see. Can
anyone confirm this finding? If so, I'm happy to provide a patch via gerrit
to solve the comments.

Best,
Floris
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