Basic Information on imdiff

Task: imdiff
Purpose: Shift and expand an image to make it match another.
Categories: image-analysis

IMDIFF is a MIRIAD task which finds optimum parameters (in a maximum
likelihood sense) for making one image approximate another image. The
parameters are an amplitude scale factor, dc offset, shifts in x and y
direction and an expansion. Alternately any of these parameters can be
fixed at a given value, and the others allowed to vary. This task
assumes that the rms noise levels in both images are identical (If this
is not the case then one of the images should be scaled to make it the
case). Also noise in the two images is assumed to be uncorrelated. If
either of these conditions are violated, the algorithm will not perform
optimally when the signal to noise ratio is poor.

This task is intended to find only small shifts and expansions. It
will probably fail if large shifts and expansions are present. The
task SHIFTY can handle large, integral, shifts. This task currently
only handles a rectangular region of interest. It does not handle
blanked pixels correctly.

Key: in1
The first input image. This is considered to be the ``reference''
image. No default.

Key: in2
The second input image. This is considered to be the image that must be
adjusted to make it look like IN1. No default.

Key: adjust
This gives the name of an output image data-set, being a version
of second image after adjustment. The default is not to  output an
adjusted image.

Key: resid
An output image consisting of the difference between IN1 and the
adjusted form of IN2. It is generally best to look at the residuals,
to check that they are noise-like. The residuals are defined as:

  Resid(x,y) = In1(x,y) - Amp*In2(Expand*x+Xshift,Expand*y+YShift)
               - Offset

(Here X and Y are relative to IN2's reference pixel). The default is
not to create the residuals.

Key: region
The region of interest. Currently this must be a rectangular
region. The default is the entire image.

Key: guard
The task cannot work right to the edge of the image, because the
shifts and expansions might mean that there is no corresponding point
between images near the image edges. Consequently a guard band is
needed. GUARD gives the width of this band, in pixels. In the output
residuals, this band will be set to zero. Default is 10.

The following five parameters give the initial estimates for the
various parameters that IMDIFF is trying to find. If the parameters
are fixed, then IMDIFF will not change them from their initial
settings.

Key: xshift
This gives the initial value, in pixels, of the x-shift.
This should be only +/- a few pixels. Default is 0.

Key: yshift
This gives the initial value, in pixels, of the y-shift. Default is 0.

Key: expand
This gives the initial expansion. Only small expansions can be
handled (e.g. approximately in the range 0.95 to 1.05). This should
be adequate for proper motion studies.  Give two values if you
are allowing the x- and y-expansions to vary independently, one
value otherwise.   Default is 1.

Key: amp
This gives the initial amplitude scale factor. Default is 1.

Key: offset
This gives the initial dc offset to apply to IN2. The default
is 0.

Key: options
Task enrichment options.  Minimum match is active. These determine
what to solve for. Possible values are:
  noamplitude Do not adjust the amplitude of the second image.
              The default is to adjust the amplitude.
  nooffset    Do not adjust the offset of the second image.
              The default is to adjust the offset.
  noxshift    Do not shift the second image in x.
              The default is to optimise the shift in x.
  noyshift    Do not shift the second image in y.
              The default is to optimise the shift in y.
  noexpand    Do not apply an expansion factor to the second image.
              The default is to apply an expansion factor in y.
  expand      Expand the x and y axes independently. The default
              is that the expansions for the two axes are the same.

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