On Fri, Sep 12, 2014 at 03:32:04PM +0200, Mathias Müller wrote:
> in my C-code I want to apply the gwyddion fft-transformation to SPM data.
>
> the code sniplet is:
>
> data_field_fft_r = gwy_data_field_new_alike(data_field, TRUE);
> data_field_fft_i = gwy_data_field_new_alike(data_field, TRUE);
> gwy_data_field_2dfft(data_field,NULL,data_field_fft_r,data_field_fft_i,GWY_WINDOWING_NONE,GWY_TRANSFORM_DIRECTION_FORWARD,GWY_INTERPOLATION_ROUND,TRUE,1)
> ;
> gwy_data_field_2dfft_humanize(data_field_fft_r) ;
>
> where data_field is obtained from real space data.
>
> When displaying the artifical data, it looks pretty good, e.g. like a
> hexagonal structure. Comparing the coordinates within the differently
> obtained FFT images, it looks
> like all the x and y values are way larger in the c-code 2d-FFT (no window)
> then the one from the gwyddion module. I.e. the very center of the
> gwyddion-performed fft-image
> is nearly at (0.002,0.004) nm^-1. But the data reasulting from
> gwy_data_field_2dfft_humanize(data_field_fft_r) show a center at approx.
> (3.43886e-08,5.05459e-08)...I guess m^-1.
Functions such as gwy_data_field_2dfft() only perform the transforms.
They do not set units nor real fiels dimensions. This is somewhat
unfortunate, but backward compatible. See for instance
fft_prosptocess() in the FFT module:
https://sourceforge.net/p/gwyddion/code/HEAD/tree/trunk/gwyddion/modules/process/fft.c#l182
Regards,
Yeti
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