Fourier shell correlation of two volumes.
 
   INPUT
 
         (no input arguments)  :   if this function  is called without arguments, an user GUI is opened.
 
        a         density map in direct space
                  (as variable, filename, or databse query for a volume)
 
        b         density map in direct space
                  (as variable, filename, or databse query for a volume)
 
 
    Parameter/Values
 
    * for computation
 
    'nshells'   : number of shells
 
    'fmask'     : to restrict the computation to a subset of fourier
                  coefficients.
 
    'apix'      :  angstroms/pixel
                    -default:  1
 
    'nmask'     : normalization mask
                  - volume variable
                  - filename
                  - two values to pass a spheric mask: [radius,smoothing]
 
    'mask'      : mask (can be a soft mask)
                  both volumes are multiplied against this mask.
                  - volume variable
                  - filename
                  - two values to pass a spheric mask: [radius,smoothing]
 
    'sym'        : a symmetry operator (optional)
                   It will be used during the computation of resolution
                   criteria.
                   - default: c1
 
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   * Resolution criterion
 
     'compute_halfbit'   true/false
                    Computes the curve for halfbit and sigma criteria
                    - default: false
 
     'halfbit'     passes a precomputed halfbit curve
 
     'csigma'      passes a precomputed sigma curve
 
 
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   * for depiction
 
    'marks'       resolutions to be marked graphically in the horizontal
                    axis
                  These are to be given in Angstroms.
 
 
    'show'        boolean variable
                  1: shows a depiction
                  0: just runs the computation
                  - default: 0
                  If fsc is called without output arguments, 'show' will be
                  set to 1
 
    'nfig'        directs the graphical output to a given window
 
    'hold'        to direct the plot to a previous figure without deleting
                  its contents.
 
    'color'       to pass a specific color.
 
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   * for output
 
    'o'           writes the fsc in an output file
 
    'ofull'       includes in the output structure the intermediate
                  volumes generated.
 
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   OUTPUT
 
    output
 
    .fsc       the Fourier Shell Correlation curve of the two volumes
 
    .res_05    resolution (in A^-1) according to the 0.5-curve criterion
 
 
 
   SEE ALSO
 
 
 
        dynamo_cone
                % for studies on anisotropic resolution.
 
        dynamo_fsc_show
                %    Basic generic function for printing and annotating.
 
 
        dynamo_compare_fsc
                %    (to generate annotated depictions)
 
 
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   EXAMPLES of SYNTAX
 
    output=dynamo_fsc(a,b);
 
    or simply:
 
    dfsc a b -ws output;
 
    (in this case a and b are filenames or database queries).
 
    % for applications from system
    dynamo_fsc(a,b,'o','my_file.txt');
 
    % to see the curve
    dfsc a b -show on;