Using the task return value

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6.7.1 Using the task return value

The contents of the return value of imstat are in a Python dictionary of key-value sets. For example,

xstat = imstat()

will assign this to the Python variable xstat.

The keys for xstat are then:

   KEYS
   blc          - absolute PIXEL coordinate of the bottom left corner of
                  the bounding box surrounding the selected region
   blcf         - Same as blc, but uses WORLD coordinates instead of pixels
   trc          - the absolute PIXEL coordinate of the top right corner
                  of the bounding box surrounding the selected region
   trcf         - Same as trc, but uses WORLD coordinates instead of pixels
   flux         - the integrated flux density if the beam is defined and
                  the if brightness units are $Jy/beam$
   npts         - the number of unmasked points used
   max          - the maximum pixel value
   min          - minimum pixel value
   maxpos       - absolute PIXEL coordinate of maximum pixel value
   maxposf      - Same as maxpos, but uses WORLD coordinates instead of pixels
   minpos       - absolute pixel coordinate of minimum pixel value
   minposf      - Same as minpos, but uses WORLD coordinates instead of pixels
   sum          - the sum of the pixel values: $\sum I_i$
   sumsq        - the sum of the squares of the pixel values: $\sum I_i^2$
   mean         - the mean of pixel values:
                  $ar{I} = \sum I_i / n$
   sigma        - the standard deviation about the mean:
                  $\sigma^2 = (\sum I_i -ar{I})^2 / (n-1)$
   rms          - the root mean square:
                  $\sqrt {\sum I_i^2 / n}$
   median       - the median pixel value (if robust=T)
   medabsdevmed - the median of the absolute deviations from the
                  median (if robust=T)
   quartile     - the inter-quartile range (if robust=T). Find the points
                  which are 25% largest and 75% largest (the median is
                  50% largest), find their difference and divide that
                  difference by 2.

For example, an imstat call might be

   default(’imstat’)
   imagename = ’ngc5921.demo.cleanimg.image’  #  The NGC5921 image cube
   box       = ’108,108,148,148’              #  20 pixels around the center
   chans     = ’21’                           #  channel 21

   xstat = imstat()

In the terminal window, imstat reports:

Statistics on  ngc5921.usecase.clean.image

Region ---
   -- bottom-left corner (pixel) [blc]: [108, 108, 0, 21]
   -- top-right corner (pixel) [trc]:   [148, 148, 0, 21]
   -- bottom-left corner (world) [blcf]: 15:22:20.076, +04.58.59.981, I, 1.41332e+09Hz
   -- top-right corner( world) [trcf]: 15:21:39.919, +05.08.59.981, I, 1.41332e+09Hz

Values --
   -- flux [flux]:              0.111799236126
   -- number of points [npts]:  1681.0
   -- maximum value [max]:      0.029451508075
   -- minimum value [min]:     -0.00612453464419
   -- position of max value (pixel) [maxpos]:  [124, 131, 0, 21]
   -- position of min value (pixel) [minpos]:  [142, 110, 0, 21]
   -- position of max value (world) [maxposf]: 15:22:04.016, +05.04.44.999, I, 1.41332e+09Hz
   -- position of min value (world) [minposf]: 15:21:45.947, +04.59.29.990, I, 1.41332e+09Hz
   -- Sum of pixel values [sum]: 1.32267159822
   -- Sum of squared pixel values [sumsq]: 0.0284534543692

Statistics ---
   -- Mean of the pixel values [mean]:       0.000786836167885
   -- Standard deviation of the Mean [sigma]: 0.00403944306904
   -- Root mean square [rms]:               0.00411418313161
   -- Median of the pixel values [median]:     0.000137259965413
   -- Median of the deviations [medabsdevmed]:       0.00152346317191
   -- Quartile [quartile]:                       0.00305395200849

The return value in xstat is

CASA <152>: xstat
  Out[152]:
{’blc’: array([108, 108,   0,  21]),
’blcf’: ’15:22:20.076, +04.58.59.981, I, 1.41332e+09Hz’,
’flux’: array([ 0.11179924]),
’max’: array([ 0.02945151]),
’maxpos’: array([124, 131,   0,  21]),
’maxposf’: ’15:22:04.016, +05.04.44.999, I, 1.41332e+09Hz’,
’mean’: array([ 0.00078684]),
’medabsdevmed’: array([ 0.00152346]),
’median’: array([ 0.00013726]),
’min’: array([-0.00612453]),
’minpos’: array([142, 110,   0,  21]),
’minposf’: ’15:21:45.947, +04.59.29.990, I, 1.41332e+09Hz’,
’npts’: array([ 1681.]),
’quartile’: array([ 0.00305395]),
’rms’: array([ 0.00411418]),
’sigma’: array([ 0.00403944]),
’sum’: array([ 1.3226716]),
’sumsq’: array([ 0.02845345]),
’trc’: array([148, 148,   0,  21]),
’trcf’: ’15:21:39.919, +05.08.59.981, I, 1.41332e+09Hz’}

ALERT: The return dictionary currently includes NumPy array values, which have to be accessed by an array index to get the array value. To access these dictionary elements, use the standard Python dictionary syntax, e.g.

xstat[<key string>][<array index>]

For example, to extract the standard deviation as a number

mystddev = xstat[’sigma’][0]
print ’Sigma = ’+str(xstat[’sigma’][0])

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