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imdiff - Shift and expand an image to make it match another.  






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.  


The first input image. This is considered to be the ``reference'' image. No default.
The second input image. This is considered to be the image that must be adjusted to make it look like IN1. No default.
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.
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.

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

This gives the initial value, in pixels, of the x-shift. This should be only +/- a few pixels. Default is 0.
This gives the initial value, in pixels, of the y-shift. Default is 0.
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.
This gives the initial amplitude scale factor. Default is 1.
This gives the initial dc offset to apply to IN2. The default is 0.
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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Time: 18:35:38 GMT, July 05, 2011