xftoxg(1)                   General Commands Manual                  xftoxg(1)

       xftoxg - Converts transforms between sections into alignment transforms

       xftoxg  [options]  input_file  [output_file]

       Xftoxg takes a list of transformations (f) from each section to the
       previous one, and computes a list of xforms (g) to apply to each sec-
       tion to obtain a single consistent set of alignments.  Transforms can
       be simple 6-component linear transforms or warping transformations.

       Three different modes of operation can be selected.    In the first,
       all sections will be transformed to a single position in the "center"
       of the series.    Here "position" refers to the entire set of parame-
       ters describing the transformation, i.e. not just the x,y coordinates
       but the rotation and stretch as well.  This mode, sometimes referred as
       giving a global alignment, can be invoked with the option "-n 0".  This
       mode will remove any consistent trends in the data stack, such as a
       progressive rotation, compression or shift.  Because of this effect,
       this mode is often inappropriate for large sets of serial sections.
       For example, if the alignment is dominated by a structure (such as a
       cell boundary) that becomes actually smaller toward one end of the
       series, the global alignment will erroneously expand sections at the
       end of the series to keep the structure the same size.

       In the second mode, the "center" position is allowed to change progres-
       sively over the series.  For each section, the program fits polynomials
       to the transformation parameters over the selected distance (centered
       on the given section, if possible), and uses these polynomial fits to
       define the position to which that section should be transformed.  This
       mode will retain trends in the data stack that occur over the range of
       the specified distance; for the example in the last paragraph, the
       trend for the cell to get smaller would be retained and sections would
       not be expanded inappropriately.  This is the default mode of opera-
       tion, with linear fits over 7 adjacent sections.

       In the third mode, the "center" position is assumed to change progres-
       sively over the whole series, based on a polynomial fit to the series,
       and a transformation will be computed for each section that will align
       it to a fitted shifting center position.  If a polynomial order of 1 is
       selected, the center position will shift linearly from one end to the
       other of the data stack.  This mode will retain trends in the data
       stack to the extent that they can be fit by the order of polynomial
       chosen.  For example, if order 1 is chosen, the fit is a linear one,
       and the resulting transforms will retain trends that occur at a con-
       stant rate from one end to the other.

       It is also possible to do a mixed or hybrid alignment in which trends
       are retained for some parameters while they are eliminated for others.
       Trends can be eliminated just for X and Y translations, just for rota-
       tions, for both translations and rotations, or for overall size changes
       as well.  The most appropriate choice would be to eliminate trends for
       rotations and possibly for translations as well, unless the rotation
       trends in the data are meaningful and should be preserved.  Eliminating
       trends in translation can result in progressive shifts that become
       extensive over a long series of sections.  This hybrid method can be
       used with either local or global polynomial fits (the second or third
       mode of operation).

       Sometimes a small minority of sections are rotated at a large angle
       from the rest.  Including such sections in the computation of the "cen-
       ter" position would skew the average rotation.  In such cases, the
       "-range" option can be used to exclude sections whose rotation angles
       fall outside of the given range from the averaging and fitting proce-
       dures.  For example, if a range of 45 degrees is given, then the pro-
       gram will find the largest group of sections whose angles span a range
       of 45 degrees or less, and use only these sections to compute the
       global average alignment.  However, if there are large blocks of sec-
       tions at a variety of different rotations, this approach will not work,
       and the recourse is to use the mixed alignment to eliminate trends in
       rotation, thus aligning all sections to a single global rotation.

       The input file can contain warping transformations, which can be speci-
       fied either as displacements at a set of control points, or as dis-
       placements on a regular grid.  In either case, the output will consist
       of a regular grid of displacements, with the same grid locations for
       every section.  The program first analyzes each warping transform to
       remove the linear component from it and combine that with the separate
       linear transformation.  The linear transforms are treated as usual,
       according to the various options that are given, including the choice
       for a hybrid alignment.  The remaining warp transforms are analyzed
       similarly to the linear transforms.  When a global alignment is being
       done, the program forms a cumulative warping transform by multiplying
       successive transforms, determines the average cumulative transform and
       multiplies each cumulative transform by this average.  This method pro-
       duces an acceptable result for a small number of sections.  When local
       or global polynominal fits are used for the linear transforms, a simi-
       lar fitting method is applied at each position of the warping trans-
       forms.  This method keeps the converted warp vectors from building up
       over large numbers of sections.

       The program can also do robust fitting, in which it attempts to elimi-
       nate or down-weight transformation components that appear to be out-
       liers.  This is an experimental feature intended to deal with situa-
       tions where a few isolated images need a much bigger shift, rotation,
       size change, or stretch than others to bring them into alignment.  With
       the default linear fitting to 7 sections, an aberrant image would be
       brought 6/7 of the way into alignment with its neighbors, and the
       neighbors would be pulled out of their proper alignment by the remain-
       ing 1/7.  In principle, with robust fitting, the aberrant image would
       be eliminated from local fits, the neighbors' alignments would be
       undisturbed, and the aberrant one would be brought to their alignment.

       To do the averaging and fitting, the program converts the A matrix of
       the g transforms to the "semi-natural" parameters: global rotation and
       magnification, and differences between Y and X axis rotation and
       stretch.  This conversion allows the hybrid alignments to be done.  It
       also means that if input transformations involve only a restricted set
       of these parameters (e.g., rotation only), this restriction will be
       retained in the final g transforms.

       The ability to use both the local fits and fits of order higher than 1
       is somewhat redundant.  If local fitting is used, start with order 1
       and go to higher orders only if necessary.

       Xftoxg uses the PIP package for input (see the manual page for pip)
       and can take also input interactively to maintain compatibility with
       old command files.  The following options can be specified either as
       command line arguments (with the -) or one per line in a command file
       (without the -).  Options can be abbreviated to unique letters; the
       currently valid abbreviations for short names are shown in parentheses.

       -input (-in) OR -InputFile     File name
              Input file with f transforms

       -goutput (-g) OR -GOutputFile       File name
              Output file for g transforms.  If an output filename is omitted
              and the input filename ends in xf, the input filename will be
              used as output with the last letter converted from f to g.

       -nfit (-n) OR -NumberToFit     Integer
              Number of adjacent sections to fit, 0 for global alignment, or 1
              for fit to all sections

       -ref (-re) OR -ReferenceSection     Integer
              Do a global alignment to the given section; this will give the
              reference section a unit transform and keep it from being trans-
              formed.  Sections are numbered from 1.

       -order (-o) OR -OrderOfPolynomialFit     Integer
              Order of the polynomial fit to the data. The default is 1 (lin-
              ear fit).

       -mixed (-mi) OR -HybridFits    Integer
              Number of parameters to eliminate trends for with a hybrid
              alignment: 1 for rotations only, 2 for translations only, 3 for
              translations and rotations, 4 for size changes also.

       -range (-ra) OR -RangeOfAnglesInAverage       Floating point
              Compute the global average center position, and the center posi-
              tion for local fits, using the largest group of sections whose
              rotation angles fall within the given range.

       -robust (-ro) OR -RobustFit
              Use robust weighting when fitting to find the center position.
              The fits will be done iteratively, with a weight assigned to
              each point in the fit and the weight adjusted after each itera-
              tion based on the fitting error of that point.  Weights are
              adjusted together in a fit to the four rotation/magnification
              parameters, and independently adjusted in a separate fit to the
              X and Y shift values.

       -kfactor (-k) OR -KFactorScaling    Integer
              Amount to scale the K factor that controls how many points are
              down-weighted in the robust fitting.  The default scaling of 1
              gives a K factor of 4.685, the factor commonly used for the
              Tukey bisquare weighting.  A smaller factor will down-weight and
              eliminate more points.

       -max (-ma) OR -MaximumIterations    Integer
              The maximum number of iterations in the robust fitting.  The
              default is 200.  A negative number can be entered to obtain out-
              put about the changes in weights after each iteration.

       -iteration (-it) OR -IterationParams     Three floats
              This option sets three values that control the robust fitting:
              (1) The maximum fraction of points that are allowed to have zero
              weight.  The actual limit for a particular fit is the nearest
              integer of this value times the number of points being fit.  The
              default is 0.2.  (2) The maximum change in weights allowed
              before considering that a fit has converged, if the fit is not
              oscillating between two states.  The default is 0.02.  (3) The
              maximum change in weights allowed for terminating the fit if it
              is oscillating between two states; the change in weights in one
              state must also be less than the maximum described in (2).  The
              default is 0.04.

       -help (-h) OR -usage
              Print help output

              Read parameter entries from standard input.

       If the program is started with no command line arguments, it reverts to
       interactive  input with the following entries:

       0 to align all sections to a single average central alignment, 1 to
       align to an average alignment fit to a single polynomial, N > 1 to
       align each section to an average alignment fit to the nearest N sec-
       tions, or -1 or -N to do a hybrid of central and shifting alignments
       with fits to a single polynomial or to locally fitted polynomials

       IF you entered a negative number, next enter 2 for central alignment of
       translations only, 3 for central alignment of translations and rota-
       tions, or 4 for central alignment of translations, rotations and size

       IF you entered a nonzero number to the first query, next enter the
       order of the polynomials to fit (1 for linear fits)

       Name of input file of F transforms

       Name of output file of G transforms

       Written by David Mastronarde, 1988
       Added hybrid method, 7/26/01
       Converted to PIP input, 1/25/04
       Added support for warping transforms, June 2011

       Email bug reports to mast at colorado dot edu.

IMOD                                4.12.32                          xftoxg(1)