[Insight-users] Base question on derivative filters

[Oleksandr Dzyubak]

Don't you change the units while convolving with the Gaussian?

Alex

Kishore Mosaliganti wrote:
> The resulting units are in world coordinates. You will not need to 
> divide by spacing. The sigma is also in world coordinates.
>
> Kishore
>
> On Tue, Sep 8, 2009 at 11:49 AM, Gomez Herrero, Alberto 
> <alberto.gomez-herrero at philips.com 
> <mailto:alberto.gomez-herrero at philips.com>> wrote:
>
>     Hello all,
>
>      
>
>     this is a question regarding derivative filters in general and the
>     RecursiveGaussianImageFilter  in particular.
>
>      
>
>     When we compute, say, the first order derivative of, say, a 2D
>      image “ I“ we get two images of the same size, “Ix”, and “Iy”. My
>     question is,  the resulting value (i.e. the Ix and Iy gray levels)
>     is expressed in what units?, I mean, if the gradient is defined as
>     increment_of_gray_level /
>     increment_of_distance_in_world_coordinates, everything is ok; but
>     if this filter works as a simple pixelwise convolution mask, then
>     the gradient is in increment_of_gray_level / increment_of_pixels.
>     In that case, I need to divide by the spacing to get the right units.
>
>      
>
>     I guess this is a very basic question, but I really need to make
>     sure that this filters work as the first definition. I can’t find
>     it in the code, I only know that the sigma for the smoothing is in
>     world coordinates though…
>
>      
>
>     Anyone can clarify this please?
>
>      
>
>     Thanks,
>
>      
>
>     Alberto
>
>
>     ------------------------------------------------------------------------
>     The information contained in this message may be confidential and
>     legally protected under applicable law. The message is intended
>     solely for the addressee(s). If you are not the intended
>     recipient, you are hereby notified that any use, forwarding,
>     dissemination, or reproduction of this message is strictly
>     prohibited and may be unlawful. If you are not the intended
>     recipient, please contact the sender by return e-mail and destroy
>     all copies of the original message.
>
>     _____________________________________
>     Powered by www.kitware.com <http://www.kitware.com>
>
>     Visit other Kitware open-source projects at
>     http://www.kitware.com/opensource/opensource.html
>
>     Please keep messages on-topic and check the ITK FAQ at:
>     http://www.itk.org/Wiki/ITK_FAQ
>
>     Follow this link to subscribe/unsubscribe:
>     http://www.itk.org/mailman/listinfo/insight-users
>
>
> ------------------------------------------------------------------------
>
> _____________________________________
> Powered by www.kitware.com
>
> Visit other Kitware open-source projects at
> http://www.kitware.com/opensource/opensource.html
>
> Please keep messages on-topic and check the ITK FAQ at: http://www.itk.org/Wiki/ITK_FAQ
>
> Follow this link to subscribe/unsubscribe:
> http://www.itk.org/mailman/listinfo/insight-users
>