[Chimera-users] MRC map values -128 to 127

Tom Goddard goddard at cgl.ucsf.edu
Fri Sep 7 13:11:00 PDT 2007

Hi Bernard,

  Thanks for the info on the history of signed versus unsigned 8-bit MRC 
files.  I see that CCP4 clearly uses signed 8-bit and most MRC used for 
EM data uses unsigned.  Is there any definitive specification for MRC 
format?  I just made up that bit about an "MRC standard".  I'm not aware 
of anyone publishing a standard.  The Chimera change was motivate by the 
EM Databank using CCP4 for their maps.  A possible more explicit 
handling as you suggest is to assume signed 8-bit for file suffixes 
".ccp4" and ".map" (used by EMDB) and unsigned 8-bit for the file suffix 
".mrc" -- basically distinguish CCP4 from MRC format in Chimera.  I'm 
not sure if the ".map" suffix used by EMDB to mean CCP4 is in use by 
other EM labs to mean MRC.


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