FRC - Fourier ring correlation and resolution determination ||

(01/15/16)

PURPOSE

Calculates the Fourier Ring Correlation (FRC) curve between two images. Stores the FRC curve in a document file and determines the resolution at which FRC falls below requested value (default is 0.5). Takes real or Fourier input images. Can apply a soft mask to real input before FSC calculation.   Example.

SEE ALSO

FSC [Fourier shell correlation and resolution determination ||]
RF [Phase Residual & Fourier ring correlation, 2D ||]
FT [Fourier Transform ||]
MO [Create Model image]

USAGE

.OPERATION: FRC [half],[sf],[resol]

[This operation can return threee optional register variables:
Variable Example Receives
First [half] Interpolated pixel where the FRC falls below requested value
Second [sf] Approximate resolution (in spatial frequency units) at this level
Third [resol] Resolution (in Angstroms) at this 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 IMAGE: IMG001
[Enter the name of the first input image. File can be real or Fourier format. ]

.SECOND INPUT IMAGE: IMG002
[Enter the name of the second input image. File must be of same type and dimensions as first input file.]

If both input files are real format then SPIDER asks:


.RING 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 for details. If radius is zero or missing then masking is skipped.]

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

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

.PIXEL SIZE (A) & RESOLUTION CUTOFF: 0.42, .5
[Enter the pixel size in Angstroms for images. If unknown, enter 1.0 and resolution column will be reported in pixel units. Enter resolution cutoff (if missing the default is 0.5).]

.FRC OUTPUT DOCUMENT FILE: FRC001
[Enter name of document file in which results are to be saved.]

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

NOTES

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

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

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

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

  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

RFACTSDO, PR3DB, RFACTSD2, FMRS_2

CALLER

FOUR1

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