[Insight-users] Base question on derivative filters

[Gomez Herrero, Alberto]

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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