[GRASS5] Problem compiling with gdal and/or fftw

Glynn Clements glynn at gclements.plus.com
Mon Feb 13 23:42:36 EST 2006

Michael Rensing wrote:

> Thanks everyone. I buess I was fooled into thinking that fftw3 would 
> replace fftw2 in all cases when yum update automagically replaced it. 
> I'll see if I can re-install fftw2.
> Has anyone got a quick explanation why the two versions of fftw are not 
> compatible?

According to the FFTW FAQ:

	Question 3.1. Why not support the FFTW 2 interface in FFTW 3?
	FFTW 3 has semantics incompatible with earlier versions: its plans can
	only be used for a given stride, multiplicity, and other
	characteristics of the input and output arrays; these stronger
	semantics are necessary for performance reasons. Thus, it is
	impossible to efficiently emulate the older interface (whose plans can
	be used for any transform of the same size). We believe that it should
	be possible to upgrade most programs without any difficulty, however.

Note that the FFT functionality in GRASS creates a new plan for each
transform and discards it afterwards, so it would be a simple matter
to update lib/gmath/fft.c to use the FFTW v3 API instead.

Supporting both versions of the API would require more work, most of
which would be changes the configuration and build system.

Glynn Clements <glynn at gclements.plus.com>

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