[eigen] Issues regarding Quaternion-alignment and const Maps

[Christoph Hertzberg <chtz@xxxxxxxxxxxxxxxxxxxxxxxx>]

Hi,

first of all great work, everyone!

I have two issues with Eigen2, which I'm not sure Eigen3 has solved/will 
solve (I did look at the new source, but might have overlooked something).

First, I guess there is no (easy) possibility to have an not-aligned 
quaternion (without disabling alignment globally).
For ordanary matrices there is a Eigen::DontAlign option, which I 
haven't found for Quaternions. And currently I don't see a very lot of 
gain in vectorization of Quaternions, especially (quaternion * vector3) 
is not vectorized, and (quaternion * quaternion) only for float (at 
least in Eigen2).


The second issue is a bit more complicated:
Eigen::Map severely violates const-correctness. I.e., it is possible to 
create a non-const Map from a (const double*).
If you still accept API-changes, I would suggest having a Map and a 
ConstMap (similar to iterators in STL).

Ok, and actually a third very minor issue regarding Maps:
Maps of fixed sizes should boil down to a single pointer, but they don't 
due to the ei_variable_if_dynamic-object, which is not allowed to have 
size 0, even if it contains no members.
One possible solution (without to much code-rewriting) would be to 
combine the sizes and the m_data pointer to a single object, providing 
getter-functions for rows and cols, and some template-specializations if 
sizes are Dynamic (similar to how ei_matrix_storage is implemented)

Best regards,
Christoph

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Dipl.-Inf. Christoph Hertzberg
Cartesium 0.051
Universität Bremen
Enrique-Schmidt-Straße 5
28359 Bremen

Tel: (+49) 421-218-64252
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