Possibly because the "Fast" aspect of the "FFT" requires the map size to have a prime factorization of small factors, generally combinations of powers of 2, 3, 5, 7, and 11, in order for the algorithm to work. Here's some basic documentation from the web: http://www.fftw.org/fftw2_doc/fftw_3.html

Nick

Nicholas K. Sauter, Ph. D.
Senior Scientist, Molecular Biophysics & Integrated Bioimaging Division

Lawrence Berkeley National Laboratory
1 Cyclotron Rd., Bldg. 33R0345
Berkeley, CA 94720
(510) 486-5713

On Tue, Feb 18, 2020 at 9:43 AM GEORGE N PHILLIPS <george.phillips@wisc.edu> wrote:

I am confused about grid sizes of maps.

When i specify a grid size for an fft_map but then pull the numpy array out for direct manimpulation, it's always 2 larger in the last dimension.

Why is it not the same size? Any help appreciated.

George

y# Now make initial (smoothed) map using random phases with Fobs
f_obs_B = f_obs1.apply_debye_waller_factors( b_iso=biso_init)
phases = f_obs_B.random_phases_compatible_with_phase_restrictions(deg=False)
phases_data = phases.data()
e_map_coeff = f_obs_B.phase_transfer(phase_source = phases_data)
e_map = e_map_coeff.fft_map(crystal_gridding = crystal_gridding, symmetry_flags = maptbx.use_space_group_symmetry)
print("cgridding", crystal_gridding.n_real())
# pick initial envelope
e_map_array = e_map.real_map().as_numpy_array()
print("e_map", e_map_array.shape)

cgridding (90, 90, 80)
e_map (90, 90, 82)

George N. Phillips, Jr., Ph.D.
Professor Emeritus
University of Wisconsin-Madison
433 Babcock Dr. Madison, Wi 53706
Phone 713 348 6951 (Houston)

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