MEDSKY-Take the median of a number of images

Description:

MEDSKY takes a series of images and produces a single image in which each pixel is the median of the corresponding pixels in the series of images. Note that this is a time-consuming operation. The algorithm used attempts to optimise itself for the amount of memory available and will run much faster if the user has a large working set.

Parameters:

FILES

The name of a text file, each line of which should give the name of an input image. These are the images that will be used to form the output median image.

OUTPUT

The name of the file to be created, in which the value of each pixel is the median value of the corresponding pixels in the input images. Note that OUTPUT must not have the same name as any of the input images - the results will be incorrect if this is the case.

IMGLOG

If LOG is set, IMGLOG supplies the name of the 'log' file produced. This has nothing but a data array containing the numbers from 1 to N of the images that supplied the corresponding median values in the output median image. (Note that if more than one image contained a given median value, the number chosen will be the lowest of the possible values - i.e. the first image has precedence)

SCALED

If set, then MEDSKY calculates the overall median value for each image in turn, and before using the data from an image, multiplies it by a scaling factor given by the median value of the first image divided by the median value of the image being used. This means that all images are conceptually scaled to the median level of the first image before being used. If SCALED is set a file - MEDSKY.LOG - is written giving the file name, median value, and scaling factor used for each image.

LOG

LOG can be set to produce a diagnostic image called a 'log' image as well as the normal output median image. A 'log' image is an integer array of the same size as the output image, in which each pixel contains the image number in the order given in the file giving the list of images) of the image that supplied the median value used. If LOG is set, the name of the 'log' image is supplied by the IMGLOG parameter.

See also:

FIGARO: BCLEAN, CLEAN, COSREJ, MEDFILT.
KAPPA: MEDIAN, MSTATS.

Source comments:

 M E D S K Y

 MEDSKY is a program for constructing sky flats for direct imaging,
 using an algorithm due to Schneider, Schmidt and Gunn.

 A list of images is read in from file,and the medians of
 equivalent pixels in all the images are found, eg. for N images
 (of any size), the first pixel ( the top-left one) in each image
 is fetched : the median of these N pixels is then found, the
 result becoming the first pixel in the output image. This process
 is repeated for all the pixels in the input images to construct a
 complete output image. In practice, more than one pixel is
 analysed per pass through the image file list, the number being
 determined by the memory available to the program. MEDSKY attempts
 to minimise the page faulting which would occur by accessing too
 much virtual memory at once by processing fewer pixels per pass,
 on the assumption that the extra overheads in doing more passes
 will be more than offset by fewer page faults. This approach seems
 to have been justified, but performance is very sensitive to
 changes in the proportions of memory allocated, and it is possible
 that improvements could be made.

 Command parameters -

 FILES  The name of a .DAT file containing a list of names
        of images. All these images must have dimensions
        equal to those of the first image in the list, FIRST.

 OUTPUT The name of the result of the operation.  This can
        be the same as for FIRST.  If not, a new structure
        is created, with everything but the data a direct
        copy of the input.

 IMGLOG Only used if the LOG keyword is set.  This supplies
        the name of the 'log' image created.  This will contain
        a single integer data array.

 Important
 ~~~~~~~~~
 MEDSKY does not check that the output image name is not a member
 of the image file list.

 Command keywords  -

 SCALED If set, MEDSKY attempts to compensate for differences
        in data scale between the different images.  It does this
        by conceptually scaling all images so that they have the
        same median value as the first image.

 LOG    If set, MEDSKY creates a 'log' image.  This is an
        image with the same dimensions as the output image, with
        each pixel a number from 1 to N (the number of files)
        showing the image number which for each output pixel had
        the data value closest to the median value calculated.
        This can be used to see if any image dominates the others,
        or if there is any trend across the image. This is a rather
        specialised option, and is not expected to be used much.

 User variables used - None
                                  DJA / AAO. 16th Aug 1987