[Insight-users] Re: VersorRigid3DTransform

[Luis Ibanez]

Hi Radhika,

The Center of rotation was removed because it
is not really needed for the optimization and
it costs 50% more on computing time to attempt
to optimize the center (9 parameters versus
6 parameters).

In practice it is enough to get close to the
real center of rotation in order stabilize the
optimization. Note that in principle, we could
have stick to always use the origin of coordinates
as center of rotation. Mathematically it is enough
to rotate around the origin and to apply a translation
in order to get any rigid transformation.

The only reason for introducing an arbitrary center
of rotation is that it reshapes the cost function in
such a way that is is easy to perform a gradient
descent on it.

The Center of rotation is set to the origin by default.
I *strongly* suggest you to set the Center of rotation
with the helper class

      CenteredTransformInitializer
http://www.itk.org/Insight/Doxygen/html/classitk_1_1CenteredTransformInitializer.html

as is illustrated in the Example file

   Insight/Examples/Registration/ImageRegistration8.cxx


Regards,


    Luis


---------------------------------

Radhika Sivaramakrishna wrote:

> Hi Luiz,
> Thanks for making this code available so fast. I tried to use the new 
> VersorRigid3DTransformOptimizer for my problem. I noticed that it only 
> has 6 parameters. Why was the center of rotation removed?
> 
> Also, does it assume origin is the center of rotation?
> 
> Thanks
> Radhika
> 
> 
> -----Original Message-----
> From: Luis Ibanez [mailto:luis.ibanez at kitware.com]
> Sent: Monday, January 05, 2004 1:59 PM
> To: Radhika Sivaramakrishna
> Cc: 'insight-users at itk.org'
> Subject: Re: VersorRigid3DTransform
> 
> 
> Hi Radhika,
> 
> 
> Bug #475  has been fixed & closed.
> 
> The new class
> 
>       itkVersorRigid3DTransformOptimizer
> 
> is now available under
> 
>       Insight/Code/Numerics
> 
> Its test is passing and an example
> on its use was setup under
> 
>    Insight/Examples/Registration/
>                 ImageRegistration8.cxx
> 
> 
> Please let us know if you encounter
> any problems using this class.
> 
> 
>    Thanks
> 
> 
>       Luis
> 
> 
> -------------------------------
> Radhika Sivaramakrishna wrote:
> 
>  > Hi Luis,
>  >
>  > Thanks for your detailed responses on the optimizer for the
>  > VersorRigid3DTransform.
>  >
>  > 
>  >
>  > Could you let me know the timeframe when you anticipate this optimizer
>  > to be available. Is there a way to check for it on my own for eg. on the
>  > dashboard?
>  >
>  > 
>  >
>  > 
>  >
> 
> 
>                                                     
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