FSC - Fourier shell correlation and resolution determination ||

(04/5/16)

PURPOSE

Calculates the Fourier Shell Correlation (FSC) curve between two volumes. Stores the FSC curve in a document file and determines the resolution at which FSC falls below requested value (default is 0.5). Can apply a soft mask to real input before FSC calculation. Takes real or Fourier input volumes. This obsolete operation is superceeded by 'FSC NEW'.   Example.

SEE ALSO

FSC NEW [Fourier shell correlation and resolution determination ||]
FRC [Fourier ring correlation and resolution determination ||]
RF 3 [Phase Residual & Fourier shell correlation, 3D ||]
FT [Fourier Transform ||]
MO 3 [Create Model volume]

USAGE

.OPERATION: FSC [v],[sf],[resol]

[This operation can return up to three optional register variables:
Variable Example Receives
First [v] Interpolated voxel where the FSC falls below requested cutoff value
Second [sf] Approximate resolution (in spatial frequency units) at this cutoff level
Third [resol] Resolution (in Angstroms) at this cutoff level

If there are multiple downward crossings of the FSC cutoff line then the values are reported for the LAST crossing. If there is no such crossing the minimal values from the FSC curve are reported.]

.FIRST INPUT VOLUME: SUB001
[Enter name of the first input volume. File can be real or Fourier format.]

.SECOND INPUT VOLUME: SUB002
[Enter name of the second input volume. File must be same size as first input file.]

If both input files are real format then SPIDER asks:


.SHELL WIDTH (RECIPROCAL SPACE UNITS) & MASKING RADIUS (PIXELS) : 0.5, 66
[Enter the shell width in reciprocal space sampling units i.e. (1/number of pixels). Enter radius for second order super-Gaussian masking. See MO 3 for details. If radius is zero or missing then masking is skipped.]

Instead, if both input files not real format then SPIDER asks:

.SHELL WIDTH (RECIPROCAL SPACE UNITS): 0.5
[Enter the shell width in reciprocal space sampling units.]

.VOXEL SIZE (A) & RESOLUTION CUTOFF: 0.41, .5
[Enter the voxel size in Angstroms. If unknown, enter 1.0 and resolution column will be reported in voxel units. Enter resolution cutoff (if missing the default is 0.5). ]

.FSC OUTPUT DOCUMENT FILE: DOC001
[Enter name for the output document file containing the FSC curve.]

.GNUPLOT FILE: res-plot.gn
[Enter name for file which will contain Gnuplot commands which can plot the FSC curve. Enter '*' if you do not want a plotting file.]

NOTES

  1. The following measures are reported for each shell:
    a) Normalized Frequency.  |NORM-FREQ| ---- Doc. file register: #1

    a) Resolution in Angstroms.  |Resolution| ---- Doc. file register: #2

    c) Fourier Shell Correlation  |FSC| ---- Doc. file register: #3
    [SUM(F1 * CONJ(F2))] / [SQRT{SUM(ABS(F1)**2) * SUM(ABS(F2)**2)}]
    where 'CONJ' denotes complex conjugate.

    d) Square root of Fourier Shell Correlation  |SQRT-FSC| ---- Doc. file register: #4
    [SQRT(FSC)]

    e) Square root (2 * Fourier Shell Correlation / (FSC + 1))  |SQRT-2FSC/(FSC+1)| ---- Doc. file register: #5
    [SQRT(2*FSC/(FSC+1)]

  2. If you do not want a results file listing of the FSC values use 'MD' operation with option 'VERBOSE OFF'.

  3. References:
    Saxton and Baumeister, J. of Microscopy, 127,(1982) 127-138.
    M. van Heel, Ultramicroscopy , 21, (1987) 95-100.
    Unser, et. al., Ultramicroscopy , 23, (1987) 39-52.

  4. Pawel Penczek has notes on interpreting FRC/FSC curves.

SUBROUTINES

PR3D, PR3DB, RFACTSD2, FMRS_3

CALLER

FOUR1