[Chimera-users] fourier transforms

Tom Goddard goddard at cgl.ucsf.edu
Thu Jan 31 11:03:15 PST 2008

Hi Matt,

  The Chimera fourier transform menu entry uses the displayed step size 
for the volume data.  So if you are showing a 512^3 map with step size 4 
then you will get a 128^3 fourier transform.  Set the step size on the 
original map to 1 and then use the fourier transform menu entry to get a 
512^3 fourier transform.

  This will take a lot of memory and time.  The FT code is not very 
efficient, using 16 bytes per voxel -- so it will need 2 Gbytes for 
computing the FT plus the memory of the original map.  Also rendering a 
512^3 map at full resolution is likely to bring your graphics card to a 
crawl and uses a great deal of memory too.

  You should also be aware that the resulting FT is the magnitude of the 
fourier coefficients, the phases are thrown away.  Also the zero 
frequency term is set to zero because it is usually very large making 
the volume histogram unuseful.  So this FT is only intended for visual 
interpretation as described in the documentation:



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