[Insight-users] Base question on derivative filters

[Luis Ibanez]

The RecursiveGaussian filter doesn't use convolution.

The value of sigma is set in physical coordinates.

The output pixel values will have units of :

       input intensity values /  physical coordinates

    *IF* you use the FirstOrder setting.

Note that there are also ZeroOrder and SecondOrder
settings.

In which case the intensity units of the output will be:

ZeroOrder:      input intensity units

Second Order :  input intensity units / (physical units)^2



    Regards,


         Luis



-------------------------------------------------------------------------------
On Wed, Sep 9, 2009 at 9:51 AM, Oleksandr Dzyubak <adzyubak at gmail.com>wrote:

> 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
>>
>>
>>
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