On Fri, Jan 17, 2014 at 11:03:10AM +0100, Mathias Müller wrote:
> it seems I found an issue regarding the 2D-FFT transformation in the
> current Gwyddion release (v2.34).
>
> I have (again) data with a physical dimension of 1 x 1 q-microns with
> a pixel aspect ratio of 2:1.
>
> Performing standard operations like leveling etc everything works
> fine. But when continuing with FFT transformation the output relates
> to the aspect ratio of the pixel size. Not only in the aspect ratio of
> the displayed image is falsified but also its physical dimension.
> Here, the xrange goes from [-256:256] invers microns, but the yrange
> is [-126:126] invers microns.
> This should be equal.
There might be some subtle bug, but basically the output seems correct.
How things correspond between the direct and requency space?
Direct space Frequency space
───────────────────────────────────────────────────────────
Pixel size Δx Image size 2π/Δx
Pixel size Δy Image size 2π/Δy
Image size Lx Pixel size 2π/Lx
Image size Ly Pixel size 2π/Ly
Your direct-space image has equal dimensions (1µm), so your FFT image
should have *square pixels*.
Your direct-space image has non-square pixels, so your FFT image should
have *non-equal dimensions*.
And this is indeed what you observe.
> Fruther, the toggle menu in the left corner (change between physically
> square and pixelwise square) does not respond.
So, of course the toggle works because the pixels are square.
Regards,
Yeti
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