Steven G. Johnson wrote: > Andrew Jaffe wrote: >> numpy returns n/2+1 complex numbers (so the first and last numbers are >> actually real) with the frequencies equivalent to the positive part of >> the fftfreq, whereas scipy returns n real numbers with the frequencies >> as in rfftfreq (i.e., two real numbers at the same frequency, except for >> the highest and lowest) [All of the above for even n; but the difference >> between numpy and scipy remains for odd n.] >> >> I think the numpy behavior makes more sense, as it doesn't require any >> unpacking after the fact, at the expense of a tiny amount of wasted >> space. But would this in fact require scipy doing extra work from >> whatever the 'native' real_fft (fftw, I assume) produces? > > As an author of FFTW, let me interject a couple of points into this > discussion. > > First, if you are using FFTW, then its real-input r2c routines > "natively" produce output in the "unpacked" numpy format as described > above: an array of n/2+1 complex numbers. Any "packed" format would > require some data permutations. Other FFT implementations use a > variety of formats. > > Second, the *reason* why FFTW's r2c routines produce unpacked output is > largely because "packed" formats do not generalize well to > multi-dimensional FFTs, while the "unpacked" format does. (Packed > formats are *possible* for multidimensional transforms, but become > increasingly intricate as you add more dimensions.) Additionally, I > personally find the unpacked format more convenient in most > applications. > > I hope this is helpful.
OK -- so it appears that all (three) of the votes so far are in support of the numpy convention -- a complex result. If this is something that can be done in pure python, I'm willing to give it a stab, but I'm probably not capable of handling any python/C issues. Does anyone out there understand the interaction between fftpack (C & python?), fftw (C), scipy and numpy in this context well enough to give some advice? Yours, Andrew ------------------------------------------------------------------------- Using Tomcat but need to do more? Need to support web services, security? Get stuff done quickly with pre-integrated technology to make your job easier Download IBM WebSphere Application Server v.1.0.1 based on Apache Geronimo http://sel.as-us.falkag.net/sel?cmd=lnk&kid=120709&bid=263057&dat=121642 _______________________________________________ Numpy-discussion mailing list Numpy-discussion@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/numpy-discussion
