Documentation 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.