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Section 3. Using FFTW
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Q3.1 FFTW seems really slow.
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Q3.2 FFTW gives results different from my old FFT.
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Q3.3 Can I save FFTW's plans?
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Q3.4 Why does your inverse transform return a scaled result?
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Q3.5 How can I make FFTW put the origin (zero frequency) at the center
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Q3.6 How do I FFT an image/audio file in *foobar* format?
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Q3.7 My program does not link (on Unix).
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Q3.2 FFTW slows down after repeated calls.
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Q3.3 An FFTW routine is crashing when I call it.
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Q3.4 My Fortran program crashes when calling FFTW.
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Q3.5 FFTW gives results different from my old FFT.
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Q3.6 Your in-place transform gives incorrect results.
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Q3.7 Can I save FFTW's plans?
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Q3.8 Why does your inverse transform return a scaled result?
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Q3.9 How can I make FFTW put the origin (zero frequency) at the center
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Q3.10 How do I FFT an image/audio file in *foobar* format?
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Q3.11 My program does not link (on Unix).
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Q3.12 My program crashes, complaining about stack space.
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Section 4. Internals of FFTW
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Q4.1 How does FFTW work?
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Q5.8 FFTW 2.1.1's MPI test programs crash with MPICH.
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Q5.9 FFTW 2.1.2's multi-threaded transforms don't work on AIX.
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Q5.10 FFTW 2.1.2's complex transforms give incorrect results for large p
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Q5.11 FFTW 2.1.3 crashes on AIX
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===============================================================================
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successful compilations of FFTW using Borland's C/C++ compilers on
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The Portland Group PGCC compiler, version 4.0 for Linux/x86, reportedly
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produces incorrect code for FFTW 2.1.3.
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Visual C++ 4.0 crashes when compiling FFTW 1.2 with all optimizations
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turned on. Visual C++ 5.0 reportedly produces incorrect code for the real
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transforms in FFTW 2.x when the option "Maximize speed" is set. We are
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-xO5 option in Sun's SC4.0 C compiler. egcs 1.0.2 produced incorrect code
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for FFTW on the PowerPC (corrected in egcs 1.1).
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The DEC C compiler, V5.8-009 on Digital UNIX V4.0 (Rev. 1091) is known to
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have bugs with its EV6 tuning. Specifically, compiling FFTW with -arch
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host (the default) or -arch ev6 on an Alpha EV6 can cause FFTW (e.g.
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fftw_test -s 256) to crash.
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-------------------------------------------------------------------------------
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Question 2.4. FFTW does not compile on Solaris, complaining about const.
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code, documented in the FFTW manual, is included with FFTW (versions 1.3
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By default, FFTW configures its wrapper routines to work with the first
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compiler it finds, e.g. g77. To configure for a different, incompatible
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Fortran compiler foobar, use ./configure F77=foobar when installing FFTW.
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-------------------------------------------------------------------------------
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Question 2.7. Can I call FFTW from C++?
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Section 3. Using FFTW
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Q3.1 FFTW seems really slow.
256
Q3.2 FFTW gives results different from my old FFT.
257
Q3.3 Can I save FFTW's plans?
258
Q3.4 Why does your inverse transform return a scaled result?
259
Q3.5 How can I make FFTW put the origin (zero frequency) at the center
260
Q3.6 How do I FFT an image/audio file in *foobar* format?
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Q3.7 My program does not link (on Unix).
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Q3.2 FFTW slows down after repeated calls.
275
Q3.3 An FFTW routine is crashing when I call it.
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Q3.4 My Fortran program crashes when calling FFTW.
277
Q3.5 FFTW gives results different from my old FFT.
278
Q3.6 Your in-place transform gives incorrect results.
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Q3.7 Can I save FFTW's plans?
280
Q3.8 Why does your inverse transform return a scaled result?
281
Q3.9 How can I make FFTW put the origin (zero frequency) at the center
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Q3.10 How do I FFT an image/audio file in *foobar* format?
283
Q3.11 My program does not link (on Unix).
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Q3.12 My program crashes, complaining about stack space.
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-------------------------------------------------------------------------------
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is significant, you have two options. First, you can use the
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FFTW_ESTIMATE option in the planner, which uses heuristics instead of
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runtime measurements and produces a good plan in a short time. Second,
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you can use the wisdom feature to precompute the plan; see Q3.3 `Can I
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you can use the wisdom feature to precompute the plan; see Q3.7 `Can I
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save FFTW's plans?'
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-------------------------------------------------------------------------------
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Question 3.2. FFTW gives results different from my old FFT.
306
Question 3.2. FFTW slows down after repeated calls.
308
Probably, NaNs or similar are creeping into your data, and the slowdown is
309
due to the resulting floating-point exceptions. For example, be aware
310
that repeatedly FFTing the same array is a diverging process (because FFTW
311
computes the unnormalized transform).
313
-------------------------------------------------------------------------------
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Question 3.3. An FFTW routine is crashing when I call it.
317
You almost certainly have a bug in your code. For example, you could be
318
passing invalid arguments (such as wrongly-sized arrays) to FFTW, or you
319
could simply have memory corruption elsewhere in your program that causes
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random crashes later on. Learn to debug, and don't complain to us unless
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you can come up with a minimal program (preferably under 30 lines) that
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illustrates the problem.
324
-------------------------------------------------------------------------------
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Question 3.4. My Fortran program crashes when calling FFTW.
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As described in the manual, on 64-bit machines you must store the plans in
329
variables large enough to hold a pointer, for example integer*8.
331
-------------------------------------------------------------------------------
333
Question 3.5. FFTW gives results different from my old FFT.
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People follow many different conventions for the DFT, and you should be
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sure to know the ones that we use (described in the FFTW manual). In
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You should also know that we compute an unnormalized transform. In
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contrast, Matlab is an example of program that computes a normalized
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transform. See Q3.4 `Why does your inverse transform return a scaled
343
transform. See Q3.8 `Why does your inverse transform return a scaled
296
-------------------------------------------------------------------------------
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Question 3.3. Can I save FFTW's plans?
346
Finally, note that floating-point arithmetic is not exact, so different
347
FFT algorithms will give slightly different results (on the order of the
348
numerical accuracy; typically a fractional difference of 1e-15 or so).
350
-------------------------------------------------------------------------------
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Question 3.6. Your in-place transform gives incorrect results.
354
As described in the FFTW manual, the output array argument has a special
355
meaning for FFTW_INPLACE transforms; you should not pass the input array
358
-------------------------------------------------------------------------------
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Question 3.7. Can I save FFTW's plans?
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Yes. Starting with version 1.2, FFTW provides the wisdom mechanism for
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saving plans. See Q4.3 `What is this wisdom thing?' and the FFTW manual.
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365
-------------------------------------------------------------------------------
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Question 3.4. Why does your inverse transform return a scaled result?
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Question 3.8. Why does your inverse transform return a scaled result?
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Computing the forward transform followed by the backward transform (or
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vice versa) yields the original array scaled by the size of the array.
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-------------------------------------------------------------------------------
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Question 3.5. How can I make FFTW put the origin (zero frequency) at the center of its output?
383
Question 3.9. How can I make FFTW put the origin (zero frequency) at the center of its output?
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For human viewing of a spectrum, it is often convenient to put the origin
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in frequency space at the center of the output array, rather than in the
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-------------------------------------------------------------------------------
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Question 3.6. How do I FFT an image/audio file in *foobar* format?
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Question 3.10. How do I FFT an image/audio file in *foobar* format?
335
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FFTW performs an FFT on an array of floating-point values. You can
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certainly use it to compute the transform of an image or audio stream, but
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-------------------------------------------------------------------------------
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Question 3.7. My program does not link (on Unix).
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Question 3.11. My program does not link (on Unix).
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Please use the exact order in which libraries are specified by the FFTW
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manual (e.g. -lrfftw -lfftw -lm). Also, note that the libraries must be
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*A* uses *B*, then *A* must be listed before *B* in the link command.).
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For example, switching the order to -lfftw -lrfftw -lm will fail.
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-------------------------------------------------------------------------------
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Question 3.12. My program crashes, complaining about stack space.
416
You cannot declare large arrays statically; you should use malloc (or
417
equivalent) to allocate the arrays you want to transform if they are
418
larger than a few hundred elements.
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===============================================================================
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Section 4. Internals of FFTW
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Q5.8 FFTW 2.1.1's MPI test programs crash with MPICH.
427
497
Q5.9 FFTW 2.1.2's multi-threaded transforms don't work on AIX.
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498
Q5.10 FFTW 2.1.2's complex transforms give incorrect results for large p
499
Q5.11 FFTW 2.1.3 crashes on AIX
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-------------------------------------------------------------------------------
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that caused incorrect results for many primes greater than 32768 (on
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32-bit machines). (Sizes without large prime factors are not affected.)
582
-------------------------------------------------------------------------------
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Question 5.11. FFTW 2.1.3 crashes on AIX
586
The FFTW 2.1.3 configure script picked incorrect compiler flags for the
587
xlc compiler on newer IBM processors. This is fixed in FFTW 2.1.4.
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FAQ/fftw-faq.ascii
