[Insight-users] Very strange link error while using vnl_Quaternion

[Karthik Krishnan]

Di Xiao wrote:

> Dear all:
>
>  I got a very strange link error while using vnl_Quaternion (coming 
> with itk lib).
>  While I use some function in vnl_Quaternion, like .size(), 
> .normalize() or .real() etc., there is no link error. But while I use 
> .angle(), .inverse() and .rotate() etc. functions, there will have 
> undefined reference errors, as  undefined reference to 
> `vnl_quaternion<float>::operator*(vnl_quaternion<float> const&) const' .
>
vnl uses explicit template instantiation to deal with allowed types. See

Utilities/vxl/core/vnl/Templates contains the list of supported templates.

vnl_quaternion< double > is the only supported type.

About the linker errors:
The quaternion, derives from vnl_vector_fixed (of length 4) . The 
methods normalize, size etc belong to the vnl_vector_fixed which is 
explicitly instantiated over float and is ok. The methods "angle" etc 
belong to the quaternion and you get a linker error.

I can't think of  a good reason for representing quaternions in any data 
type other than double, but if you can, I can add it the templates, so 
it will get compiled into the vnl library.

> Any suggestion for solving it is welcomed.
>
> Thanks
>
> Xiao Di
>
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